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The sea, connected as the world ocean or simply the
ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...
, is the body of salty water that covers approximately 71% of the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
's surface. The word sea is also used to denote second-order sections of the sea, such as the
Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on ...
, as well as certain large, entirely landlocked,
saltwater lake A salt lake or saline lake is a landlocked body of water that has a concentration of salts (typically sodium chloride) and other dissolved minerals significantly higher than most lakes (often defined as at least three grams of salt per litre) ...
s, such as the
Caspian Sea The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central A ...
. The sea moderates Earth's
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
and has important roles in the
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
,
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
, and nitrogen cycles. Humans harnessing and studying the sea have been recorded since ancient times, and evidenced well into
prehistory Prehistory, also known as pre-literary history, is the period of human history between the use of the first stone tools by hominins 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The us ...
, while its modern scientific study is called
oceanography Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynami ...
. The most abundant solid dissolved in
seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
is
sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
. The water also contains
salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
of
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
,
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
,
potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
, and mercury, amongst many other elements, some in minute concentrations.
Salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
varies widely, being lower near the surface and the mouths of large
rivers A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...
and higher in the depths of the ocean; however, the relative proportions of dissolved salts vary little across the oceans. Winds blowing over the surface of the sea produce
waves Waves most often refers to: * Waves, oscillations accompanied by a transfer of energy that travel through space or mass. * Wind waves, surface waves that occur on the free surface of bodies of water. Waves may also refer to: Music *Waves (ban ...
, which
break Break or Breaks or The Break may refer to: Time off from duties * Recess (break), time in which a group of people is temporarily dismissed from its duties * Break (work), time off during a shift/recess ** Coffee break, a short mid-morning rest ...
when they enter the shallow water near
land Land, also known as dry land, ground, or earth, is the solid terrestrial surface of the planet Earth that is not submerged by the ocean or other bodies of water. It makes up 29% of Earth's surface and includes the continents and various isla ...
. Winds also create
surface currents An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth conto ...
through friction, setting up slow but stable circulations of water throughout the oceans. The directions of the circulation are governed by several factors including the shapes of the continents and
Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star Po ...
(through the
Coriolis effect In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
). Deep-sea currents, known as the
global conveyor belt Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temper ...
, carry cold water from near the poles to every ocean.
Tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...
s, the generally twice-daily rise and fall of
sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardis ...
s, are caused by Earth's rotation and the gravitational effects of the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
and, to a lesser extent, of the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. Tides may have a very high range in
bays A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a gulf, sea, sound, or bight. A cove is a small, circular bay with a na ...
or
estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environme ...
.
Submarine earthquake A submarine, undersea, or underwater earthquake is an earthquake that occurs underwater at the bottom of a body of water, especially an ocean. They are the leading cause of tsunamis. The magnitude can be measured scientifically by the use of the ...
s arising from
tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
movements under the oceans can lead to destructive
tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater exp ...
s, as can volcanoes, huge
landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environmen ...
s, or the impact of large
meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...
s. A wide variety of organisms, including
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
,
protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...
s,
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
, plants,
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
, and animals, lives in the sea, which offers a wide range of
marine habitat Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea (the term ''marine'' comes from the Latin ''mare'', meaning sea or ocean). A habitat is an ecological or environmental ...
s and
ecosystems An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
, ranging vertically from the sunlit surface and
shoreline A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past a ...
to the great depths and pressures of the cold, dark
abyssal zone The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word , meaning bottomless. At depths of , this zone remains in perpetual darkness. It covers 83% of the total area of the ocean an ...
, and in latitude from the cold waters under
polar ice cap A polar ice cap or polar cap is a high-latitude region of a planet, dwarf planet, or natural satellite that is covered in ice. There are no requirements with respect to size or composition for a body of ice to be termed a polar ice cap, no ...
s to the warm waters of
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...
s in tropical regions. Many of the major groups of organisms evolved in the sea and life may have started there. The sea provides substantial supplies of food for humans, mainly
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
, but also
shellfish Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environ ...
,
mammals Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
and
seaweed Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and '' Chlorophyta'' (green) macroalgae. Seaweed species such as ...
, whether caught by fishermen or
farmed Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...
underwater. Other human uses of the sea include
trade Trade involves the transfer of goods and services from one person or entity to another, often in exchange for money. Economists refer to a system or network that allows trade as a market. An early form of trade, barter, saw the direct exc ...
, travel, mineral extraction,
power generation Electricity generation is the process of generating electric power from sources of primary energy. For utilities in the electric power industry, it is the stage prior to its delivery ( transmission, distribution, etc.) to end users or its stor ...
,
warfare War is an intense armed conflict between states, governments, societies, or paramilitary groups such as mercenaries, insurgents, and militias. It is generally characterized by extreme violence, destruction, and mortality, using regu ...
, and leisure activities such as
swimming Swimming is the self-propulsion of a person through water, or other liquid, usually for recreation, sport, exercise, or survival. Locomotion is achieved through coordinated movement of the limbs and the body to achieve hydrodynamic thrust that r ...
,
sailing Sailing employs the wind—acting on sails, wingsails or kites—to propel a craft on the surface of the ''water'' (sailing ship, sailboat, raft, windsurfer, or kitesurfer), on ''ice'' (iceboat) or on ''land'' ( land yacht) over a chose ...
, and
scuba diving Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for " Self-Contained Underwater Breathing Apparatus", was coined by Chr ...
. Many of these activities create
marine pollution Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residential waste, particles, noise, excess carbon dioxide or invasive organisms enter the ocean and cause harmful effects there. The major ...
. The sea has been an integral element for humans throughout history and culture.


Definition

The sea is the interconnected system of all the Earth's oceanic waters, including the
Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
,
Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...
,
Indian Indian or Indians may refer to: Peoples South Asia * Indian people, people of Indian nationality, or people who have an Indian ancestor ** Non-resident Indian, a citizen of India who has temporarily emigrated to another country * South Asia ...
,
Southern Southern may refer to: Businesses * China Southern Airlines, airline based in Guangzhou, China * Southern Airways, defunct US airline * Southern Air, air cargo transportation company based in Norwalk, Connecticut, US * Southern Airways Express, M ...
and
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
s. "Sea." ''Merriam-Webster.com Dictionary'', Merriam-Webster, https://www.merriam-webster.com/dictionary/sea. Accessed 14 March 2021. However, the word "sea" can also be used for many specific, much smaller bodies of seawater, such as the
North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian ...
or the
Red Sea The Red Sea ( ar, البحر الأحمر - بحر القلزم, translit=Modern: al-Baḥr al-ʾAḥmar, Medieval: Baḥr al-Qulzum; or ; Coptic: ⲫⲓⲟⲙ ⲛ̀ϩⲁϩ ''Phiom Enhah'' or ⲫⲓⲟⲙ ⲛ̀ϣⲁⲣⲓ ''Phiom ǹšari''; ...
. There is no sharp distinction between seas and
ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...
s, though generally seas are smaller, and are often partly (as
marginal sea This is a list of seas of the World Ocean, including marginal seas, areas of water, various gulfs, bights, bays, and straits. Terminology * Ocean – the four to seven largest named bodies of water in the World Ocean, all of which have "Ocea ...
s or particularly as the
Mediterranean sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on ...
) or wholly (as inland seas) enclosed by
land Land, also known as dry land, ground, or earth, is the solid terrestrial surface of the planet Earth that is not submerged by the ocean or other bodies of water. It makes up 29% of Earth's surface and includes the continents and various isla ...
. However, an exception to this is the
Sargasso Sea The Sargasso Sea () is a region of the Atlantic Ocean bounded by four currents forming an ocean gyre. Unlike all other regions called seas, it has no land boundaries. It is distinguished from other parts of the Atlantic Ocean by its charac ...
which has no coastline and lies within a circular current, the North Atlantic Gyre. Seas are generally larger than lakes and contain salt water, but the
Sea of Galilee The Sea of Galilee ( he, יָם כִּנֶּרֶת, Judeo-Aramaic: יַמּא דטבריא, גִּנֵּיסַר, ar, بحيرة طبريا), also called Lake Tiberias, Kinneret or Kinnereth, is a freshwater lake in Israel. It is the lowest ...
is a
freshwater lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...
. The
United Nations Convention on the Law of the Sea The United Nations Convention on the Law of the Sea (UNCLOS), also called the Law of the Sea Convention or the Law of the Sea Treaty, is an international agreement that establishes a legal framework for all marine and maritime activities. , 167 ...
states that all of the ocean is "sea".


Physical science

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
is the only known
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
with seas of liquid
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
on its surface, although
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
possesses
ice caps In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets. Description Ice caps are not constrained by topographical features ...
and similar planets in other solar systems may have oceans. Earth's of sea contain about 97.2 percent of its known water and cover approximately 71 percent of its surface. Another 2.15% of Earth's water is frozen, found in the sea ice covering the
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
, the ice cap covering
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
and its adjacent seas, and various
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...
s and surface deposits around the world. The remainder (about 0.65% of the whole) form underground reservoirs or various stages of the
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly co ...
, containing the
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...
encountered and used by most
terrestrial life The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', for ''gigaannum'') and evide ...
:
vapor In physics, a vapor (American English) or vapour (British English and Canadian English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Her ...
in the
air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
, the
cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...
s it slowly forms, the
rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
falling from them, and the
lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
s and
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...
s spontaneously formed as its waters flow again and again to the sea.
NOAA The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...
.
Lesson 7: The Water Cycle
" in ''Ocean Explorer''.
The scientific study of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
and Earth's
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly co ...
is
hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...
;
hydrodynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) a ...
studies the
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
of water in motion. The more recent study of the sea in particular is
oceanography Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynami ...
. This began as the study of the shape of the ocean's currents but has since expanded into a large and
multidisciplinary Interdisciplinarity or interdisciplinary studies involves the combination of multiple academic disciplines into one activity (e.g., a research project). It draws knowledge from several other fields like sociology, anthropology, psychology, ec ...
field:Monkhouse, F.J. (1975) ''Principles of Physical Geography''. pp. 327–28. Hodder & Stoughton. . it examines the properties of
seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
; studies
waves Waves most often refers to: * Waves, oscillations accompanied by a transfer of energy that travel through space or mass. * Wind waves, surface waves that occur on the free surface of bodies of water. Waves may also refer to: Music *Waves (ban ...
,
tides Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables c ...
, and currents; charts coastlines and maps the seabeds; and studies
marine life Marine life, sea life, or ocean life is the plants, animals and other organisms that live in the salt water of seas or oceans, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. ...
. The subfield dealing with the sea's motion, its forces, and the forces acting upon it is known as
physical oceanography Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters. Physical oceanography is one of several sub-domains into which oceanography is div ...
.
Marine biology Marine biology is the scientific study of the biology of marine life, organisms in the sea. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifi ...
(biological oceanography) studies the
plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
,
animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
, and other organisms inhabiting marine ecosystems. Both are informed by
chemical oceanography Marine chemistry, also known as ocean chemistry or chemical oceanography, is influenced by plate tectonics and seafloor spreading, turbidity currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. The fiel ...
, which studies the behavior of elements and
molecules A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
within the oceans: particularly, at the moment, the ocean's role in the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major compon ...
and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
's role in the increasing acidification of seawater. Marine and maritime
geography Geography (from Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, an ...
charts the shape and shaping of the sea, while
marine geology Marine geology or geological oceanography is the study of the history and structure of the ocean floor. It involves geophysical, geochemical, sedimentological and paleontological investigations of the ocean floor and coastal zone. Marine geolog ...
(geological oceanography) has provided evidence of
continental drift Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of pl ...
and the
composition Composition or Compositions may refer to: Arts and literature *Composition (dance), practice and teaching of choreography *Composition (language), in literature and rhetoric, producing a work in spoken tradition and written discourse, to include v ...
and
structure of the Earth The internal structure of Earth is the solid portion of the Earth, excluding its atmosphere and hydrosphere. The structure consists of an outer silicate solid crust, a highly viscous asthenosphere and solid mantle, a liquid outer core whos ...
, clarified the process of
sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...
, and assisted the study of
volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
and
earthquakes An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fro ...
.


Seawater


Salinity

A characteristic of seawater is that it is salty. Salinity is usually measured in parts per thousand (
Per mille (from Latin , "in each thousand") is an expression that means parts per thousand. Other recognised spellings include per mil, per mill, permil, permill, or permille. The associated sign is written , which looks like a percent ...
or per mil), and the open ocean has about solids per litre, a salinity of 35 ‰. The Mediterranean Sea is slightly higher at 38 ‰, while the salinity of the northern Red Sea can reach 41‰. In contrast, some landlocked
hypersaline lake A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride, brines, and other salts, with saline levels surpassing that of ocean water (3.5%, i.e. ). Specific microbial species can thrive in ...
s have a much higher salinity, for example the
Dead Sea The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Ban ...
has dissolved solids per litre (300 ‰). While the constituents of table salt (
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
and
chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...
) make up about 85 percent of the solids in solution, there are also other metal ions such as
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
and
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
, and negative ions including sulphate,
carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
, and
bromide A bromide ion is the negatively charged form (Br−) of the element bromine, a member of the halogens group on the periodic table. Most bromides are colorless. Bromides have many practical roles, being found in anticonvulsants, flame-retardant ...
. Despite variations in the levels of salinity in different seas, the relative composition of the dissolved salts is stable throughout the world's oceans. Seawater is too saline for humans to drink safely, as the
kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...
s cannot excrete urine as salty as seawater. Although the amount of salt in the ocean remains relatively constant within the scale of millions of years, various factors affect the salinity of a body of water. Evaporation and by-product of ice formation (known as "brine rejection") increase salinity, whereas
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, sea ice melt, and runoff from land reduce it. The
Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and from ...
, for example, has many rivers flowing into it, and thus the sea could be considered as
brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estua ...
. Meanwhile, the
Red Sea The Red Sea ( ar, البحر الأحمر - بحر القلزم, translit=Modern: al-Baḥr al-ʾAḥmar, Medieval: Baḥr al-Qulzum; or ; Coptic: ⲫⲓⲟⲙ ⲛ̀ϩⲁϩ ''Phiom Enhah'' or ⲫⲓⲟⲙ ⲛ̀ϣⲁⲣⲓ ''Phiom ǹšari''; ...
is very salty due to its high evaporation rate.NOAA (11 Jan 2013).
Drinking Seawater Can Be Deadly to Humans
".


Temperature

Sea temperature depends on the amount of solar radiation falling on its surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over while near the poles the temperature in equilibrium with the sea ice is about . There is a continuous circulation of water in the oceans. Warm surface currents cool as they move away from the tropics, and the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, before eventually welling up again towards the surface. Deep seawater has a temperature between and in all parts of the globe. Seawater with a typical salinity of 35 ‰ has a freezing point of about −1.8 °C (28.8 °F). When its temperature becomes low enough,
ice crystals Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth, n ...
form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as
frazil Frazil ice is a collection of loose, randomly oriented ice crystals millimeter and sub-millimeter in size, with various shapes, e.g. elliptical disks, dendrites, needles and of an irregular nature. Frazil ice forms during the winter in open-wat ...
. In calm conditions this freezes into a thin flat sheet known as nilas, which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join into flat discs known as pancakes. These slide under each other and coalesce to form floes. In the process of freezing, salt water and air are trapped between the ice crystals. Nilas may have a salinity of 12–15 ‰, but by the time the
sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's o ...
is one year old, this falls to 4–6 ‰.


Oxygen concentration

The amount of oxygen found in seawater depends primarily on the plants growing in it. These are mainly algae, including
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...
, with some
vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...
s such as
seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families ( Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the ...
es. In daylight the photosynthetic activity of these plants produces oxygen, which dissolves in the seawater and is used by marine animals. At night, photosynthesis stops, and the amount of dissolved oxygen declines. In the deep sea, where insufficient light penetrates for plants to grow, there is very little dissolved oxygen. In its absence, organic material is broken down by
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
bacteria producing
hydrogen sulphide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
.
Climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
is likely to reduce levels of oxygen in surface waters, since the solubility of oxygen in water falls at higher temperatures.
Ocean deoxygenation Ocean deoxygenation is the reduction of the oxygen content of the global oceans and coastal zones due to human activities as a consequence of anthropogenic emissions of carbon dioxide and eutrophication-driven excess production. It is manifest ...
is projected to increase hypoxia by 10%, and triple suboxic waters (oxygen concentrations 98% less than the mean surface concentrations), for each 1 °C of upper ocean warming.


Light

The amount of light that penetrates the sea depends on the angle of the sun, the weather conditions and the
turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...
of the water. Much light gets reflected at the surface, and red light gets absorbed in the top few metres. Yellow and green light reach greater depths, and blue and violet light may penetrate as deep as . There is insufficient light for photosynthesis and plant growth beyond a depth of about .


Sea level

Over most of geologic time, the sea level has been higher than it is today. The main factor affecting sea level over time is the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term. At the
last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eu ...
, some 20,000 years ago, the sea level was about lower than in present times (2012).Sea Level and Climate
. USGS. By Richard Z. Poore, Richard S. Williams, Jr., and Christopher Tracey.
For at least the last 100 years, sea level has been rising at an average rate of about per year. Most of this rise can be attributed to an increase in the temperature of the sea due to
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
, and the resulting slight thermal expansion of the upper of water. Additional contributions, as much as one quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.


Waves

Wind blowing over the surface of a body of water forms
waves Waves most often refers to: * Waves, oscillations accompanied by a transfer of energy that travel through space or mass. * Wind waves, surface waves that occur on the free surface of bodies of water. Waves may also refer to: Music *Waves (ban ...
that are perpendicular to the direction of the wind. The friction between air and water caused by a gentle breeze on a pond causes
ripples Ripple may refer to: Science and technology * Capillary wave, commonly known as ripple, a wave traveling along the phase boundary of a fluid ** Ripple, more generally a disturbance, for example of spacetime in gravitational waves * Ripple (electri ...
to form. A strong blow over the ocean causes larger waves as the moving air pushes against the raised ridges of water. The waves reach their maximum height when the rate at which they are travelling nearly matches the speed of the wind. In open water, when the wind blows continuously as happens in the Southern Hemisphere in the
Roaring Forties The Roaring Forties are strong westerly winds found in the Southern Hemisphere, generally between the latitudes of 40°S and 50°S. The strong west-to-east air currents are caused by the combination of air being displaced from the Equator ...
, long, organised masses of water called swell roll across the ocean. If the wind dies down, the wave formation is reduced, but already-formed waves continue to travel in their original direction until they meet land. The size of the waves depends on the
fetch Fetch may refer to: Books * ''Fetch'', a 2012 book by Alan MacDonald and David Roberts * ''The Fetch'', a 2006 book by Chris Humphreys * ''The Fetch'', a 2009 book by Laura Whitcomb * ''The Fetch'', a 1991 book by Robert Holdstock * ''Fazbear ...
, the distance that the wind has blown over the water and the strength and duration of that wind. When waves meet others coming from different directions, interference between the two can produce broken, irregular seas.
Constructive interference In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...
can cause individual (unexpected)
rogue wave Rogue waves (also known as freak waves, monster waves, episodic waves, killer waves, extreme waves, and abnormal waves) are unusually large, unpredictable, and suddenly appearing surface waves that can be extremely dangerous to ships, even to lar ...
s much higher than normal.Garrison, Tom (2012)
''Essentials of Oceanography''
. 6th ed. pp. 204 ff. Brooks/Cole, Belmont. .
Most waves are less than high and it is not unusual for strong storms to double or triple that height;
offshore construction Offshore construction is the installation of structures and facilities in a marine environment, usually for the production and transmission of electricity, oil, gas and other resources. It is also called maritime engineering. Construction a ...
such as
wind farms Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few h ...
and
oil platform An oil platform (or oil rig, offshore platform, oil production platform, and similar terms) is a large structure with facilities to extract and process petroleum and natural gas that lie in rock formations beneath the seabed. Many oil platfor ...
s use
metocean In offshore and coastal engineering, metocean refers to the syllabic abbreviation of meteorology and (physical) oceanography. Metocean study In various stages of an offshore or coastal engineering project a metocean study will be undertaken. ...
statistics from measurements in computing the wave forces (due to for instance the
hundred-year wave A hundred-year wave is a statistically projected water wave, the height of which, on average, is met or exceeded once in a hundred years for a given location. The likelihood of this wave height being attained at least once in the hundred-year peri ...
) they are designed against. Rogue waves, however, have been documented at heights above . The top of a wave is known as the crest, the lowest point between waves is the trough and the distance between the crests is the wavelength. The wave is pushed across the surface of the sea by the wind, but this represents a transfer of energy and not a horizontal movement of water. As waves approach land and move into shallow water, they change their behavior. If approaching at an angle, waves may bend (
refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomen ...
) or wrap rocks and headlands (
diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
). When the wave reaches a point where its deepest oscillations of the water contact the
seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most o ...
, they begin to slow down. This pulls the crests closer together and increases the waves' height, which is called wave shoaling. When the ratio of the wave's height to the water depth increases above a certain limit, it "
breaks Break or Breaks or The Break may refer to: Time off from duties * Recess (break), time in which a group of people is temporarily dismissed from its duties * Break (work), time off during a shift/recess ** Coffee break, a short mid-morning rest ...
", toppling over in a mass of foaming water. This rushes in a sheet up the beach before retreating into the sea under the influence of gravity.


Tsunami

A tsunami is an unusual form of wave caused by an infrequent powerful event such as an underwater earthquake or landslide, a meteorite impact, a volcanic eruption or a collapse of land into the sea. These events can temporarily lift or lower the surface of the sea in the affected area, usually by a few feet. The potential energy of the displaced seawater is turned into kinetic energy, creating a shallow wave, a tsunami, radiating outwards at a velocity proportional to the square root of the depth of the water and which therefore travels much faster in the open ocean than on a continental shelf. In the deep open sea, tsunamis have wavelengths of around , travel at speeds of over 600 miles per hour (970 km/hr) and usually have a height of less than three feet, so they often pass unnoticed at this stage. In contrast, ocean surface waves caused by winds have wavelengths of a few hundred feet, travel at up to and are up to high. As a tsunami moves into shallower water its speed decreases, its wavelength shortens and its amplitude increases enormously, behaving in the same way as a wind-generated wave in shallow water, but on a vastly greater scale. Either the trough or the crest of a tsunami can arrive at the coast first. In the former case, the sea draws back and leaves subtidal areas close to the shore exposed which provides a useful warning for people on land. When the crest arrives, it does not usually break but rushes inland, flooding all in its path. Much of the destruction may be caused by the flood water draining back into the sea after the tsunami has struck, dragging debris and people with it. Often several tsunami are caused by a single geological event and arrive at intervals of between eight minutes and two hours. The first wave to arrive on shore may not be the biggest or most destructive.


Currents

Wind blowing over the surface of the sea causes
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of ...
at the interface between air and sea. Not only does this cause waves to form but it also makes the surface seawater move in the same direction as the wind. Although winds are variable, in any one place they predominantly blow from a single direction and thus a surface current can be formed. Westerly winds are most frequent in the mid-latitudes while easterlies dominate the tropics. When water moves in this way, other water flows in to fill the gap and a circular movement of surface currents known as a gyre is formed. There are five main gyres in the world's oceans: two in the Pacific, two in the Atlantic and one in the Indian Ocean. Other smaller gyres are found in lesser seas and a single gyre flows around
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
. These gyres have followed the same routes for millennia, guided by the
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sc ...
of the land, the wind direction and the
Coriolis effect In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
. The surface currents flow in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The water moving away from the equator is warm, and that flowing in the reverse direction has lost most of its heat. These currents tend to moderate the Earth's climate, cooling the equatorial region and warming regions at higher latitudes. Global climate and weather forecasts are powerfully affected by the world ocean, so
global climate model A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms ...
ling makes use of ocean circulation models as well as models of other major components such as the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
, land surfaces, aerosols and sea ice. Ocean models make use of a branch of physics, geophysical fluid dynamics, that describes the large-scale flow of fluids such as seawater. Surface currents only affect the top few hundred metres of the sea, but there are also large-scale flows in the ocean depths caused by the movement of deep water masses. A main deep ocean current flows through all the world's oceans and is known as the
thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temp ...
or global conveyor belt. This movement is slow and is driven by differences in density of the water caused by variations in salinity and temperature. At high latitudes the water is chilled by the low atmospheric temperature and becomes saltier as sea ice crystallizes out. Both these factors make it denser, and the water sinks. From the deep sea near Greenland, such water flows southwards between the continental landmasses on either side of the Atlantic. When it reaches the Antarctic, it is joined by further masses of cold, sinking water and flows eastwards. It then splits into two streams that move northwards into the Indian and Pacific Oceans. Here it is gradually warmed, becomes less dense, rises towards the surface and loops back on itself. It takes a thousand years for this circulation pattern to be completed. Besides gyres, there are temporary surface currents that occur under specific conditions. When waves meet a shore at an angle, a longshore current is created as water is pushed along parallel to the coastline. The water swirls up onto the beach at right angles to the approaching waves but drains away straight down the slope under the effect of gravity. The larger the breaking waves, the longer the beach and the more oblique the wave approach, the stronger is the longshore current. These currents can shift great volumes of sand or pebbles, create spits and make beaches disappear and water channels silt up. A
rip current A rip current, often simply called a rip (or misleadingly a ''rip tide''), is a specific kind of water current that can occur near beaches with breaking waves. A rip is a strong, localized, and narrow current of water which moves directly away ...
can occur when water piles up near the shore from advancing waves and is funnelled out to sea through a channel in the seabed. It may occur at a gap in a
sandbar In oceanography, geomorphology, and geoscience, a shoal is a natural submerged ridge, bank, or bar that consists of, or is covered by, sand or other unconsolidated material and rises from the bed of a body of water to near the surface. ...
or near a man-made structure such as a
groyne A groyne (in the U.S. groin) is a rigid hydraulic structure built perpendicularly from an ocean shore (in coastal engineering) or a river bank, interrupting water flow and limiting the movement of sediment. It is usually made out of wood, concr ...
. These strong currents can have a velocity of per second, can form at different places at different stages of the tide and can carry away unwary bathers. Temporary upwelling currents occur when the wind pushes water away from the land and deeper water rises to replace it. This cold water is often rich in nutrients and creates blooms of phytoplankton and a great increase in the productivity of the sea.


Tides

Tides are the regular rise and fall in water level experienced by seas and oceans in response to the
gravitational In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...
influences of the Moon and the Sun, and the effects of the Earth's rotation. During each tidal cycle, at any given place the water rises to a maximum height known as "high tide" before ebbing away again to the minimum "low tide" level. As the water recedes, it uncovers more and more of the
foreshore The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various specie ...
, also known as the intertidal zone. The difference in height between the high tide and low tide is known as the tidal range or tidal amplitude. Most places experience two high tides each day, occurring at intervals of about 12 hours and 25 minutes. This is half the 24 hours and 50 minute period that it takes for the Earth to make a complete revolution and return the Moon to its previous position relative to an observer. The Moon's mass is some 27 million times smaller than the Sun, but it is 400 times closer to the Earth.
Tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
or tide-raising force decreases rapidly with distance, so the moon has more than twice as great an effect on tides as the Sun. A bulge is formed in the ocean at the place where the Earth is closest to the Moon, because it is also where the effect of the Moon's gravity is stronger. On the opposite side of the Earth, the lunar force is at its weakest and this causes another bulge to form. As the Moon rotates around the Earth, so do these ocean bulges move around the Earth. The gravitational attraction of the Sun is also working on the seas, but its effect on tides is less powerful than that of the Moon, and when the Sun, Moon and Earth are all aligned (full moon and new moon), the combined effect results in the high "spring tides". In contrast, when the Sun is at 90° from the Moon as viewed from Earth, the combined gravitational effect on tides is less causing the lower "neap tides". A
storm surge A storm surge, storm flood, tidal surge, or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low-pressure weather systems, such as cyclones. It is measured as the rise in water level above the ...
can occur when high winds pile water up against the coast in a shallow area and this, coupled with a low pressure system, can raise the surface of the sea at high tide dramatically.


Ocean basins

The Earth is composed of a magnetic central
core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...
, a mostly liquid
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
and a hard rigid outer shell (or
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...
), which is composed of the Earth's rocky crust and the deeper mostly solid outer layer of the mantle. On land the crust is known as the
continental crust Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial'' be ...
while under the sea it is known as the
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...
. The latter is composed of relatively dense
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
and is some five to ten kilometres (three to six miles) thick. The relatively thin lithosphere floats on the weaker and hotter mantle below and is fractured into a number of
tectonic plates Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
. In mid-ocean, magma is constantly being thrust through the seabed between adjoining plates to form mid-oceanic ridges and here convection currents within the mantle tend to drive the two plates apart. Parallel to these ridges and nearer the coasts, one oceanic plate may slide beneath another oceanic plate in a process known as
subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
. Deep
trenches A trench is a type of excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit). In geology, trenches result from erosi ...
are formed here and the process is accompanied by friction as the plates grind together. The movement proceeds in jerks which cause earthquakes, heat is produced and
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...
is forced up creating underwater mountains, some of which may form chains of volcanic islands near to deep trenches. Near some of the boundaries between the land and sea, the slightly denser oceanic plates slide beneath the continental plates and more subduction trenches are formed. As they grate together, the continental plates are deformed and buckle causing mountain building and seismic activity. The Earth's deepest trench is the
Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maximum known ...
which extends for about across the seabed. It is near the
Mariana Islands The Mariana Islands (; also the Marianas; in Chamorro: ''Manislan Mariånas'') are a crescent-shaped archipelago comprising the summits of fifteen longitudinally oriented, mostly dormant volcanic mountains in the northwestern Pacific Ocean, betw ...
, a volcanic
archipelago An archipelago ( ), sometimes called an island group or island chain, is a chain, cluster, or collection of islands, or sometimes a sea containing a small number of scattered islands. Examples of archipelagos include: the Indonesian Arch ...
in the West Pacific. Its deepest point is 10.994 kilometres (nearly 7 miles) below the surface of the sea.


Coasts

The zone where land meets sea is known as the
coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...
and the part between the lowest spring tides and the upper limit reached by splashing waves is the
shore A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past a ...
. A
beach A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles, etc., or biological sources, such as mollusc s ...
is the accumulation of sand or shingle on the shore. A
headland A headland, also known as a head, is a coastal landform, a point of land usually high and often with a sheer drop, that extends into a body of water. It is a type of promontory. A headland of considerable size often is called a cape.Whittow, J ...
is a point of land jutting out into the sea and a larger
promontory A promontory is a raised mass of land that projects into a lowland or a body of water (in which case it is a peninsula). Most promontories either are formed from a hard ridge of rock that has resisted the erosive forces that have removed the ...
is known as a
cape A cape is a clothing accessory or a sleeveless outer garment which drapes the wearer's back, arms, and chest, and connects at the neck. History Capes were common in medieval Europe, especially when combined with a hood in the chaperon. Th ...
. The indentation of a coastline, especially between two headlands, is a
bay A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a gulf, sea, sound, or bight. A cove is a small, circular bay with a na ...
, a small bay with a narrow inlet is a
cove A cove is a small type of bay or coastal inlet. Coves usually have narrow, restricted entrances, are often circular or oval, and are often situated within a larger bay. Small, narrow, sheltered bays, inlets, creeks, or recesses in a coast are o ...
and a large bay may be referred to as a
gulf A gulf is a large inlet from the ocean into the landmass, typically with a narrower opening than a bay, but that is not observable in all geographic areas so named. The term gulf was traditionally used for large highly-indented navigable bodie ...
. Coastlines are influenced by a number of factors including the strength of the waves arriving on the shore, the gradient of the land margin, the composition and hardness of the coastal rock, the inclination of the off-shore slope and the changes of the level of the land due to local uplift or submergence. Normally, waves roll towards the shore at the rate of six to eight per minute and these are known as constructive waves as they tend to move material up the beach and have little erosive effect. Storm waves arrive on shore in rapid succession and are known as destructive waves as the
swash Swash, or forewash in geography, is a turbulence, turbulent layer of water that washes up on the beach after an incoming ocean surface wave, wave has broken. The swash action can move beach materials up and down the beach, which results in the ...
moves beach material seawards. Under their influence, the sand and shingle on the beach is ground together and abraded. Around high tide, the power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure. At the same time, sand and pebbles have an erosive effect as they are thrown against the rocks. This tends to undercut the cliff, and normal
weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...
processes such as the action of frost follows, causing further destruction. Gradually, a wave-cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave-erosion. Material worn from the margins of the land eventually ends up in the sea. Here it is subject to attrition as currents flowing parallel to the coast scour out channels and transport sand and pebbles away from their place of origin. Sediment carried to the sea by rivers settles on the seabed causing deltas to form in estuaries. All these materials move back and forth under the influence of waves, tides and currents. Dredging removes material and deepens channels but may have unexpected effects elsewhere on the coastline. Governments make efforts to prevent flooding of the land by the building of
breakwaters A breakwater is a permanent structure constructed at a coastal area to protect against tides, currents, waves, and storm surges. Part of a coastal management system, breakwaters are installed to minimize erosion, and to protect anchorages, h ...
,
seawall A seawall (or sea wall) is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservation ...
s, dykes and levees and other sea defences. For instance, the
Thames Barrier The Thames Barrier is a retractable barrier system built to protect the floodplain of most of Greater London from exceptionally high tides and storm surges moving up from the North Sea. It has been operational since 1982. When needed, it is c ...
is designed to protect London from a storm surge, while the failure of the dykes and levees around
New Orleans New Orleans ( , ,New Orleans
during
Hurricane Katrina Hurricane Katrina was a destructive Category 5 Atlantic hurricane that caused over 1,800 fatalities and $125 billion in damage in late August 2005, especially in the city of New Orleans and the surrounding areas. It was at the time the cost ...
created a
humanitarian crisis A humanitarian crisis (or sometimes humanitarian disaster) is defined as a singular event or a series of events that are threatening in terms of health, safety or well-being of a community or large group of people. It may be an internal or extern ...
in the United States.


Water cycle

The sea plays a part in the water or hydrological cycle, in which water
evaporates Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
from the ocean, travels through the atmosphere as vapour, condenses, falls as rain or snow, thereby sustaining life on land, and largely returns to the sea. Even in the
Atacama Desert The Atacama Desert ( es, Desierto de Atacama) is a desert plateau in South America covering a 1,600 km (990 mi) strip of land on the Pacific coast, west of the Andes Mountains. The Atacama Desert is the driest nonpolar desert in th ...
, where little rain ever falls, dense clouds of fog known as the camanchaca blow in from the sea and support plant life. In central Asia and other large land masses, there are
endorheic basin An endorheic basin (; also spelled endoreic basin or endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water, such as rivers or oceans, but drainage converges instead into lakes ...
s which have no outlet to the sea, separated from the ocean by mountains or other natural geologic features that prevent the water draining away. The
Caspian Sea The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central A ...
is the largest one of these. Its main inflow is from the River Volga, there is no outflow and the evaporation of water makes it saline as dissolved minerals accumulate. The
Aral Sea The Aral Sea ( ; kk, Арал теңізі, Aral teñızı; uz, Орол денгизи, Orol dengizi; kaa, Арал теңизи, Aral teńizi; russian: Аральское море, Aral'skoye more) was an endorheic lake lying between Kazak ...
in Kazakhstan and Uzbekistan, and Pyramid Lake in the western United States are further examples of large, inland saline water-bodies without drainage. Some endorheic lakes are less salty, but all are sensitive to variations in the quality of the inflowing water.


Carbon cycle

Oceans contain the greatest quantity of actively cycled carbon in the world and are second only to the
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...
in the amount of carbon they store. The oceans' surface layer holds large amounts of
dissolved organic carbon Dissolved organic carbon (DOC) is the fraction of organic carbon operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometers. The fraction remaining on the filter is called partic ...
that is exchanged rapidly with the atmosphere. The deep layer's concentration of dissolved inorganic carbon is about 15 percent higher than that of the surface layer and it remains there for much longer periods of time.
Thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temp ...
exchanges carbon between these two layers. Carbon enters the ocean as atmospheric carbon dioxide dissolves in the surface layers and is converted into carbonic acid,
carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
, and
bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochemi ...
: :CO2 (gas) CO2 (aq) :CO2 (aq) + H2O H2CO3 :H2CO3 HCO3 + H+ :HCO3 CO32− + H+ It can also enter through rivers as dissolved organic carbon and is converted by photosynthetic organisms into organic carbon. This can either be exchanged throughout the food chain or precipitated into the deeper, more carbon rich layers as dead soft tissue or in shells and bones as calcium carbonate. It circulates in this layer for long periods of time before either being deposited as sediment or being returned to surface waters through thermohaline circulation.


Life in the sea

The oceans are home to a diverse collection of life forms that use it as a habitat. Since sunlight illuminates only the upper layers, the major part of the ocean exists in permanent darkness. As the different depth and temperature zones each provide habitat for a unique set of species, the marine environment as a whole encompasses an immense diversity of life. Marine habitats range from surface water to the deepest
oceanic trench Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tren ...
es, including coral reefs,
kelp forest Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Ea ...
s,
seagrass meadow A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) plants found in shallow coastal waters and in the brackish waters of estuaries. Seagrasses are flowering plants with stems an ...
s, tidepools, muddy, sandy and rocky seabeds, and the open
pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...
zone. The organisms living in the sea range from
whales Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
30 metres (100 ft) long to microscopic phytoplankton and
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
, fungi, and bacteria. Marine life plays an important part in the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major compon ...
as photosynthetic organisms convert dissolved carbon dioxide into organic carbon and it is economically important to humans for providing
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
for use as food. Life may have originated in the sea and all the major groups of animals are represented there. Scientists differ as to precisely where in the sea life arose: the Miller-Urey experiments suggested a dilute chemical "soup" in open water, but more recent suggestions include volcanic hot springs, fine-grained clay sediments, or deep-sea "
black smoker A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...
" vents, all of which would have provided protection from damaging ultraviolet radiation which was not blocked by the early Earth's atmosphere.


Marine habitats

Marine habitats can be divided horizontally into coastal and open ocean habitats. Coastal habitats extend from the shoreline to the edge of the
continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
. Most marine life is found in coastal habitats, even though the shelf area occupies only 7 percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf. Alternatively, marine habitats can be divided vertically into
pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...
(open water),
demersal The demersal zone is the part of the sea or ocean (or deep lake) consisting of the part of the water column near to (and significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer ...
(just above the seabed) and
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
(sea bottom) habitats. A third division is by
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north ...
: from polar seas with ice shelves, sea ice and icebergs, to temperate and tropical waters. Coral reefs, the so-called "rainforests of the sea", occupy less than 0.1 percent of the world's ocean surface, yet their ecosystems include 25 percent of all marine species. The best-known are
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
coral reefs such as Australia's
Great Barrier Reef The Great Barrier Reef is the world's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, A ...
, but cold water reefs harbour a wide array of species including corals (only six of which contribute to reef formation).


Algae and plants

Marine primary producers — plants and microscopic organisms in the plankton — are widespread and very essential for the ecosystem. It has been estimated that half of the world's oxygen is produced by phytoplankton. About 45 percent of the sea's
primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
of living material is contributed by
diatoms A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
. Much larger algae, commonly known as
seaweed Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and '' Chlorophyta'' (green) macroalgae. Seaweed species such as ...
s, are important locally; ''Sargassum'' forms floating drifts, while
kelp Kelps are large brown algae seaweeds that make up the order Laminariales. There are about 30 different genera. Despite its appearance, kelp is not a plant - it is a heterokont, a completely unrelated group of organisms. Kelp grows in "underwa ...
form seabed forests.
Flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants t ...
s in the form of seagrasses grow in "
meadow A meadow ( ) is an open habitat, or field, vegetated by grasses, herbs, and other non- woody plants. Trees or shrubs may sparsely populate meadows, as long as these areas maintain an open character. Meadows may be naturally occurring or arti ...
s" in sandy shallows,
mangrove A mangrove is a shrub or tree that grows in coastal saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evolution in severa ...
s line the coast in tropical and subtropical regions and salt-tolerant plants thrive in regularly inundated
salt marsh A salt marsh or saltmarsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It is domin ...
es. All of these habitats are able to sequester large quantities of carbon and support a
biodiverse Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic ('' genetic variability''), species ('' species diversity''), and ecosystem ('' ecosystem diversity'') ...
range of larger and smaller animal life. Light is only able to penetrate the top so this is the only part of the sea where plants can grow. The surface layers are often deficient in biologically active nitrogen compounds. The marine
nitrogen cycle The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biolo ...
consists of complex microbial transformations which include the fixation of nitrogen, its assimilation,
nitrification ''Nitrification'' is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate occurring through separate organisms or direct ammonia oxidation to nitrate in comammox bacteria. The transformation of ...
,
anammox Anammox, an abbreviation for anaerobic ammonium oxidation, is a globally important microbial process of the nitrogen cycle that takes place in many natural environments. The bacteria mediating this process were identified in 1999, and were a gre ...
and denitrification. Some of these processes take place in deep water so that where there is an upwelling of cold waters, and also near estuaries where land-sourced nutrients are present, plant growth is higher. This means that the most productive areas, rich in plankton and therefore also in fish, are mainly coastal.


Animals and other marine life

There is a broader spectrum of higher animal
taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
in the sea than on land, many marine species have yet to be discovered and the number known to science is expanding annually. Some
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s such as
seabird Seabirds (also known as marine birds) are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same envir ...
s,
seals Seals may refer to: * Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly: ** Earless seal, or "true seal" ** Fur seal * Seal (emblem), a device to impress an emblem, used as a means of a ...
and
sea turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhe ...
s return to the land to breed but fish,
cetacean Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel th ...
s and
sea snake Sea snakes, or coral reef snakes, are elapid snakes that inhabit marine environments for most or all of their lives. They belong to two subfamilies, Hydrophiinae and Laticaudinae. Hydrophiinae also includes Australasian terrestrial snakes, w ...
s have a completely aquatic lifestyle and many invertebrate
phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...
are entirely marine. In fact, the oceans teem with life and provide many varying microhabitats. One of these is the surface film which, even though tossed about by the movement of waves, provides a rich environment and is home to bacteria,
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
,
microalgae Microalgae or microphytes are microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine systems, living in both the water column and sediment. They are unicellular species which exist indiv ...
,
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
, fish eggs and various larvae. The pelagic zone contains
macro- Macro (or MACRO) may refer to: Science and technology * Macroscopic, subjects visible to the eye * Macro photography, a type of close-up photography * Image macro, a picture with text superimposed * Monopole, Astrophysics and Cosmic Ray Observat ...
and microfauna and myriad zooplankton which drift with the currents. Most of the smallest organisms are the larvae of fish and marine invertebrates which liberate eggs in vast numbers because the chance of any one embryo surviving to maturity is so minute. The zooplankton feed on phytoplankton and on each other and form a basic part of the complex food chain that extends through variously sized fish and other
nekton Nekton or necton (from the ) refers to the actively swimming aquatic organisms in a body of water. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the passive organisms t ...
ic organisms to large
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...
,
shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s,
porpoise Porpoises are a group of fully aquatic marine mammals, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). Although similar in appearance to dolphins, they are more closely related to narwhals a ...
s,
dolphin A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the b ...
s and
whale Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
s. Some marine creatures make large migrations, either to other regions of the ocean on a seasonal basis or vertical migrations daily, often ascending to feed at night and descending to safety by day. Ships can introduce or spread
invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species adv ...
through the discharge of
ballast Ballast is material that is used to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship ...
water or the transport of organisms that have accumulated as part of the fouling community on the hulls of vessels. The demersal zone supports many animals that feed on benthic organisms or seek protection from predators and the seabed provides a range of habitats on or under the surface of the substrate which are used by creatures adapted to these conditions. The tidal zone with its periodic exposure to the dehydrating air is home to
barnacle A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosiv ...
s,
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is est ...
s and
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...
s. The
neritic zone The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated ...
has many organisms that need light to flourish. Here, among algal encrusted rocks live
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...
s,
echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
s,
polychaete Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made ...
worms,
sea anemone Sea anemones are a group of predatory marine invertebrates of the order Actiniaria. Because of their colourful appearance, they are named after the '' Anemone'', a terrestrial flowering plant. Sea anemones are classified in the phylum Cnidaria, ...
s and other invertebrates. Corals often contain photosynthetic
symbionts Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
and live in shallow waters where light penetrates. The extensive calcareous skeletons they extrude build up into coral reefs which are an important feature of the seabed. These provide a
biodiverse Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic ('' genetic variability''), species ('' species diversity''), and ecosystem ('' ecosystem diversity'') ...
habitat for reef dwelling organisms. There is less sea life on the floor of deeper seas but marine life also flourishes around
seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...
s that rise from the depths, where fish and other animals congregate to spawn and feed. Close to the seabed live
demersal fish Demersal fish, also known as groundfish, live and feed on or near the bottom of seas or lakes (the demersal zone).Walrond Carl . "Coastal fish - Fish of the open sea floor"Te Ara - the Encyclopedia of New Zealand. Updated 2 March 2009 They oc ...
that feed largely on pelagic organisms or
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
invertebrates. Exploration of the deep sea by submersibles revealed a new world of creatures living on the seabed that scientists had not previously known to exist. Some like the detrivores rely on organic material falling to the ocean floor. Others cluster round deep sea
hydrothermal vent A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspo ...
s where mineral-rich flows of water emerge from the seabed, supporting communities whose primary producers are sulphide-oxidising chemoautotrophic bacteria, and whose consumers include specialised bivalves, sea anemones, barnacles, crabs, worms and fish, often found nowhere else. A dead whale sinking to the bottom of the ocean provides food for an assembly of organisms which similarly rely largely on the actions of sulphur-reducing bacteria. Such places support unique biomes where many new microbes and other lifeforms have been discovered.


Humans and the sea


History of navigation and exploration

Humans have travelled the seas since they first built sea-going craft. Mesopotamians were using
bitumen Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...
to
caulk Caulk or, less frequently, caulking is a material used to seal joints or seams against leakage in various structures and piping. The oldest form of caulk consisted of fibrous materials driven into the wedge-shaped seams between boards on ...
their reed boats and, a little later, masted
sail A sail is a tensile structure—which is made from fabric or other membrane materials—that uses wind power to propel sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and even sail-powered land vehicles. Sails ma ...
s.Carter, Robert (2012). ''A Companion to the Archaeology of the Ancient Near East''. Ch. 19: "Watercraft", pp. 347 ff. Wiley-Blackwell. . By c. 3000 BC,
Austronesians The Austronesian peoples, sometimes referred to as Austronesian-speaking peoples, are a large group of peoples in Taiwan, Maritime Southeast Asia, Micronesia, coastal New Guinea, Island Melanesia, Polynesia, and Madagascar that speak Au ...
on Taiwan had begun spreading into
maritime Southeast Asia Maritime Southeast Asia comprises the countries of Brunei, Indonesia, Malaysia, the Philippines, Singapore, and East Timor. Maritime Southeast Asia is sometimes also referred to as Island Southeast Asia, Insular Southeast Asia or Oceanic Sout ...
. Subsequently, the Austronesian "
Lapita The Lapita culture is the name given to a Neolithic Austronesian people and their material culture, who settled Island Melanesia via a seaborne migration at around 1600 to 500 BCE. They are believed to have originated from the northern Phili ...
" peoples displayed great feats of navigation, reaching out from the
Bismarck Archipelago The Bismarck Archipelago (, ) is a group of islands off the northeastern coast of New Guinea in the western Pacific Ocean and is part of the Islands Region of Papua New Guinea. Its area is about 50,000 square km. History The first inhabitants o ...
to as far away as
Fiji Fiji ( , ,; fj, Viti, ; Fiji Hindi: फ़िजी, ''Fijī''), officially the Republic of Fiji, is an island country in Melanesia, part of Oceania in the South Pacific Ocean. It lies about north-northeast of New Zealand. Fiji consis ...
,
Tonga Tonga (, ; ), officially the Kingdom of Tonga ( to, Puleʻanga Fakatuʻi ʻo Tonga), is a Polynesian country and archipelago. The country has 171 islands – of which 45 are inhabited. Its total surface area is about , scattered over in ...
, and
Samoa Samoa, officially the Independent State of Samoa; sm, Sāmoa, and until 1997 known as Western Samoa, is a Polynesian island country consisting of two main islands ( Savai'i and Upolu); two smaller, inhabited islands ( Manono and Apolima); ...
. Their descendants continued to travel thousands of miles between tiny islands on
outrigger canoes An outrigger is a projecting structure on a boat, with specific meaning depending on types of vessel. Outriggers may also refer to legs on a wheeled vehicle that are folded out when it needs stabilization, for example on a crane that lifts ...
, and in the process they found many new islands, including
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
,
Easter Island Easter Island ( rap, Rapa Nui; es, Isla de Pascua) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is most famous for its nearl ...
(Rapa Nui), and New Zealand. The Ancient Egyptians and
Phoenicians Phoenicia () was an ancient thalassocratic civilization originating in the Levant region of the eastern Mediterranean, primarily located in modern Lebanon. The territory of the Phoenician city-states extended and shrank throughout their his ...
explored the
Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on ...
and Red Sea with the Egyptian Hannu reaching the
Arabian Peninsula The Arabian Peninsula, (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") or Arabia, is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plat ...
and the African Coast around 2750 BC. In the first millennium BC, Phoenicians and Greeks established colonies throughout the Mediterranean and the
Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...
. Around 500 BC, the Carthaginian navigator Hanno left a detailed
periplus A periplus (), or periplous, is a manuscript document that lists the ports and coastal landmarks, in order and with approximate intervening distances, that the captain of a vessel could expect to find along a shore. In that sense, the periplus wa ...
of an Atlantic journey that reached at least
Senegal Senegal,; Wolof: ''Senegaal''; Pulaar: 𞤅𞤫𞤲𞤫𞤺𞤢𞥄𞤤𞤭 (Senegaali); Arabic: السنغال ''As-Sinighal'') officially the Republic of Senegal,; Wolof: ''Réewum Senegaal''; Pulaar : 𞤈𞤫𞤲𞤣𞤢𞥄𞤲𞤣𞤭 ...
and possibly
Mount Cameroon Mount Cameroon is an active volcano in the South West region of Cameroon next to the city of Buea near the Gulf of Guinea. Mount Cameroon is also known as Cameroon Mountain or Fako (the name of the higher of its two peaks) or by its indigenous n ...
. In the early Mediaeval period, the
Vikings Vikings ; non, víkingr is the modern name given to seafaring people originally from Scandinavia (present-day Denmark, Norway and Sweden), who from the late 8th to the late 11th centuries raided, pirated, traded and se ...
crossed the North Atlantic and even reached the northeastern fringes of North America. Novgorodians had also been sailing the
White Sea The White Sea (russian: Белое море, ''Béloye móre''; Karelian and fi, Vienanmeri, lit. Dvina Sea; yrk, Сэрако ямʼ, ''Serako yam'') is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is su ...
since the 13th century or before. Meanwhile, the seas along the eastern and southern Asian coast were used by Arab and Chinese traders. The Chinese
Ming Dynasty The Ming dynasty (), officially the Great Ming, was an Dynasties in Chinese history, imperial dynasty of China, ruling from 1368 to 1644 following the collapse of the Mongol Empire, Mongol-led Yuan dynasty. The Ming dynasty was the last ort ...
had a fleet of 317 ships with 37,000 men under
Zheng He Zheng He (; 1371–1433 or 1435) was a Chinese mariner, explorer, diplomat, fleet admiral, and court eunuch during China's early Ming dynasty. He was originally born as Ma He in a Muslim family and later adopted the surname Zheng conferred ...
in the early fifteenth century, sailing the Indian and Pacific Oceans. In the late fifteenth century, Western European mariners started making longer voyages of exploration in search of trade.
Bartolomeu Dias Bartolomeu Dias ( 1450 – 29 May 1500) was a Portuguese mariner and explorer. In 1488, he became the first European navigator to round the southern tip of Africa and to demonstrate that the most effective southward route for ships lay in the o ...
rounded the
Cape of Good Hope The Cape of Good Hope ( af, Kaap die Goeie Hoop ) ;''Kaap'' in isolation: pt, Cabo da Boa Esperança is a rocky headland on the Atlantic coast of the Cape Peninsula in South Africa. A common misconception is that the Cape of Good Hope is t ...
in 1487 and
Vasco da Gama Vasco da Gama, 1st Count of Vidigueira (; ; c. 1460s – 24 December 1524), was a Portuguese explorer and the first European to reach India by sea. His initial voyage to India by way of Cape of Good Hope (1497–1499) was the first to link E ...
reached India via the Cape in 1498.
Christopher Columbus Christopher Columbus * lij, Cristoffa C(or)ombo * es, link=no, Cristóbal Colón * pt, Cristóvão Colombo * ca, Cristòfor (or ) * la, Christophorus Columbus. (; born between 25 August and 31 October 1451, died 20 May 1506) was a ...
sailed from Cadiz in 1492, attempting to reach the eastern lands of India and Japan by the novel means of travelling westwards. He made landfall instead on an island in the
Caribbean Sea The Caribbean Sea ( es, Mar Caribe; french: Mer des Caraïbes; ht, Lanmè Karayib; jam, Kiaribiyan Sii; nl, Caraïbische Zee; pap, Laman Karibe) is a sea of the Atlantic Ocean in the tropics of the Western Hemisphere. It is bounded by Mexico ...
and a few years later, the Venetian navigator
John Cabot John Cabot ( it, Giovanni Caboto ; 1450 – 1500) was an Italian navigator and explorer. His 1497 voyage to the coast of North America under the commission of Henry VII of England is the earliest-known European exploration of coastal North ...
reached
Newfoundland Newfoundland and Labrador (; french: Terre-Neuve-et-Labrador; frequently abbreviated as NL) is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region ...
. The Italian Amerigo Vespucci, after whom America was named, explored the South American coastline in voyages made between 1497 and 1502, discovering the mouth of the
Amazon River The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile. The headwaters of t ...
. In 1519 the
Portuguese Portuguese may refer to: * anything of, from, or related to the country and nation of Portugal ** Portuguese cuisine, traditional foods ** Portuguese language, a Romance language *** Portuguese dialects, variants of the Portuguese language ** Portu ...
navigator
Ferdinand Magellan Ferdinand Magellan ( or ; pt, Fernão de Magalhães, ; es, link=no, Fernando de Magallanes, ; 4 February 1480 – 27 April 1521) was a Portuguese explorer. He is best known for having planned and led the 1519 Spanish expedition to the East ...
led the Spanish
Magellan-Elcano expedition The Magellan expedition, also known as the Magellan–Elcano expedition, was the first voyage around the world in recorded history. It was a 16th century Spanish expedition initially led by Portuguese explorer Ferdinand Magellan to the Maluku I ...
which would be the first to sail around the world. As for the history of
navigational instrument Navigational instruments are instruments used by nautical navigators and pilots as tools of their trade. The purpose of navigation is to ascertain the present position and to determine the speed, direction, etc. to arrive at the port or point o ...
, a
compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
was first used by the ancient Greeks and Chinese to show where north lies and the direction in which the ship is heading. The latitude (an angle which ranges from 0° at the equator to 90° at the poles) was determined by measuring the angle between the Sun, Moon or a specific star and the horizon by the use of an
astrolabe An astrolabe ( grc, ἀστρολάβος ; ar, ٱلأَسْطُرلاب ; persian, ستاره‌یاب ) is an ancient astronomical instrument that was a handheld model of the universe. Its various functions also make it an elaborate inclin ...
,
Jacob's staff The term Jacob's staff is used to refer to several things, also known as cross-staff, a ballastella, a fore-staff, a ballestilla, or a balestilha. In its most basic form, a Jacob's staff is a stick or pole with length markings; most staffs ar ...
or
sextant A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celes ...
. The
longitude Longitude (, ) is a geographic coordinate that specifies the east–west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter l ...
(a line on the globe joining the two poles) could only be calculated with an accurate
chronometer A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and th ...
to show the exact time difference between the ship and a fixed point such as the
Greenwich Meridian The historic prime meridian or Greenwich meridian is a geographical reference line that passes through the Royal Observatory, Greenwich, Royal Observatory, Greenwich, in London, England. The modern IERS Reference Meridian widely used today ...
. In 1759,
John Harrison John Harrison ( – 24 March 1776) was a self-educated English Carpentry, carpenter and clockmaker who invented the marine chronometer, a long-sought-after device for solving the History of longitude, problem of calculating longitude while at s ...
, a clockmaker, designed such an instrument and James Cook used it in his voyages of exploration. Nowadays, the
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
(GPS) using over thirty satellites enables accurate navigation worldwide. With regards to maps that are vital for navigation, in the second century,
Ptolemy Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importanc ...
mapped the whole known world from the "Fortunatae Insulae",
Cape Verde , national_anthem = () , official_languages = Portuguese , national_languages = Cape Verdean Creole , capital = Praia , coordinates = , largest_city = capital , demonym ...
or
Canary Islands The Canary Islands (; es, Canarias, ), also known informally as the Canaries, are a Spanish autonomous community and archipelago in the Atlantic Ocean, in Macaronesia. At their closest point to the African mainland, they are west of Morocc ...
, eastward to the
Gulf of Thailand The Gulf of Thailand, also known as the Gulf of Siam, is a shallow inlet in the southwestern South China Sea, bounded between the southwestern shores of the Indochinese Peninsula and the northern half of the Malay Peninsula. It is around in l ...
. This map was used in 1492 when Christopher Columbus set out on his voyages of discovery. Subsequently,
Gerardus Mercator Gerardus Mercator (; 5 March 1512 – 2 December 1594) was a 16th-century geographer, cosmographer and Cartography, cartographer from the County of Flanders. He is most renowned for creating the Mercator 1569 world map, 1569 world map based on ...
made a practical map of the world in 1538, his map projection conveniently making
rhumb lines In navigation, a rhumb line, rhumb (), or loxodrome is an arc crossing all meridians of longitude at the same angle, that is, a path with constant bearing as measured relative to true north. Introduction The effect of following a rhumb li ...
straight. By the eighteenth century better maps had been made and part of the objective of
James Cook James Cook (7 November 1728 Old Style date: 27 October – 14 February 1779) was a British explorer, navigator, cartographer, and captain in the British Royal Navy, famous for his three voyages between 1768 and 1779 in the Pacific Ocean an ...
on his voyages was to further map the ocean. Scientific study has continued with the depth recordings of the ''
Tuscarora Tuscarora may refer to the following: First nations and Native American people and culture * Tuscarora people **''Federal Power Commission v. Tuscarora Indian Nation'' (1960) * Tuscarora language, an Iroquoian language of the Tuscarora people * ...
'', the oceanic research of the Challenger voyages (1872–1876), the work of the Scandinavian seamen
Roald Amundsen Roald Engelbregt Gravning Amundsen (, ; ; 16 July 1872 – ) was a Norwegian explorer of polar regions. He was a key figure of the period known as the Heroic Age of Antarctic Exploration. Born in Borge, Østfold, Norway, Amundsen bega ...
and
Fridtjof Nansen Fridtjof Wedel-Jarlsberg Nansen (; 10 October 186113 May 1930) was a Norwegian polymath and Nobel Peace Prize laureate. He gained prominence at various points in his life as an explorer, scientist, diplomat, and humanitarian. He led the team t ...
, the
Michael Sars Michael Sars (30 August 1805 – 22 October 1869) was a Norwegian theologian and biologist. Biography Sars was born in Bergen, Norway. He studied natural history and theology at Royal Frederick University from 1823 and completed a cand.theol. ...
expedition in 1910, the
German Meteor expedition The German Meteor expedition (German: ''Deutsche Atlantik Expedition'') was an oceanographic expedition that explored the South Atlantic ocean from the equatorial region to Antarctica in 1925–1927. Depth soundings, water temperature studies, wat ...
of 1925, the Antarctic survey work of ''
Discovery II ''Discovery II'', built in 1971, is the second of three Discovery sternwheel riverboats operated by the Riverboat Discovery company. ''Discovery II'' is still in use as a tour vessel on the Chena and Tanana rivers near Fairbanks, Alaska. Hist ...
'' in 1932, and others since. Furthermore, in 1921, the
International Hydrographic Organization The International Hydrographic Organization (IHO) is an intergovernmental organisation representing hydrography. , the IHO comprised 98 Member States. A principal aim of the IHO is to ensure that the world's seas, oceans and navigable waters a ...
(IHO) was set up, and it constitutes the world authority on hydrographic surveying and nautical charting. A fourth edition draft was published in 1986 but so far several naming disputes (such as the one over the
Sea of Japan The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, it h ...
) have prevented its ratification.


History of oceanography and deep sea exploration

Scientific oceanography began with the voyages of Captain James Cook from 1768 to 1779, describing the Pacific with unprecedented precision from 71 degrees South to 71 degrees North. John Harrison's chronometers supported Cook's accurate navigation and charting on two of these voyages, permanently improving the standard attainable for subsequent work. Other expeditions followed in the nineteenth century, from Russia, France, the Netherlands and the United States as well as Britain. On HMS ''Beagle'', which provided
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
with ideas and materials for his 1859 book ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'', the ship's captain,
Robert FitzRoy Vice-Admiral Robert FitzRoy (5 July 1805 – 30 April 1865) was an English officer of the Royal Navy and a scientist. He achieved lasting fame as the captain of during Charles Darwin's famous voyage, FitzRoy's second expedition to Tierra de ...
, charted the seas and coasts and published his four-volume report of the ship's three voyages in 1839.
Edward Forbes Edward Forbes FRS, FGS (12 February 1815 – 18 November 1854) was a Manx naturalist. In 1846, he proposed that the distributions of montane plants and animals had been compressed downslope, and some oceanic islands connected to the mainlan ...
's 1854 book, ''Distribution of Marine Life'' argued that no life could exist below around 600 metres (2000 feet). This was proven wrong by the British biologists W. B. Carpenter and
C. Wyville Thomson Sir Charles Wyville Thomson (5 March 1830 – 10 March 1882) was a Scottish natural historian and marine zoologist. He served as the chief scientist on the Challenger expedition; his work there revolutionized oceanography and led to his knight ...
, who in 1868 discovered life in deep water by dredging. Wyville Thompson became chief scientist on the Challenger expedition of 1872–1876, which effectively created the science of oceanography. On her journey round the globe, ''HMS Challenger'' discovered about 4,700 new marine species, and made 492 deep sea soundings, 133 bottom dredges, 151 open water trawls and 263 serial water temperature observations. In the southern Atlantic in 1898/1899,
Carl Chun Carl Chun (1 October 1852 – 11 April 1914) was a German marine biologist. Chun was born in Höchst, today a part of Frankfurt, and studied zoology at the University of Leipzig, where from 1878 to 1883 he was privat-docent of zoology and an a ...
on the ''Valdivia'' brought many new life forms to the surface from depths of over . The first observations of deep-sea animals in their natural environment were made in 1930 by
William Beebe Charles William Beebe ( ; July 29, 1877 – June 4, 1962) was an American naturalist, ornithologist, marine biologist, entomologist, explorer, and author. He is remembered for the numerous expeditions he conducted for the New York Zoological ...
and
Otis Barton Frederick Otis Barton Jr. (June 5, 1899 – April 15, 1992) was an American deep-sea diver, inventor and actor. Early life and career Born in New York, the independently wealthy Barton designed the first bathysphere and made a dive with W ...
who descended to in the spherical steel
Bathysphere The Bathysphere (Greek: , , "deep" and , , "sphere") was a unique spherical deep-sea submersible which was unpowered and lowered into the ocean on a cable, and was used to conduct a series of dives off the coast of Bermuda from 1930 to 1934. The ...
. This was lowered by cable but by 1960 a self-powered submersible,
Trieste Trieste ( , ; sl, Trst ; german: Triest ) is a city and seaport in northeastern Italy. It is the capital city, and largest city, of the autonomous region of Friuli Venezia Giulia, one of two autonomous regions which are not subdivided into provi ...
developed by
Jacques Piccard Jacques Piccard (28 July 19221 November 2008) was a Swiss oceanographer and engineer, known for having developed underwater submarines for studying ocean currents. In the Challenger Deep, he and Lt. Don Walsh of the United States Navy were the ...
, took Piccard and
Don Walsh Don Walsh (born November 2, 1931) is an American oceanographer, explorer and marine policy specialist. He and Jacques Piccard were aboard the bathyscaphe ''Trieste'' when it made a record maximum descent into the Challenger Deep on January 2 ...
to the deepest part of the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
's oceans, the
Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maximum known ...
in the Pacific, reaching a record depth of about , a feat not repeated until 2012 when
James Cameron James Francis Cameron (born August 16, 1954) is a Canadian filmmaker. A major figure in the post-New Hollywood era, he is considered one of the industry's most innovative filmmakers, regularly pushing the boundaries of cinematic capability w ...
piloted the
Deepsea Challenger ''Deepsea Challenger'' (DCV 1) is a deep-diving submersible designed to reach the bottom of Challenger Deep, the deepest-known point on Earth. On 26 March 2012, Canadian film director James Cameron piloted the craft to accomplish this goal i ...
to similar depths. An
atmospheric diving suit An atmospheric diving suit (ADS) is a small one-person articulated submersible which resembles a suit of armour, with elaborate pressure joints to allow articulation while maintaining an internal pressure of one atmosphere. An ADS can enable di ...
can be worn for deep sea operations, with a new world record being set in 2006 when a US Navy diver descended to in one of these articulated, pressurized suits. At great depths, no light penetrates through the water layers from above and the pressure is extreme. For deep sea exploration it is necessary to use specialist vehicles, either
remotely operated underwater vehicle A remotely operated underwater vehicle (technically ROUV or just ROV) is a tethered underwater mobile device, commonly called ''underwater robot''. Definition This meaning is different from remote control vehicles operating on land or in the ai ...
s with lights and cameras or crewed
submersible A submersible is a small watercraft designed to operate underwater. The term "submersible" is often used to differentiate from other underwater vessels known as submarines, in that a submarine is a fully self-sufficient craft, capable of ind ...
s. The battery-operated Mir submersibles have a three-person crew and can descend to 20,000 feet (6,000 m). They have viewing ports, 5,000-watt lights, video equipment and manipulator arms for collecting samples, placing probes or pushing the vehicle across the sea bed when the thrusters would stir up excessive sediment.
Bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...
is the mapping and study of the
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sc ...
of the ocean floor. Methods used for measuring the depth of the sea include single or multibeam
echosounder Echo sounding or depth sounding is the use of sonar for ranging, normally to determine the depth of water (bathymetry). It involves transmitting acoustic waves into water and recording the time interval between emission and return of a pulse; ...
s,
laser airborne depth sounder The Laser Airborne Depth Sounder (LADS) Flight was a unit of the Royal Australian Navy. Unlike the rest of the flying units of the RAN, it was not controlled by the Fleet Air Arm from , but instead fell under the operational control of the Austral ...
s and the calculation of depths from satellite remote sensing data. This information is used for determining the routes of undersea cables and pipelines, for choosing suitable locations for siting oil rigs and offshore wind turbines and for identifying possible new fisheries. Ongoing oceanographic research includes marine lifeforms, conservation, the marine environment, the chemistry of the ocean, the studying and modelling of climate dynamics, the air-sea boundary, weather patterns, ocean resources, renewable energy, waves and currents, and the design and development of new tools and technologies for investigating the deep. Whereas in the 1960s and 1970s research could focus on taxonomy and basic biology, in the 2010s attention has shifted to larger topics such as climate change. Researchers make use of satellite-based
remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
for surface waters, with research ships, moored observatories and autonomous underwater vehicles to study and monitor all parts of the sea.


Law

"Freedom of the seas" is a principle in
international law International law (also known as public international law and the law of nations) is the set of rules, norms, and standards generally recognized as binding between states. It establishes normative guidelines and a common conceptual framework for ...
dating from the seventeenth century. It stresses freedom to navigate the oceans and disapproves of war fought in
international waters The terms international waters or transboundary waters apply where any of the following types of bodies of water (or their drainage basins) transcend international boundaries: oceans, large marine ecosystems, enclosed or semi-enclosed regiona ...
. Today, this concept is enshrined in the United Nations Convention on the Law of the Sea (UNCLOS), the third version of which came into force in 1994. Article 87(1) states: "The high seas are open to all states, whether coastal or
land-locked A landlocked country is a country that does not have territory connected to an ocean or whose coastlines lie on endorheic basins. There are currently 44 landlocked countries and 4 landlocked de facto states. Kazakhstan is the world's largest ...
." Article 87(1) (a) to (f) gives a non-exhaustive list of freedoms including navigation, overflight, the laying of
submarine cables Submarine cable is any electrical cable that is laid on the seabed, although the term is often extended to encompass cables laid on the bottom of large freshwater bodies of water. Examples include: *Submarine communications cable *Submarine power ...
, building artificial islands, fishing and scientific research. The safety of shipping is regulated by the
International Maritime Organization The International Maritime Organization (IMO, French: ''Organisation maritime internationale'') is a specialised agency of the United Nations responsible for regulating shipping. The IMO was established following agreement at a UN conference ...
. Its objectives include developing and maintaining a regulatory framework for shipping, maritime safety, environmental concerns, legal matters, technical co-operation and maritime security. UNCLOS defines various areas of water. "Internal waters" are on the landward side of a baseline and foreign vessels have no right of passage in these. "Territorial waters" extend to 12 nautical miles (22 kilometres; 14 miles) from the coastline and in these waters, the coastal state is free to set laws, regulate use and exploit any resource. A "contiguous zone" extending a further 12 nautical miles allows for
hot pursuit Hot pursuit is a legal term. Hot Pursuit may also refer to: Film and television * ''Hot Pursuit'' (1984 TV series), a 1984 NBC television series * ''Hot Pursuit'' (2006 TV series), a 2006 Court TV television series * ''Hot Pursuit'' (1987 film) ...
of vessels suspected of infringing laws in four specific areas: customs, taxation, immigration and pollution. An "exclusive economic zone" extends for 200 nautical miles (370 kilometres; 230 miles) from the baseline. Within this area, the coastal nation has sole exploitation rights over all natural resources. The "continental shelf" is the
natural prolongation The natural prolongation principle or principle of natural prolongation is a legal concept introduced in maritime claims submitted to the United Nations. The phrase denotes a concept of political geography and international law that a nation's mar ...
of the land territory to the
continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
's outer edge, or 200 nautical miles from the coastal state's baseline, whichever is greater. Here the coastal nation has the exclusive right to harvest minerals and also living resources "attached" to the seabed.


War

Control of the sea is important to the security of a maritime nation, and the naval
blockade A blockade is the act of actively preventing a country or region from receiving or sending out food, supplies, weapons, or communications, and sometimes people, by military force. A blockade differs from an embargo or sanction, which are le ...
of a port can be used to cut off food and supplies in time of war. Battles have been fought on the sea for more than 3,000 years. In about 1210 B.C., Suppiluliuma II, the king of the
Hittites The Hittites () were an Anatolian people who played an important role in establishing first a kingdom in Kussara (before 1750 BC), then the Kanesh or Nesha kingdom (c. 1750–1650 BC), and next an empire centered on Hattusa in north-centra ...
, defeated and burned a fleet from
Alashiya Alashiya ( akk, 𒀀𒆷𒅆𒅀 ''Alašiya'' -la-ši-ia uga, 𐎀𐎍𐎘𐎊 ''ẢLṮY''; Linear B: 𐀀𐀨𐀯𐀍 ''Alasios'' -ra-si-jo, also spelled Alasiya, also known as the Kingdom of Alashiya, was a state which existed in the Middle ...
(modern
Cyprus Cyprus ; tr, Kıbrıs (), officially the Republic of Cyprus,, , lit: Republic of Cyprus is an island country located south of the Anatolian Peninsula in the eastern Mediterranean Sea. Its continental position is disputed; while it is geo ...
). In the decisive 480 B.C.
Battle of Salamis The Battle of Salamis ( ) was a naval battle fought between an alliance of Greek city-states under Themistocles and the Persian Empire under King Xerxes in 480 BC. It resulted in a decisive victory for the outnumbered Greeks. The battle was ...
, the Greek general
Themistocles Themistocles (; grc-gre, Θεμιστοκλῆς; c. 524–459 BC) was an Athenian politician and general. He was one of a new breed of non-aristocratic politicians who rose to prominence in the early years of the Athenian democracy. A ...
trapped the far larger fleet of the Persian king Xerxes in a narrow channel and attacked vigorously, destroying 200 Persian ships for the loss of 40 Greek vessels. At the end of the
Age of Sail The Age of Sail is a period that lasted at the latest from the mid-16th (or mid- 15th) to the mid- 19th centuries, in which the dominance of sailing ships in global trade and warfare culminated, particularly marked by the introduction of naval ...
, the British Royal Navy, led by
Horatio Nelson Vice-Admiral Horatio Nelson, 1st Viscount Nelson, 1st Duke of Bronte (29 September 1758 – 21 October 1805) was a British flag officer in the Royal Navy. His inspirational leadership, grasp of strategy, and unconventional tactics brought abo ...
, broke the power of the combined French and Spanish fleets at the 1805
Battle of Trafalgar The Battle of Trafalgar (21 October 1805) was a naval engagement between the British Royal Navy and the combined fleets of the French and Spanish Navies during the War of the Third Coalition (August–December 1805) of the Napoleonic Wars (180 ...
. With steam and the industrial production of steel plate came greatly increased firepower in the shape of the
dreadnought The dreadnought (alternatively spelled dreadnaught) was the predominant type of battleship in the early 20th century. The first of the kind, the Royal Navy's , had such an impact when launched in 1906 that similar battleships built after her ...
battleships A battleship is a large armored warship with a main battery consisting of large caliber guns. It dominated naval warfare in the late 19th and early 20th centuries. The term ''battleship'' came into use in the late 1880s to describe a type of ...
armed with long-range guns. In 1905, the Japanese fleet decisively defeated the Russian fleet, which had travelled over , at the
Battle of Tsushima The Battle of Tsushima (Japanese:対馬沖海戦, Tsushimaoki''-Kaisen'', russian: Цусимское сражение, ''Tsusimskoye srazheniye''), also known as the Battle of Tsushima Strait and the Naval Battle of Sea of Japan (Japanese: 日 ...
. Dreadnoughts fought inconclusively in the
First World War World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
at the 1916
Battle of Jutland The Battle of Jutland (german: Skagerrakschlacht, the Battle of the Skagerrak) was a naval battle fought between Britain's Royal Navy Grand Fleet, under Admiral John Jellicoe, 1st Earl Jellicoe, Sir John Jellicoe, and the Imperial German Navy ...
between the
Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against F ...
's
Grand Fleet The Grand Fleet was the main battlefleet of the Royal Navy during the First World War. It was established in August 1914 and disbanded in April 1919. Its main base was Scapa Flow in the Orkney Islands. History Formed in August 1914 from the ...
and the
Imperial German Navy The Imperial German Navy or the Imperial Navy () was the navy of the German Empire, which existed between 1871 and 1919. It grew out of the small Prussian Navy (from 1867 the North German Federal Navy), which was mainly for coast defence. Wilhel ...
's
High Seas Fleet The High Seas Fleet (''Hochseeflotte'') was the battle fleet of the German Imperial Navy and saw action during the First World War. The formation was created in February 1907, when the Home Fleet (''Heimatflotte'') was renamed as the High Seas ...
. In the
Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, the British victory at the 1940
Battle of Taranto The Battle of Taranto took place on the night of 11–12 November 1940 during the Second World War between British naval forces, under Admiral Andrew Cunningham, and Italian naval forces, under Admiral Inigo Campioni. The Royal Navy launched ...
showed that naval air power was sufficient to overcome the largest warships, foreshadowing the decisive sea-battles of the
Pacific War The Pacific War, sometimes called the Asia–Pacific War, was the theater of World War II that was fought in Asia, the Pacific Ocean, the Indian Ocean, and Oceania. It was geographically the largest theater of the war, including the vast ...
including the Battles of the
Coral Sea The Coral Sea () is a marginal sea of the South Pacific off the northeast coast of Australia, and classified as an interim Australian bioregion. The Coral Sea extends down the Australian northeast coast. Most of it is protected by the Fre ...
, Midway, the Philippine Sea, and the climactic
Battle of Leyte Gulf The Battle of Leyte Gulf ( fil, Labanan sa golpo ng Leyte, lit=Battle of Leyte gulf; ) was the largest naval battle of World War II and by some criteria the largest naval battle in history, with over 200,000 naval personnel involved. It was fou ...
, in all of which the dominant ships were
aircraft carrier An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a ...
s. Submarines became important in naval warfare in World War I, when German submarines, known as
U-boat U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare role ...
s, sank nearly 5,000 Allied merchant ships, including the
RMS Lusitania RMS ''Lusitania'' (named after the Roman province in Western Europe corresponding to modern Portugal) was a British ocean liner that was launched by the Cunard Line in 1906 and that held the Blue Riband appellation for the fastest Atlanti ...
, which helped to bring the United States into the war. In World War II, almost 3,000 Allied ships were sunk by U-boats attempting to block the flow of supplies to Britain, but the Allies broke the blockade in the
Battle of the Atlantic The Battle of the Atlantic, the longest continuous military campaign in World War II, ran from 1939 to the defeat of Nazi Germany in 1945, covering a major part of the naval history of World War II. At its core was the Allied naval blockade ...
, which lasted the whole length of the war, sinking 783 U-boats. Since 1960, several nations have maintained fleets of nuclear-powered
ballistic missile submarine A ballistic missile submarine is a submarine capable of deploying submarine-launched ballistic missiles (SLBMs) with nuclear warheads. The United States Navy's hull classification symbols for ballistic missile submarines are SSB and SSBN – t ...
s, vessels equipped to launch
ballistic missile A ballistic missile is a type of missile that uses projectile motion to deliver warheads on a target. These weapons are guided only during relatively brief periods—most of the flight is unpowered. Short-range ballistic missiles stay within the ...
s with
nuclear warhead A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions (thermonuclear bomb), producing a nuclear explosion. Both bomb ...
s from under the sea. Some of these are kept permanently on patrol.


Travel

Sailing ships or packets carried mail overseas, one of the earliest being the Dutch service to
Batavia Batavia may refer to: Historical places * Batavia (region), a land inhabited by the Batavian people during the Roman Empire, today part of the Netherlands * Batavia, Dutch East Indies, present-day Jakarta, the former capital of the Dutch East In ...
in the 1670s. These added passenger accommodation, but in cramped conditions. Later, scheduled services were offered but the time journeys took depended much on the weather. When steamships replaced sailing vessels, ocean-going liners took over the task of carrying people. By the beginning of the twentieth century, crossing the Atlantic took about five days and shipping companies competed to own the largest and fastest vessels. The
Blue Riband The Blue Riband () is an unofficial accolade given to the passenger liner crossing the Atlantic Ocean in regular service with the record highest average speed. The term was borrowed from horse racing and was not widely used until after 1910. T ...
was an unofficial accolade given to the fastest liner crossing the Atlantic in regular service. The ''
Mauretania Mauretania (; ) is the Latin name for a region in the ancient Maghreb. It stretched from central present-day Algeria westwards to the Atlantic, covering northern present-day Morocco, and southward to the Atlas Mountains. Its native inhabitants, ...
'' held the title with 26.06 knots (48.26 km/h) for twenty years from 1909. The
Hales Trophy The Hales Trophy (officially the North Atlantic Blue Riband Challenge Trophy) is an award for the fastest Atlantic crossing by a commercial passenger vessel. The award was created in 1935 when Harold K. Hales, a British politician and owner of Ha ...
, another award for the fastest commercial crossing of the Atlantic, was won by the ''
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
'' in 1952 for a crossing that took three days, ten hours and forty minutes. The great liners were comfortable but expensive in fuel and staff. The age of the trans-Atlantic liners waned as cheap intercontinental flights became available. In 1958, a regular scheduled air service between New York and Paris taking seven hours doomed the Atlantic ferry service to oblivion. One by one the vessels were laid up, some were scrapped, others became cruise ships for the leisure industry and still others floating hotels.


Trade

Maritime trade has existed for millennia. The
Ptolemaic dynasty The Ptolemaic dynasty (; grc, Πτολεμαῖοι, ''Ptolemaioi''), sometimes referred to as the Lagid dynasty (Λαγίδαι, ''Lagidae;'' after Ptolemy I's father, Lagus), was a Macedonian Greek royal dynasty which ruled the Ptolemaic ...
had developed trade with India using the Red Sea ports and in the first millennium BC the Arabs, Phoenicians,
Israelites The Israelites (; , , ) were a group of Semitic-speaking tribes in the ancient Near East who, during the Iron Age, inhabited a part of Canaan. The earliest recorded evidence of a people by the name of Israel appears in the Merneptah Stele o ...
and Indians traded in luxury goods such as spices, gold, and precious stones. The Phoenicians were noted sea traders and under the Greeks and Romans, commerce continued to thrive. With the collapse of the Roman Empire, European trade dwindled but it continued to flourish among the kingdoms of Africa, the Middle East, India, China and southeastern Asia. From the 16th to the 19th centuries, over a period of 400 years, about 12–13 million Africans were shipped across the Atlantic to be sold as slaves in the Americas as part of the
Atlantic slave trade The Atlantic slave trade, transatlantic slave trade, or Euro-American slave trade involved the transportation by slave traders of enslaved African people, mainly to the Americas. The slave trade regularly used the triangular trade route and i ...
. Large quantities of goods are transported by sea, especially across the Atlantic and around the Pacific Rim. A major trade route passes through the
Pillars of Hercules The Pillars of Hercules ( la, Columnae Herculis, grc, Ἡράκλειαι Στῆλαι, , ar, أعمدة هرقل, Aʿmidat Hiraql, es, Columnas de Hércules) was the phrase that was applied in Antiquity to the promontories that flank t ...
, across the Mediterranean and the
Suez Canal The Suez Canal ( arz, قَنَاةُ ٱلسُّوَيْسِ, ') is an artificial sea-level waterway in Egypt, connecting the Mediterranean Sea to the Red Sea through the Isthmus of Suez and dividing Africa and Asia. The long canal is a popular ...
to the Indian Ocean and through the
Straits of Malacca The Strait of Malacca is a narrow stretch of water, 500 mi (800 km) long and from 40 to 155 mi (65–250 km) wide, between the Malay Peninsula (Peninsular Malaysia) to the northeast and the Indonesian island of Sumatra to the southwest, connec ...
; much trade also passes through the
English Channel The English Channel, "The Sleeve"; nrf, la Maunche, "The Sleeve" (Cotentinais) or ( Jèrriais), (Guernésiais), "The Channel"; br, Mor Breizh, "Sea of Brittany"; cy, Môr Udd, "Lord's Sea"; kw, Mor Bretannek, "British Sea"; nl, Het Kana ...
. Shipping lanes are the routes on the open sea used by cargo vessels, traditionally making use of trade winds and currents. Over 60 percent of the world's container traffic is conveyed on the top twenty trade routes. Increased melting of Arctic ice since 2007 enables ships to travel the
Northwest Passage The Northwest Passage (NWP) is the sea route between the Atlantic and Pacific oceans through the Arctic Ocean, along the northern coast of North America via waterways through the Canadian Arctic Archipelago. The eastern route along the Arct ...
for some weeks in summertime, avoiding the longer routes via the Suez Canal or the
Panama Canal The Panama Canal ( es, Canal de Panamá, link=no) is an artificial waterway in Panama that connects the Atlantic Ocean with the Pacific Ocean and divides North and South America. The canal cuts across the Isthmus of Panama and is a conduit ...
. Shipping is supplemented by
air freight Air cargo is any property carried or to be carried in an aircraft. Air cargo comprises air freight, air express and airmail. Aircraft types Different cargo can be transported by passenger, cargo or combi aircraft: * Passenger aircraft use the ...
, a more expensive process mostly used for particularly valuable or perishable cargoes. Seaborne trade carries more than US$4 trillion worth of goods each year.
Bulk cargo Bulk cargo is commodity cargo that is transported unpackaged in large quantities. Description Bulk cargo refers to material in either liquid or granular, particulate form, as a mass of relatively small solids, such as petroleum/ crude oi ...
in the form of liquids, powder or particles are carried loose in the holds of
bulk carrier A bulk carrier or bulker is a merchant ship specially designed to transport unpackaged bulk cargo — such as grains, coal, ore, steel coils, and cement — in its cargo holds. Since the first specialized bulk carrier was built in 1852, econom ...
s and include
crude oil Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...
,
grain A grain is a small, hard, dry fruit (caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legum ...
,
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dea ...
,
ore Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...
,
scrap metal Scrap consists of recyclable materials, usually metals, left over from product manufacturing and consumption, such as parts of vehicles, building supplies, and surplus materials. Unlike waste, scrap has monetary value, especially recovered me ...
,
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
and
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
. Other cargo, such as manufactured goods, is usually transported within standard sized, lockable containers, loaded on purpose-built
container ship A container ship (also called boxship or spelled containership) is a cargo ship that carries all of its load in truck-size intermodal containers, in a technique called containerization. Container ships are a common means of commercial intermodal ...
s at dedicated terminals. Before the rise of
containerization Containerization is a system of intermodal freight transport using intermodal containers (also called shipping containers and ISO containers). Containerization is also referred as "Container Stuffing" or "Container Loading", which is the pro ...
in the 1960s, these goods were loaded, transported and unloaded piecemeal as break-bulk cargo. Containerization greatly increased the efficiency and decreased the cost of moving goods by sea, and was a major factor leading to the rise of
globalization Globalization, or globalisation (Commonwealth English; see spelling differences), is the process of interaction and integration among people, companies, and governments worldwide. The term ''globalization'' first appeared in the early 20t ...
and exponential increases in
international trade International trade is the exchange of capital, goods, and services across international borders or territories because there is a need or want of goods or services. (see: World economy) In most countries, such trade represents a significant ...
in the mid-to-late 20th century.


Food

Fish and other fishery products are among the most widely consumed sources of protein and other essential nutrients. In 2009, 16.6% of the world's intake of animal protein and 6.5% of all protein consumed came from fish. In order to fulfill this need, coastal countries have exploited marine resources in their exclusive economic zone, although fishing vessels are increasingly venturing further afield to exploit stocks in international waters. In 2011, the total world production of fish, including
aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. lot ...
, was estimated to be 154 million tonnes, of which most was for human consumption. The harvesting of wild fish accounted for 90.4 million tonnes, while annually increasing aquaculture contributes the rest. The north west Pacific is by far the most productive area with 20.9 million tonnes (27 percent of the global marine catch) in 2010. In addition, the number of fishing vessels in 2010 reached 4.36 million, whereas the number of people employed in the primary sector of fish production in the same year amounted to 54.8 million. Modern fishing vessels include
fishing trawler A fishing trawler is a commercial fishing vessel designed to operate Trawling, fishing trawls. Trawling is a method of fishing that involves actively dragging or pulling a trawl through the water behind one or more trawlers. Trawls are fishing ...
s with a small crew, stern trawlers, purse seiners, long-line factory vessels and large
factory ship A factory ship, also known as a fish processing vessel, is a large ocean-going vessel with extensive on-board facilities for processing and freezing caught fish or whales. Modern factory ships are automated and enlarged versions of the earlier wh ...
s which are designed to stay at sea for weeks, processing and freezing great quantities of fish. The equipment used to capture the fish may be
purse seine Seine fishing (or seine-haul fishing; ) is a method of fishing that employs a surrounding net, called a seine, that hangs vertically in the water with its bottom edge held down by weights and its top edge buoyed by floats. Seine nets can be dep ...
s, other seines, trawls, dredges, gillnets and long-lines and the fish species most frequently targeted are
herring Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Oceans, i ...
,
cod Cod is the common name for the demersal fish genus '' Gadus'', belonging to the family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gadus'' is commonly not call ...
,
anchovy An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...
,
tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
,
flounder Flounders are a group of flatfish species. They are demersal fish, found at the bottom of oceans around the world; some species will also enter estuaries. Taxonomy The name "flounder" is used for several only distantly related species, thou ...
, mullet, squid and
salmon Salmon () is the common name for several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the family (biology), family Salmonidae, which are native to tributary, tributaries of the ...
.
Overexploitation Overexploitation, also called overharvesting, refers to harvesting a renewable resource to the point of diminishing returns. Continued overexploitation can lead to the destruction of the resource, as it will be unable to replenish. The term app ...
has become a serious concern; it does not only cause the depletion of fish stocks, but also substantially reduce the size of predatory fish populations. It has been estimated that "industrialized fisheries typically reduced community biomass by 80% within 15 years of exploitation." In order to avoid overexploitation, many countries have introduced
quotas Quota may refer to: Economics * Import quota, a trade restriction on the quantity of goods imported into a country * Market Sharing Quota, an economic system used in Canadian agriculture * Milk quota, a quota on milk production in Europe * Indi ...
in their own waters. However, recovery efforts often entail substantial costs to local economies or food provision.
Artisan fishing Artisanal fishing (or traditional/subsistence fishing) consists of various small-scale, low-technology, low-capital, fishing practices undertaken by individual fishing households (as opposed to commercial fishing). Many of these households are o ...
methods include rod and line, harpoons, skin diving, traps, throw nets and drag nets. Traditional fishing boats are powered by paddle, wind or outboard motors and operate in near-shore waters. The
Food and Agriculture Organization The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...
is encouraging the development of local fisheries to provide food security to coastal communities and help alleviate poverty.


Aquaculture

About 79 million tonnes (78M long tons; 87M short tons) of food and non-food products were produced by aquaculture in 2010, an all-time high. About six hundred species of plants and animals were cultured, some for use in seeding wild populations. The animals raised included finfish, aquatic
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s, crustaceans, molluscs,
sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...
s,
sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...
s, sea squirts and jellyfish. Integrated
mariculture Mariculture or marine farming is a specialized branch of aquaculture (which includes freshwater aquaculture) involving the cultivation of marine organisms for food and other animal products, in enclosed sections of the open ocean ( offshore mari ...
has the advantage that there is a readily available supply of planktonic food in the ocean, and waste is removed naturally. Various methods are employed. Mesh enclosures for finfish can be suspended in the open seas, cages can be used in more sheltered waters or ponds can be refreshed with water at each high tide.
Shrimps Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some crustaceans outside of this order are refer ...
can be reared in shallow ponds connected to the open sea. Ropes can be hung in water to grow algae, oysters and mussels. Oysters can be reared on trays or in mesh tubes. Sea cucumbers can be ranched on the seabed. Captive breeding programmes have raised
lobster Lobsters are a family (biology), family (Nephropidae, Synonym (taxonomy), synonym Homaridae) of marine crustaceans. They have long bodies with muscular tails and live in crevices or burrows on the sea floor. Three of their five pairs of legs ...
larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...
e for release of juveniles into the wild resulting in an increased lobster harvest in
Maine Maine () is a state in the New England and Northeastern regions of the United States. It borders New Hampshire to the west, the Gulf of Maine to the southeast, and the Canadian provinces of New Brunswick and Quebec to the northeast and north ...
. At least 145 species of seaweed – red, green, and brown algae – are eaten worldwide, and some have long been farmed in Japan and other Asian countries; there is great potential for additional
algaculture Algaculture is a form of aquaculture involving the farming of species of algae. The majority of algae that are intentionally cultivated fall into the category of microalgae (also referred to as phytoplankton, microphytes, or planktonic algae). M ...
. Few maritime flowering plants are widely used for food but one example is
marsh samphire ''Salicornia europaea'', known as common glasswort or just glasswort, is a halophytic annual dicot flowering plant in the family Amaranthaceae. Glasswort is a succulent herb also known as ‘Pickle weed’ or ‘ Marsh samphire’. As a succule ...
which is eaten both raw and cooked. A major difficulty for aquaculture is the tendency towards monoculture and the associated risk of widespread
disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
. Aquaculture is also associated with environmental risks; for instance,
shrimp farm Shrimp farming is an aquaculture business that exists in either a marine or freshwater environment, producing shrimp or prawns (crustaceans of the groups Caridea or Dendrobranchiata) for human consumption. Marine Commercial marine shrimp farm ...
ing has caused the destruction of important
mangrove forest Mangrove forests, also called mangrove swamps, mangrove thickets or mangals, are productive wetlands that occur in coastal intertidal zones. Mangrove forests grow mainly at tropical and subtropical latitudes because mangroves cannot withstand fr ...
s throughout
southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, south-eastern region of Asia, consistin ...
.


Leisure

Use of the sea for leisure developed in the nineteenth century, and became a significant industry in the twentieth century. Maritime leisure activities are varied, and include self-organized trips cruising,
yachting Yachting is the use of recreational boats and ships called ''yachts'' for racing or cruising. Yachts are distinguished from working ships mainly by their leisure purpose. "Yacht" derives from the Dutch word '' jacht'' ("hunt"). With sailboats, t ...
, powerboat racing and
fishing Fishing is the activity of trying to catch fish. Fish are often caught as wildlife from the natural environment, but may also be caught from stocked bodies of water such as ponds, canals, park wetlands and reservoirs. Fishing techniques inclu ...
; commercially organized voyages on
cruise ships Cruise ships are large passenger ships used mainly for vacationing. Unlike ocean liners, which are used for transport, cruise ships typically embark on round-trip voyages to various ports-of-call, where passengers may go on tours known as "s ...
; and trips on smaller vessels for
ecotourism Ecotourism is a form of tourism involving responsible travel (using sustainable transport) to natural areas, conserving the environment, and improving the well-being of the local people. Its purpose may be to educate the traveler, to provide funds ...
such as
whale watching Whale watching is the practice of observing whales and dolphins ( cetaceans) in their natural habitat. Whale watching is mostly a recreational activity (cf. birdwatching), but it can also serve scientific and/or educational purposes.Hoyt, E. 2 ...
and coastal
birdwatching Birdwatching, or birding, is the observing of birds, either as a recreational activity or as a form of citizen science. A birdwatcher may observe by using their naked eye, by using a visual enhancement device like binoculars or a telescope, b ...
.
Sea bathing The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Se ...
became the vogue in Europe in the 18th century after Dr. William Buchan advocated the practice for health reasons.
Surfing Surfing is a surface water sport in which an individual, a surfer (or two in tandem surfing), uses a board to ride on the forward section, or face, of a moving wave of water, which usually carries the surfer towards the shore. Waves suitabl ...
is a sport in which a wave is ridden by a surfer, with or without a
surfboard A surfboard is a narrow plank used in surfing. Surfboards are relatively light, but are strong enough to support an individual standing on them while riding an ocean wave. They were invented in ancient Hawaii, where they were known as ''papa he'e ...
. Other marine
water sports Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...
include
kite surfing Kiteboarding or kitesurfing is a sport that involves using wind power with a large power kite to pull a rider across a water, land, or snow surface. It combines aspects of paragliding, surfing, windsurfing, skateboarding, snowboarding, and wa ...
, where a
power kite A power kite or traction kite is a large kite designed to provide significant pull to the user. Types The two most common forms are the foil, and the leading edge inflatable. There are also other less common types of power kite including rig ...
propels a rider on a board across the water,
windsurfing Windsurfing is a wind propelled water sport that is a combination of sailing and surfing. It is also referred to as "sailboarding" and "boardsailing", and emerged in the late 1960s from the aerospace and surf culture of California. Windsurfing ga ...
, where the power is provided by a fixed, manoeuvrable sail and
water skiing Water skiing (also waterskiing or water-skiing) is a surface water sport in which an individual is pulled behind a boat or a cable ski installation over a body of water, skimming the surface on two skis or one ski. The sport requires suffic ...
, where a
powerboat A motorboat, speedboat or powerboat is a boat that is exclusively powered by an engine. Some motorboats are fitted with inboard engines, others have an outboard motor installed on the rear, containing the internal combustion engine, the gea ...
is used to pull a skier. Beneath the surface,
freediving Freediving, free-diving, free diving, breath-hold diving, or skin diving is a form of underwater diving that relies on breath-holding until resurfacing rather than the use of breathing apparatus such as scuba gear. Besides the limits of breath-h ...
is necessarily restricted to shallow descents.
Pearl divers Pearl hunting, also known as pearling, is the activity of recovering pearls from wild molluscs, usually oysters or mussels, in the sea or freshwater. Pearl hunting was prevalent in the Persian Gulf region and Japan for thousands of years. On t ...
can dive to with baskets to collect
oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not al ...
s. Human eyes are not adapted for use underwater but vision can be improved by wearing a
diving mask Diving most often refers to: * Diving (sport), the sport of jumping into deep water * Underwater diving, human activity underwater for recreational or occupational purposes Diving or Dive may also refer to: Sports * Dive (American football), a ...
. Other useful equipment includes
fins A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fin ...
and snorkels, and
scuba equipment Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for " Self-Contained Underwater Breathing Apparatus", was coined by Chri ...
allows underwater breathing and hence a longer time can be spent beneath the surface. The depths that can be reached by divers and the length of time they can stay underwater is limited by the increase of pressure they experience as they descend and the need to prevent
decompression sickness Decompression sickness (abbreviated DCS; also called divers' disease, the bends, aerobullosis, and caisson disease) is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompressio ...
as they return to the surface. Recreational divers restrict themselves to depths of beyond which the danger of
nitrogen narcosis Narcosis while diving (also known as nitrogen narcosis, inert gas narcosis, raptures of the deep, Martini effect) is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gas ...
increases. Deeper dives can be made with specialised equipment and training.


Industry


Power generation

The sea offers a very large supply of
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
carried by
ocean wave In fluid dynamics, a wind wave, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result from the wind blowing over the water surface. The contact distance in the direction o ...
s,
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide t ...
s,
salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...
differences, and ocean temperature differences which can be harnessed to generate electricity. Forms of
sustainable Specific definitions of sustainability are difficult to agree on and have varied in the literature and over time. The concept of sustainability can be used to guide decisions at the global, national, and individual levels (e.g. sustainable livin ...
marine energy include
tidal power Tidal power or tidal energy is harnessed by converting energy from tides into useful forms of power, mainly electricity using various methods. Although not yet widely used, tidal energy has the potential for future electricity generation. Ti ...
,
ocean thermal energy Ocean Thermal Energy Conversion (OTEC) uses the ocean thermal gradient between cooler deep and warmer shallow or surface seawaters to run a heat engine and produce useful work, usually in the form of electricity. OTEC can operate with a very h ...
and
wave power Wave power is the capture of energy of wind waves to do useful work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power is a wave energy converter (WEC). Waves are generated by wind p ...
. Electricity
power station A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many p ...
s are often located on the coast or beside an estuary so that the sea can be used as a heat sink. A colder heat sink enables more efficient power generation, which is important for expensive
nuclear power Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced b ...
plants in particular. Tidal power uses generators to produce electricity from tidal flows, sometimes by using a dam to store and then release seawater. The Rance barrage, long, near
St Malo Saint-Malo (, , ; Gallo: ; ) is a historic French port in Ille-et-Vilaine, Brittany, on the English Channel coast. The walled city had a long history of piracy, earning much wealth from local extortion and overseas adventures. In 1944, the Alli ...
in
Brittany Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo language, Gallo: ''Bertaèyn'' ) is a peninsula, Historical region, historical country and cultural area in the west of modern France, covering the western part of what was known ...
opened in 1967; it generates about 0.5 GW, but it has been followed by few similar schemes. The large and highly variable energy of waves gives them enormous destructive capability, making affordable and reliable wave machines problematic to develop. A small 2 MW commercial wave power plant, "Osprey", was built in Northern Scotland in 1995 about 300 metres (1000 ft) offshore. It was soon damaged by waves, then destroyed by a storm.
Offshore wind power Offshore wind power or offshore wind energy is the generation of electricity through wind farms in bodies of water, usually at sea. There are higher wind speeds offshore than on land, so offshore farms generate more electricity per amount of c ...
is captured by
wind turbine A wind turbine is a device that converts the kinetic energy of wind into electrical energy. Hundreds of thousands of large turbines, in installations known as wind farms, now generate over 650 gigawatts of power, with 60 GW added each year. ...
s placed out at sea; it has the advantage that wind speeds are higher than on land, though wind farms are more costly to construct offshore. The first offshore wind farm was installed in Denmark in 1991, and the installed capacity of worldwide offshore wind farms reached 34 GW in 2020, mainly situated in Europe.


Extractive industries

The seabed contains large reserves of minerals which can be exploited by dredging. This has advantages over land-based mining in that equipment can be built at specialised
shipyard A shipyard, also called a dockyard or boatyard, is a place where ships are built and repaired. These can be yachts, military vessels, cruise liners or other cargo or passenger ships. Dockyards are sometimes more associated with maintenance a ...
s and
infrastructure Infrastructure is the set of facilities and systems that serve a country, city, or other area, and encompasses the services and facilities necessary for its economy, households and firms to function. Infrastructure is composed of public and priv ...
costs are lower. Disadvantages include problems caused by waves and tides, the tendency for excavations to silt up and the washing away of spoil heaps. There is a risk of coastal erosion and environmental damage. Seafloor massive sulphide deposits are potential sources of
silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
,
gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
,
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
,
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
and
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
and trace metals since their discovery in the 1960s. They form when geothermally heated water is emitted from deep sea hydrothermal vents known as "black smokers". The ores are of high quality but prohibitively costly to extract. There are large deposits of
petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
, as oil and
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
, in rocks beneath the seabed.
Offshore platforms Offshore may refer to: Science and technology * Offshore (hydrocarbons) * Offshore construction, construction out at sea * Offshore drilling, discovery and development of oil and gas resources which lie underwater through drilling a well * Of ...
and
drilling rig A drilling rig is an integrated system that drills wells, such as oil or water wells, or holes for piling and other construction purposes, into the earth's subsurface. Drilling rigs can be massive structures housing equipment used to drill wat ...
s
extract An extract is a substance made by extracting a part of a raw material, often by using a solvent such as ethanol, oil or water. Extracts may be sold as tinctures, absolutes or in powder form. The aromatic principles of many spices, nuts, h ...
the oil or gas and store it for transport to land. Offshore oil and gas production can be difficult due to the remote, harsh environment. Drilling for oil in the sea has environmental impacts. Animals may be disorientated by
seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. S ...
s used to locate deposits, and there is debate as to whether this causes the beaching of whales. Toxic substances such as
mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
, lead and
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...
may be released. The infrastructure may cause damage, and oil may be spilt. Large quantities of
methane clathrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...
exist on the seabed and in
ocean sediment Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles have their origins in soil and rocks and have been transported from the land to the sea, mainly ...
, of interest as a potential energy source. Also on the seabed are
manganese nodule Polymetallic nodules, also called manganese nodules, are mineral concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. As nodules can be found in vast quantities, and contain valuable metals, de ...
s formed of layers of
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
,
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
and other hydroxides around a core. In the Pacific these may cover up to 30 percent of the deep ocean floor. The minerals precipitate from seawater and grow very slowly. Their commercial extraction for
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
was investigated in the 1970s but abandoned in favour of more convenient sources. In suitable locations,
diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...
s are gathered from the seafloor using suction hoses to bring gravel ashore. In deeper waters, mobile seafloor crawlers are used and the deposits are pumped to a vessel above. In Namibia, more diamonds are now collected from marine sources than by conventional methods on land. The sea holds large quantities of valuable dissolved minerals. The most important,
Salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
for table and industrial use has been harvested by solar evaporation from shallow ponds since prehistoric times.
Bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simila ...
, accumulated after being leached from the land, is economically recovered from the Dead Sea, where it occurs at 55,000 parts per million (ppm).


Fresh water production

Desalination Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in Soil salinity control, soil desalination, which is an issue f ...
is the technique of removing salts from seawater to leave
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
suitable for drinking or irrigation. The two main processing methods,
vacuum distillation Vacuum distillation is distillation performed under reduced pressure, which allows the purification of compounds not readily distilled at ambient pressures or simply to save time or energy. This technique separates compounds based on differences i ...
and
reverse osmosis Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pre ...
, use large quantities of energy. Desalination is normally only undertaken where fresh water from other sources is in short supply or energy is plentiful, as in the excess heat generated by power stations. The brine produced as a by-product contains some toxic materials and is returned to the sea.


Indigenous sea peoples

Several
nomad A nomad is a member of a community without fixed habitation who regularly moves to and from the same areas. Such groups include hunter-gatherers, pastoral nomads (owning livestock), tinkers and trader nomads. In the twentieth century, the popu ...
ic indigenous groups in
Maritime Southeast Asia Maritime Southeast Asia comprises the countries of Brunei, Indonesia, Malaysia, the Philippines, Singapore, and East Timor. Maritime Southeast Asia is sometimes also referred to as Island Southeast Asia, Insular Southeast Asia or Oceanic Sout ...
live in boats and derive nearly all they need from the sea. The
Moken people The Moken (also ''Mawken or'' ''Morgan''; ; th, ชาวเล, lit=sea people, translit=chao le) are an Austronesian people of the Mergui Archipelago, a group of approximately 800 islands claimed by both Myanmar and Thailand. Most of the 2,0 ...
live on the coasts of
Thailand Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is bo ...
and
Burma Myanmar, ; UK pronunciations: US pronunciations incl. . Note: Wikipedia's IPA conventions require indicating /r/ even in British English although only some British English speakers pronounce r at the end of syllables. As John Wells explai ...
and islands in the
Andaman Sea The Andaman Sea (historically also known as the Burma Sea) is a marginal sea of the northeastern Indian Ocean bounded by the coastlines of Myanmar and Thailand along the Gulf of Martaban and west side of the Malay Peninsula, and separated from ...
. The
Bajau people The Sama-Bajau include several Austronesian ethnic groups of Maritime Southeast Asia. The name collectively refers to related people who usually call themselves the Sama or Samah (formally A'a Sama, "Sama people"); or are known by the exony ...
are originally from the Sulu Archipelago,
Mindanao Mindanao ( ) ( Jawi: مينداناو) is the second-largest island in the Philippines, after Luzon, and seventh-most populous island in the world. Located in the southern region of the archipelago, the island is part of an island group of ...
and northern
Borneo Borneo (; id, Kalimantan) is the third-largest island in the world and the largest in Asia. At the geographic centre of Maritime Southeast Asia, in relation to major Indonesian islands, it is located north of Java, west of Sulawesi, and eas ...
. Some Sea Gypsies are accomplished free-divers, able to descend to depths of , though many are adopting a more settled, land-based way of life. The indigenous peoples of the Arctic such as the Chukchi,
Inuit Inuit (; iu, ᐃᓄᐃᑦ 'the people', singular: Inuk, , dual: Inuuk, ) are a group of culturally similar indigenous peoples inhabiting the Arctic and subarctic regions of Greenland, Labrador, Quebec, Nunavut, the Northwest Territories ...
,
Inuvialuit The Inuvialuit (sing. Inuvialuk; ''the real people'') or Western Canadian Inuit are Inuit who live in the western Canadian Arctic region. They, like all other Inuit, are descendants of the Thule who migrated eastward from Alaska Alask ...
and Yup'iit hunt marine mammals including seals and whales, and the
Torres Strait Islanders Torres Strait Islanders () are the Indigenous Melanesian people of the Torres Strait Islands, which are part of the state of Queensland, Australia. Ethnically distinct from the Aboriginal people of the rest of Australia, they are often grouped ...
of Australia include the Great Barrier Reef among their possessions. They live a traditional life on the islands involving hunting, fishing, gardening and trading with neighbouring peoples in Papua and mainland
Aboriginal Australians Aboriginal Australians are the various Indigenous peoples of the Australian mainland and many of its islands, such as Tasmania, Fraser Island, Hinchinbrook Island, the Tiwi Islands, and Groote Eylandt, but excluding the Torres Strait Islands ...
.


In culture

The sea appears in human culture in contradictory ways, as both powerful but serene and as beautiful but dangerous. It has its place in literature, art, poetry, film, theatre, classical music, mythology and dream interpretation. The Ancients personified it, believing it to be under the control of a
being In metaphysics, ontology is the philosophical study of being, as well as related concepts such as existence, becoming, and reality. Ontology addresses questions like how entities are grouped into categories and which of these entities exis ...
who needed to be appeased, and symbolically, it has been perceived as a hostile environment populated by fantastic creatures; the
Leviathan Leviathan (; he, לִוְיָתָן, ) is a sea serpent noted in theology and mythology. It is referenced in several books of the Hebrew Bible, including Psalms, the Book of Job, the Book of Isaiah, the Book of Amos, and, according to some ...
of the
Bible The Bible (from Koine Greek , , 'the books') is a collection of religious texts or scriptures that are held to be sacred in Christianity, Judaism, Samaritanism, and many other religions. The Bible is an anthologya compilation of texts of a ...
,
Scylla In Greek mythology, Scylla), is obsolete. ( ; grc-gre, Σκύλλα, Skúlla, ) is a legendary monster who lives on one side of a narrow channel of water, opposite her counterpart Charybdis. The two sides of the strait are within an arrow's r ...
in
Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical co ...
,
Isonade :ja:磯撫で is an enormous, shark-like sea monster said to live off the coast of Matsuurahttp://www.obakemono.com/obake/isonade/ and other places in Western Japan. Description According to the ''Ehon Hyaku Monogatari'', its body has never b ...
in
Japanese mythology Japanese mythology is a collection of traditional stories, folktales, and beliefs that emerged in the islands of the Japanese archipelago. Shinto and Buddhist traditions are the cornerstones of Japanese mythology. The history of thousands of year ...
, and the
kraken The kraken () is a legendary sea monster of enormous size said to appear off the coasts of Norway. Kraken, the subject of sailors' superstitions and mythos, was first described in the modern age at the turn of the 18th century, in a travelogu ...
of late
Norse mythology Norse, Nordic, or Scandinavian mythology is the body of myths belonging to the North Germanic peoples, stemming from Old Norse religion and continuing after the Christianization of Scandinavia, and into the Nordic folklore of the modern period ...
. The sea and ships have been depicted in art ranging from simple drawings on the walls of huts in Lamu to seascapes by Joseph Turner. In
Dutch Golden Age painting Dutch Golden Age painting is the painting of the Dutch Golden Age, a period in Dutch history roughly spanning the 17th century, during and after the later part of the Eighty Years' War (1568–1648) for Dutch independence. The new Dutch Republ ...
, artists such as Jan Porcellis, Hendrick Dubbels,
Willem van de Velde the Elder Willem van de Velde the Elder (1610/11 – 13 December 1693) was a Dutch Golden Age seascape painter, who produced many precise drawings of ships and ink paintings of fleets, but later learned to use oil paints like his son. Biography W ...
and his son, and
Ludolf Bakhuizen Ludolf BakhuizenLudolf Bakhuizen
at the
Dutch navy The Royal Netherlands Navy ( nl, Koninklijke Marine, links=no) is the naval force of the Kingdom of the Netherlands. During the 17th century, the navy of the Dutch Republic (1581–1795) was one of the most powerful naval forces in the world an ...
at the peak of its military prowess. The Japanese artist
Katsushika Hokusai , known simply as Hokusai, was a Japanese ukiyo-e artist of the Edo period, active as a painter and printmaker. He is best known for the woodblock print series '' Thirty-Six Views of Mount Fuji'', which includes the iconic print ''The Great W ...
created colour
prints In molecular biology, the PRINTS database is a collection of so-called "fingerprints": it provides both a detailed annotation resource for protein families, and a diagnostic tool for newly determined sequences. A fingerprint is a group of conserve ...
of the moods of the sea, including ''
The Great Wave off Kanagawa is a woodblock print by Japanese ''ukiyo-e'' artist Hokusai, created in late 1831 during the Edo period of Japanese history. The print depicts three boats moving through a storm-tossed sea, with a large wave forming a spiral in the centre an ...
''. Music too has been inspired by the ocean, sometimes by composers who lived or worked near the shore and saw its many different aspects.
Sea shanties A sea shanty, chantey, or chanty () is a genre of traditional folk song that was once commonly sung as a work song to accompany rhythmical labor aboard large merchant sailing vessels. The term ''shanty'' most accurately refers to a specific ...
, songs that were chanted by mariners to help them perform arduous tasks, have been woven into compositions and impressions in music have been created of calm waters, crashing waves and storms at sea. As a symbol, the sea has for centuries played a role in
literature Literature is any collection of written work, but it is also used more narrowly for writings specifically considered to be an art form, especially prose fiction, drama, and poetry. In recent centuries, the definition has expanded to include ...
,
poetry Poetry (derived from the Greek ''poiesis'', "making"), also called verse, is a form of literature that uses aesthetic and often rhythmic qualities of language − such as phonaesthetics, sound symbolism, and metre − to evoke meanings i ...
and
dream A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. Humans spend about two hours dreaming per night, and each dream lasts around 5 to 20 minutes, althou ...
s. Sometimes it is there just as a gentle background but often it introduces such themes as storm, shipwreck, battle, hardship, disaster, the dashing of hopes and death. In his
epic poem An epic poem, or simply an epic, is a lengthy narrative poem typically about the extraordinary deeds of extraordinary characters who, in dealings with gods or other superhuman forces, gave shape to the mortal universe for their descendants. ...
the ''
Odyssey The ''Odyssey'' (; grc, Ὀδύσσεια, Odýsseia, ) is one of two major Ancient Greek literature, ancient Greek Epic poetry, epic poems attributed to Homer. It is one of the oldest extant works of literature still widely read by moder ...
'', written in the eighth century BC,
Homer Homer (; grc, Ὅμηρος , ''Hómēros'') (born ) was a Greek poet who is credited as the author of the ''Iliad'' and the ''Odyssey'', two epic poems that are foundational works of ancient Greek literature. Homer is considered one of the ...
describes the ten-year voyage of the Greek hero
Odysseus Odysseus ( ; grc-gre, Ὀδυσσεύς, Ὀδυσεύς, OdysseúsOdyseús, ), also known by the Latin variant Ulysses ( , ; lat, UlyssesUlixes), is a legendary Greek king of Ithaca and the hero of Homer's epic poem the ''Odyssey''. Odysse ...
who struggles to return home across the sea's many hazards after the war described in the ''
Iliad The ''Iliad'' (; grc, Ἰλιάς, Iliás, ; "a poem about Ilium") is one of two major ancient Greek epic poems attributed to Homer. It is one of the oldest extant works of literature still widely read by modern audiences. As with the ''Odysse ...
''. The sea is a recurring theme in the
Haiku is a type of short form poetry originally from Japan. Traditional Japanese haiku consist of three phrases that contain a ''kireji'', or "cutting word", 17 '' on'' (phonetic units similar to syllables) in a 5, 7, 5 pattern, and a ''kigo'', or se ...
poems of the Japanese
Edo period The or is the period between 1603 and 1867 in the history of Japan, when Japan was under the rule of the Tokugawa shogunate and the country's 300 regional '' daimyo''. Emerging from the chaos of the Sengoku period, the Edo period was characteriz ...
poet
Matsuo Bashō born then was the most famous poet of the Edo period in Japan. During his lifetime, Bashō was recognized for his works in the collaborative '' haikai no renga'' form; today, after centuries of commentary, he is recognized as the greatest ma ...
(松尾 芭蕉) (1644–1694). In the works of psychiatrist
Carl Jung Carl Gustav Jung ( ; ; 26 July 1875 – 6 June 1961) was a Swiss psychiatrist and psychoanalyst who founded analytical psychology. Jung's work has been influential in the fields of psychiatry, anthropology, archaeology, literature, philo ...
, the sea symbolizes the personal and the
collective unconscious Collective unconscious (german: kollektives Unbewusstes) refers to the unconscious mind and shared mental concepts. It is generally associated with idealism and was coined by Carl Jung. According to Jung, the human collective unconscious is populat ...
in
dream interpretation Dream interpretation is the process of assigning meaning to dreams. Although associated with some forms of psychotherapy, there is no reliable evidence that understanding or interpreting dreams has a positive impact on one's mental health. In m ...
, the depths of the sea symbolizing the depths of the
unconscious mind The unconscious mind (or the unconscious) consists of the processes in the mind which occur automatically and are not available to introspection and include thought processes, memories, interests, and motivations. Even though these processes exis ...
.


Environmental issues

Human activities affect
marine life Marine life, sea life, or ocean life is the plants, animals and other organisms that live in the salt water of seas or oceans, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. M ...
and
marine habitat Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea (the term ''marine'' comes from the Latin ''mare'', meaning sea or ocean). A habitat is an ecological or environmental ...
s through
overfishing Overfishing is the removal of a species of fish (i.e. fishing) from a body of water at a rate greater than that the species can replenish its population naturally (i.e. the overexploitation of the fishery's existing fish stock), resulting in th ...
,
habitat loss Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...
, the introduction of
invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species adv ...
,
ocean pollution Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residential waste, particles, noise, excess carbon dioxide or invasive organisms enter the ocean and cause harmful effects there. The majorit ...
,
ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
and
ocean warming In oceanography and climatology, ocean heat content (OHC) is a term for the energy absorbed by the ocean, where it is stored for indefinite time periods as internal energy or enthalpy. The rise in OHC accounts for over 90% of Earth’s excess the ...
. These impact
marine ecosystem Marine ecosystems are the largest of Earth's aquatic ecosystems and exist in waters that have a high salt content. These systems contrast with freshwater ecosystems, which have a lower salt content. Marine waters cover more than 70% of the surf ...
s and
food webs A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one ...
and may result in consequences as yet unrecognised for the
biodiversity Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
and continuation of marine life forms.


Acidification

Seawater is slightly
alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...
and had an average pH of about 8.2 over the past 300 million years. More recently,
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
has resulted in an increase of the
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
content of the atmosphere; about 30–40% of the added CO2 is absorbed by the oceans, forming carbonic acid and lowering the pH (now below 8.1) through a process called
ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
. The pH is expected to reach 7.7 (representing a 3-fold increase in hydrogen ion concentration) by the year 2100, which is a significant change in a century. One important element for the formation of skeletal material in marine animals is
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
, but calcium carbonate becomes more soluble with pressure, so carbonate shells and
skeletons A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
dissolve below its compensation depth. Calcium carbonate also becomes more soluble at lower pH, so ocean acidification is likely to have profound effects on marine organisms with calcareous shells, such as oysters, clams, sea urchins, and corals, because their ability to form shells will be reduced, and the carbonate compensation depth will rise closer to the sea surface. Affected
plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
ic organisms will include the snail-like molluscs known as pteropods, and single-celled
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
called
coccolithophorid Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingd ...
s and
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm for catching food and ot ...
. All of these are important parts of the
food chain A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), det ...
and a diminution in their numbers will have significant consequences. In tropical regions,
coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
s are likely to be severely affected as it becomes more difficult to build their calcium carbonate skeletons, in turn adversely impacting other
reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock out ...
dwellers. The current rate of
ocean chemistry Marine chemistry, also known as ocean chemistry or chemical oceanography, is influenced by plate tectonics and seafloor spreading, turbidity currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. The fiel ...
change appears to be without precedent in Earth's geological history, making it unclear how well marine ecosystems will be able to adapt to the shifting conditions of the near future. Of particular concern is the manner in which the combination of acidification with the expected additional stressors of higher temperatures and lower oxygen levels will impact the seas.


Marine pollution

Many substances enter the sea as a result of human activities. Combustion products are transported in the air and deposited into the sea by precipitation. Industrial outflows and
sewage Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system. Sewage consists of wastewater discharged from residenc ...
contribute
heavy metals upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead Heavy metals are generally defined as ...
,
pesticide Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and lampri ...
s,
PCBs Polychlorinated biphenyls (PCBs) are highly carcinogenic chemical compounds, formerly used in industrial and consumer products, whose production was banned in the United States by the Toxic Substances Control Act in 1979 and internationally by t ...
,
disinfectant A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than st ...
s, household cleaning products and other synthetic chemicals. These become concentrated in the surface film and in marine sediment, especially estuarine mud. The result of all this contamination is largely unknown because of the large number of substances involved and the lack of information on their biological effects. The heavy metals of greatest concern are copper, lead, mercury, cadmium and zinc which may be Bio-accumulation, bio-accumulated by marine organisms and are passed up the food chain. Much floating plastic rubbish does not Biodegradation, biodegrade, instead disintegrating over time and eventually breaking down to the molecular level. Rigid plastics may float for years. In the centre of the Pacific gyre there is a permanent Great Pacific garbage patch, floating accumulation of mostly plastic waste and there is a similar North Atlantic garbage patch, garbage patch in the Atlantic. Foraging sea birds such as the albatross and petrel may mistake debris for food, and accumulate indigestible plastic in their digestive systems. Turtles and whales have been found with plastic bags and fishing line in their stomachs. Microplastics may sink, threatening filter feeders on the seabed. Most oil pollution in the sea comes from cities and industry. Oil is dangerous for marine animals. It can clog the feathers of sea birds, reducing their insulating effect and the birds' buoyancy, and be ingested when they preen themselves in an attempt to remove the contaminant. Marine mammals are less seriously affected but may be chilled through the removal of their insulation, blinded, dehydrated or poisoned. Benthos, Benthic invertebrates are swamped when the oil sinks, fish are poisoned and the food chain is disrupted. In the short term, oil spills result in wildlife populations being decreased and unbalanced, leisure activities being affected and the livelihoods of people dependent on the sea being devastated. The marine environment has self-cleansing properties and naturally occurring bacteria will act over time to remove oil from the sea. In the Gulf of Mexico, where oil-eating bacteria are already present, they take only a few days to consume spilt oil. Run-off of fertilisers from agricultural land is a major source of pollution in some areas and the discharge of raw sewage has a similar effect. The extra nutrients provided by these sources can cause eutrophication, excessive plant growth. Nitrogen is often the limiting factor in marine systems, and with added nitrogen, algal blooms and red tides can lower the oxygen level of the water and kill marine animals. Such events have created dead zones in the
Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and from ...
and the Gulf of Mexico. Some algal blooms are caused by cyanobacteria that make
shellfish Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environ ...
that Filter feeder, filter feed on them toxic, harming animals like sea otters. Nuclear facilities too can pollute. The Irish Sea was contaminated by radioactive caesium-137 from the former Sellafield nuclear fuel processing plant and nuclear accidents may also cause radioactive material to seep into the sea, as did the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011. The dumping of waste (including oil, noxious liquids, sewage and garbage) at sea is governed by international law. The Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, London Convention (1972) is a United Nations agreement to control ocean dumping which had been ratified by 89 countries by 8 June 2012. MARPOL 73/78 is a convention to minimize pollution of the seas by ships. By May 2013, 152 maritime nations had ratified MARPOL.


See also

* *List of seas *Bay *Gulf


Notes


References


External links


National Oceanic and Atmospheric Administration
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