HOME

TheInfoList




An estuary is a partially enclosed
coastal The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean The ocean (also the sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ...

coastal
body of
brackish water Brackish water, also sometimes termed brack water, is water occurring in a natural environment having more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) with fresh water together, as in est ...
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 environments and are an example of an
ecotone An ecotone is a transition area between two biological communities, where two communities meet and integrate. It may be narrow or wide, and it may be local (the zone between a field and forest) or regional (the transition between forest and gras ...
. Estuaries are subject both to marine influences such as
tides (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon and tides have a phase shift. Tides are the rise and fall of sea leve ...

tides
, waves, and the influx of saline water and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in
sediment Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is cate ...

sediment
, making estuaries among the most productive natural habitats in the world. Most existing estuaries formed during the
Holocene The Holocene ( ) is the current geological epoch In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age (geology), age but shorter than a period (geology), period. The current epoch is the Holocene E ...
epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago. Estuaries are typically classified according to their
geomorphological incised into shale at the foot of the North Caineville Plateau, Utah, within the pass carved by the Fremont River (Utah), Fremont River and known as the Blue Gate. Grove Karl Gilbert, GK Gilbert studied the landscapes of this area in great detail, ...

geomorphological
features or to water-circulation patterns. They can have many different names, such as
bay A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface ...

bay
s,
harbor A harbor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American Engl ...

harbor
s,
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow , such as s, s, s, or . Lagoons are commonly divided into ''coastal lagoons'' and '' lagoons''. They have also been identified as occurring on mixed-sand and ...

lagoon
s,
inlet An inlet is an indentation of a shoreline, usually long and narrow, such as a small bay or arm, that often leads to an enclosed body of salt water, such as a sound In physics, sound is a vibration that propagates as an acoustic wave, through ...

inlet
s, or
sound In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
s, although some of these water bodies do not strictly meet the above definition of an estuary and could be fully saline. Many estuaries suffer
degeneration Degeneracy, degenerate, or degeneration may refer to: Arts and entertainment * Degenerate (album), ''Degenerate'' (album), a 2010 album by the British band Trigger the Bloodshed * Degenerate art, a term adopted in the 1920s by the Nazi Party in ...
from a variety of factors including
soil erosion Soil erosion is the displacement of the upper layer of soil File:Stagnogley.JPG, Surface-water-Gley soil, gley developed in glacial till, Northern Ireland. Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that togeth ...

soil erosion
,
deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...

deforestation
,
overgrazing Overgrazing occurs when plants are exposed to intensive grazing In agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentism, sedentary huma ...

overgrazing
,
overfishing Overfishing is the removal of a species of 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, an ...
and the filling of wetlands.
Eutrophication Eutrophication (from Greek ''eutrophos'', "well-nourished") is the process by which an entire body of water (Lysefjord) in Norway Norway, officially the Kingdom of Norway,Names in the official and recognised languages: Bokmål Bokmå ...

Eutrophication
may lead to excessive nutrients from sewage and animal wastes; pollutants including
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 m ...
,
polychlorinated biphenyl A polychlorinated biphenyl (PCB) is an organic chlorine compound with the formula C12H10−''x''Cl''x''. Polychlorinated biphenyls were once widely deployed as dielectric A dielectric (or dielectric material) is an electrical insulator th ...
s,
radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide A nuclide (or nucleide, from atomic nucleus, nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their ...
s and
hydrocarbon In , a hydrocarbon is an consisting entirely of and . Hydrocarbons are examples of s. Hydrocarbons are generally colourless and hydrophobic with only weak odours. Because of their diverse molecular structures, it is difficult to generalize furth ...
s from sewage inputs; and diking or damming for
flood control Flood control methods are used to reduce or prevent the detrimental effects of flood A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the ...
or water diversion.


Definition

The word "estuary" is derived from the Latin word ''aestuarium'' meaning tidal inlet of the sea, which in itself is derived from the term ''aestus'', meaning tide. There have been many definitions proposed to describe an estuary. The most widely accepted definition is: "a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which seawater is measurably diluted with freshwater derived from land drainage". However, this definition excludes a number of coastal water bodies such as coastal lagoons and
brackish Brackish water, also sometimes termed brack water, is water occurring in a natural environment having more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) with fresh water together, as in estu ...
seas. A more comprehensive definition of an estuary is "a semi-enclosed body of water connected to the sea as far as the
tidal limit Head of tide, tidal limit or tidehead is the farthest point upstream where a 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 grou ...
or the salt intrusion limit and receiving freshwater runoff; however the freshwater inflow may not be perennial, the connection to the sea may be closed for part of the year and tidal influence may be negligible". This broad definition also includes
fjord In physical geography Physical geography (also known as physiography) is one of the two fields of geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the ...

fjord
s,
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow , such as s, s, s, or . Lagoons are commonly divided into ''coastal lagoons'' and '' lagoons''. They have also been identified as occurring on mixed-sand and ...

lagoon
s,
river mouth A river mouth is where a river flows into a larger body of water ( Lysefjord) in Norway Norway ( nb, ; nn, ; se, Norga; smj, Vuodna; sma, Nöörje), officially the Kingdom of Norway, is a Nordic countries, Nordic country in Nor ...

river mouth
s, and tidal creeks. An estuary is a dynamic
ecosystem 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 syste ...

ecosystem
having a connection to the open sea through which the
sea water Seawater, or salt water, is water Water (chemical formula H2O) 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 ...

sea water
enters with the rhythm of the
tide Tides are the rise and fall of s caused by the combined effects of the forces exerted by the and the , and the of the . s can be used for any given locale to find the predicted times and (or ""). The predictions are influenced by many ...

tide
s. The seawater entering the estuary is diluted by the
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 Total dissolved solids (TDS) is a measure of the dissolved combined content of all ...

fresh water
flowing from rivers and streams. The pattern of dilution varies between different estuaries and depends on the volume of freshwater, the tidal range, and the extent of evaporation of the water in the estuary.


Classification based on geomorphology


Drowned river valleys

Drowned river valleys are also known as coastal plain estuaries. In places where the sea level is rising relative to the land, sea water progressively penetrates into river valleys and the topography of the estuary remains similar to that of a river valley. This is the most common type of estuary in temperate climates. Well-studied estuaries include the
Severn Estuary , Gloucestershire Gloucestershire ( abbreviated Glos) is a Counties of England, county in South West England. The county comprises part of the Cotswold Hills, part of the flat fertile valley of the River Severn, and the entire Forest of Dea ...

Severn Estuary
in the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the United Kingdom. Some prefer to use Britain as shorth ...

United Kingdom
and the Ems Dollard along the Dutch-German border. The width-to-depth ratio of these estuaries is typically large, appearing wedge-shaped (in cross-section) in the inner part and broadening and deepening seaward. Water depths rarely exceed . Examples of this type of estuary in the U.S. are the
Hudson River The Hudson River is a river that flows from north to south primarily through eastern New York (state), New York in the United States. It originates in the Adirondack Mountains of Upstate New York and flows southward through the Hudson Valley ...

Hudson River
,
Chesapeake Bay The Chesapeake Bay ( ) is the largest in the United States. The Bay is located in the and is primarily separated from the Atlantic Ocean by the (including the parts: the / and the state of ) with its mouth of the Bay at the south end located ...
, and
Delaware Bay Delaware Bay is the estuary outlet of the Delaware River on the northeast seaboard of the United States. Approximately in area, the bay's fresh water mixes for many miles with the salt water of the Atlantic Ocean. The bay is bordered inland by t ...
along the
Mid-AtlanticMid-Atlantic or Mid Atlantic can refer to: *The middle of the Atlantic Ocean *Mid-Atlantic English, a mix between British English and American English *Mid-Atlantic Region (Little League World Series), one of the United States geographic divisions of ...
coast, and
Galveston Bay Galveston Bay ( ) is a bay in the western Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North America North America is a continent e ...

Galveston Bay
and
Tampa Bay Tampa Bay is a large natural harbor A harbor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the U ...
along the
Gulf Coast The Gulf Coast of the United States is the coastline The coast, also known as the coastline or seashore, is defined as the area where land meets the sea or ocean, or as a line that forms the boundary between the land and the ocean T ...

Gulf Coast
.


Lagoon-type or bar-built

Bar-built estuaries are found in a place where the deposition of sediment has kept pace with rising sea levels so that the estuaries are shallow and separated from the sea by sand spits or barrier islands. They are relatively common in tropical and subtropical locations. These estuaries are semi-isolated from ocean waters by barrier beaches (
barrier island Barrier islands are coastal landforms and a type of dune system that are exceptionally flat or lumpy areas of sand that form by wave and tidal action parallel to the mainland coast. They usually occur in chains, consisting of anything from a f ...
s and barrier spits). Formation of barrier beaches partially encloses the estuary, with only narrow inlets allowing contact with the ocean waters. Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. They are extensive along the Atlantic and Gulf coasts of the U.S. in areas with active coastal deposition of sediments and where tidal ranges are less than . The barrier beaches that enclose bar-built estuaries have been developed in several ways: * building up of offshore bars by wave action, in which sand from the seafloor is deposited in elongated bars parallel to the shoreline, * reworking of sediment discharge from rivers by a wave, current, and wind action into beaches, overwash flats, and dunes, * engulfment of mainland beach ridges (ridges developed from the erosion of coastal plain sediments around 5000 years ago) due to
sea level rise Tide gauge measurements show that the current global sea level rise began at the start of the 20th century. Between 1900 and 2017, the globally averaged sea level Mean sea level (MSL) (often shortened to sea level) is an average In colloqui ...

sea level rise
and resulting in the breaching of the ridges and flooding of the coastal lowlands, forming shallow lagoons, and * elongation of barrier spits from the erosion of headlands due to the action of
longshore current Longshore drift from longshore current is a geological process that consists of the transportation of s (clay, silt, pebbles, sand, shingle) along a coast parallel to the shoreline, which is dependent on the angle incoming wave direction. Obliq ...

longshore current
s, with the spits growing in the direction of the littoral drift.


Fjord-type

Fjords were formed where Pleistocene glaciers deepened and widened existing river valleys so that they become U-shaped in cross-sections. At their mouths there are typically rocks, bars or sills of
glacial deposits image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is di ...

glacial deposits
, which have the effects of modifying the estuarine circulation.
Fjord In physical geography Physical geography (also known as physiography) is one of the two fields of geography Geography (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδ ...

Fjord
-type estuaries are formed in deeply eroded valleys formed by
glacier A glacier (; ) is a persistent body of dense ice Ice is into a state. Depending on the presence of such as particles of soil or bubbles of air, it can appear transparent or a more or less bluish-white color. In the , ice is abunda ...

glacier
s. These U-shaped estuaries typically have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The estuary is shallowest at its mouth, where terminal glacial
moraine A moraine is any accumulation of unconsolidated debris (regolith and Rock (geology), rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glac ...

moraine
s or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed . The width-to-depth ratio is generally small. In estuaries with very shallow sills, tidal oscillations only affect the water down to the depth of the sill, and the waters deeper than that may remain stagnant for a very long time, so there is only an occasional exchange of the deep water of the estuary with the ocean. If the sill depth is deep, water circulation is less restricted, and there is a slow but steady exchange of water between the estuary and the ocean. Fjord-type estuaries can be found along the coasts of
Alaska Alaska (; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Central Alaskan Yup'ik language, Yup'ik: ''Alaskaq''; tli, Anáaski) is a U.S. state in the Western United States, on the northwest extremity of the country's West Coast of the United State ...

Alaska
, the
Puget Sound Puget Sound () is a of the , an inlet of the , and part of the . It is located along the northwestern coast of the of . It is a complex system of interconnected marine waterways and basins, with one major and two minor connections to the ope ...
region of western
Washington state Washington (), officially the State of Washington, is a U.S. state, state in the Pacific Northwest region of the Western United States. Named for George Washington—the first President of the United States, U.S. president—the state was form ...
,
British Columbia ( en, Splendour without diminishment) , image_map = British Columbia in Canada 2.svg , Label_map = yes , coordinates = , official_lang = None , Slogan = Beautiful British C ...

British Columbia
, eastern Canada,
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an autonomous territory An autonomous administrative division (also referred to as an autonomous area, entity, unit, region, subdivision, or territory) is a subnational administra ...

Greenland
,
Iceland Iceland ( is, Ísland; ) is a Nordic Nordic most commonly refers to: * Nordic countries, written in plural as Nordics, the northwestern European countries, including Scandinavia, Fennoscandia and the List of islands in the Atlantic Ocean#N ...

Iceland
, New Zealand, and Norway.


Tectonically produced

These estuaries are formed by subsidence or land cut off from the ocean by land movement associated with
faulting In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...
,
volcano A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet a ...

volcano
es, and
landslide Landslides, also known as landslips, are several forms of mass wasting Mass wasting, also known as mass movement, is a general term for the movement of rock (geology), rock or soil down slopes under the force of gravity. It differs from othe ...

landslide
s.
Inundation Inundation (from the Latin ''inundatio'', flood) is both the act of intentionally flood Flash flooding caused by heavy rain falling in a short amount of time. A flood is an overflow of water that submerges land that is usually dry ...

Inundation
from eustatic sea-level rise during the
Holocene The Holocene ( ) is the current geological epoch In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age (geology), age but shorter than a period (geology), period. The current epoch is the Holocene E ...
Epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-E ...
has also contributed to the formation of these estuaries. There are only a small number of
tectonically Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...
produced estuaries; one example is the
San Francisco Bay San Francisco Bay is a shallow estuary An estuary is a partially enclosed Coast, 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 ...

San Francisco Bay
, which was formed by the crustal movements of the
San Andreas fault The San Andreas Fault is a continental transform fault that extends roughly through California California is a U.S. state, state in the Western United States. With over 39.3million residents across a total area of approximately , it is th ...
system causing the inundation of the lower reaches of the
Sacramento ) , image_map = Sacramento County California Incorporated and Unincorporated areas Sacramento Highlighted.svg , mapsize = 250x200px , map_caption = Location within Sacramento ...
and San Joaquin rivers.


Classification based on water circulation


Salt wedge

In this type of estuary, river output greatly exceeds marine input and tidal effects have minor importance. Freshwater floats on top of the seawater in a layer that gradually thins as it moves seaward. The denser seawater moves landward along the bottom of the estuary, forming a wedge-shaped layer that is thinner as it approaches land. As a velocity difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater. An example of a salt wedge estuary is the
Mississippi River The Mississippi River is the second-longest river and chief 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 b ...

Mississippi River
.


Partially mixed

As tidal forcing increases, river output becomes less than the marine input. Here, current induced turbulence causes mixing of the whole water column such that salinity varies more longitudinally rather than vertically, leading to a moderately stratified condition. Examples include the
Chesapeake Bay The Chesapeake Bay ( ) is the largest in the United States. The Bay is located in the and is primarily separated from the Atlantic Ocean by the (including the parts: the / and the state of ) with its mouth of the Bay at the south end located ...
and
Narragansett Bay Narragansett Bay is a bay and estuary on the north side of Rhode Island Sound covering , of which is in Rhode Island. The bay forms New England's largest estuary, which functions as an expansive natural harbor and includes a small archipelago. Smal ...

Narragansett Bay
.


Well-mixed

Tidal mixing forces exceed river output, resulting in a well-mixed water column and the disappearance of the vertical salinity
gradient In vector calculus Vector calculus, or vector analysis, is concerned with differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Prod ...

gradient
. The freshwater-seawater boundary is eliminated due to the intense and eddy effects. The lower reaches of
Delaware Bay Delaware Bay is the estuary outlet of the Delaware River on the northeast seaboard of the United States. Approximately in area, the bay's fresh water mixes for many miles with the salt water of the Atlantic Ocean. The bay is bordered inland by t ...
and the
Raritan River The Raritan River is a major river of New Jersey New Jersey is a U.S. state, state in the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. It is bordered on the north and ea ...
in
New Jersey New Jersey is a U.S. state, state in the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. It is bordered on the north and east by the state of New York (state), New York; on the ea ...
are examples of vertically homogenous estuaries.


Inverse

Inverse estuaries occur in dry climates where evaporation greatly exceeds the inflow of freshwater. A salinity maximum zone is formed, and both riverine and oceanic water flow close to the surface towards this zone. This water is pushed downward and spreads along the bottom in both the seaward and landward direction. An example of an inverse estuary is
Spencer Gulf The Spencer Gulf is the westernmost and larger of two large inlets (the other being Gulf St Vincent) on the southern coast of Australia, in the state of South Australia, facing the Great Australian Bight. It spans from the Cape Catastrophe and ...

Spencer Gulf
, South Australia. & Hall, S.M. (2014): Spencer Gulf: Geological setting and evolution. In: ''Natural History of Spencer Gulf.'' Royal Society of South Australia Inc. p. 21.


Intermittent

Estuary type varies dramatically depending on freshwater input, and is capable of changing from a wholly marine
embayment 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 (geography), gulf, sea, sound (geography), sound, or bight (geogr ...
to any of the other estuary types.


Physiochemical variation

The most important variable characteristics of estuary water are the concentration of dissolved oxygen,
salinity Salinity () is the saltiness or amount of dissolved in a body of , called (see also ). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal to ‰). Salinity is an important ...

salinity
and
sediment Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is cate ...

sediment
load. There is extreme spatial variability in salinity, with a range of near-zero at the
tidal limit Head of tide, tidal limit or tidehead is the farthest point upstream where a 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 grou ...
of tributary rivers to 3.4% at the estuary mouth. At any one point, the salinity will vary considerably over time and seasons, making it a harsh environment for organisms. Sediment often settles in intertidal
mudflats , intertidal and subtidal zones. The most apparent character of the area is the development of tidal channels, affecting mainly the intertidal zone. In this case, the tidal flat is protected seaward by a shoal, beach barrier, but in many cases (lo ...
which are extremely difficult to colonize. No points of attachment exist for
algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grouping that includes species from multiple distinct s. Included organisms range from , such as '','' and the s, to forms, such as the , a large whi ...

algae
, so vegetation based habitat is not established. Sediment can also clog feeding and respiratory structures of species, and special adaptations exist within mudflat species to cope with this problem. Lastly,
dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group ...
variation can cause problems for life forms. Nutrient-rich sediment from man-made sources can promote primary production life cycles, perhaps leading to eventual decay removing the dissolved oxygen from the water; thus hypoxic or
anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of disso ...
zones can develop.


Implications for marine life

Estuaries are incredibly dynamic systems, where temperature, salinity, turbidity, depth and flow all change daily in response to the tides. This dynamism makes estuaries highly productive habitats, but also make it difficult for many species to survive year-round. As a result, estuaries large and small experience strong seasonal variation in their fish communities. In winter, the fish community is dominated by hardy marine residents, and in summer a variety of marine and anadromous fishes move into and out of estuaries, capitalizing on their high productivity. Estuaries provide critical habitat to a variety of species that rely on estuaries for life-cycle completion. Pacific Herring (''Clupea pallasii'') are known to lay their eggs in estuaries and bays, surfperch give birth in estuaries, juvenile flatfish and rockfish migrate to estuaries to rear, and
anadromous Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres. Fish usually animal migration, migrate to feed or to reproduce, but i ...
salmonids Salmonidae is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain the we ...
and
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of Agnatha, jawless fish of the order (biology), order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by ...

lamprey
s use estuaries as migration corridors. Also, migratory bird populations, such as the
black-tailed godwit The black-tailed godwit (''Limosa limosa'') is a large, long-legged, long-billed shorebird first described by Carl Linnaeus in 1758. It is a member of the godwit genus, ''Limosa''. There are four subspecies, all with orange head, neck and chest i ...
, rely on estuaries. Two of the main challenges of estuarine life are the variability in
salinity Salinity () is the saltiness or amount of dissolved in a body of , called (see also ). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal to ‰). Salinity is an important ...

salinity
and
sedimentation Sedimentation is the deposition of sediments Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks A rock is any naturally occurring solid mass or aggreg ...
. Many species of
fish Fish are , , -bearing animals that lack with . Included in this definition are the living , s, and and as well as various extinct related groups. Around 99% of living fish species are ray-finned fish, belonging to the class , with over 95 ...

fish
and
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s have various methods to control or conform to the shifts in salt concentrations and are termed
osmoconformer Osmoconformers are marine organisms that maintain an internal environment which is isotonic to their external environment. This means that the osmotic pressure of the organism's cells is equal to the osmotic pressure of their surrounding environm ...
s and osmoregulators. Many animals also
burrow An Eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of Animal locomotion, locomotion. Burrows provide ...
to avoid
predation Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical env ...

predation
and to live in a more stable sedimental environment. However, large numbers of bacteria are found within the sediment which has a very high oxygen demand. This reduces the levels of oxygen within the sediment often resulting in partially
anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of disso ...
conditions, which can be further exacerbated by limited water flux.
Phytoplankton Phytoplankton () are the autotrophic An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compoun ...

Phytoplankton
are key primary producers in estuaries. They move with the water bodies and can be flushed in and out with the
tide Tides are the rise and fall of s caused by the combined effects of the forces exerted by the and the , and the of the . s can be used for any given locale to find the predicted times and (or ""). The predictions are influenced by many ...

tide
s. Their productivity is largely dependent upon the
turbidity Turbidity is the cloudiness or haziness of a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (matter), phase of matter ...

turbidity
of the water. The main phytoplankton present is
diatoms Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic ...

diatoms
and
dinoflagellates The dinoflagellates ( Greek δῖνος ''dinos'' "whirling" and Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, ...

dinoflagellates
which are abundant in the sediment. It is important to remember that a primary source of food for many organisms on estuaries, including
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
, is
detritus In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...

detritus
from the settlement of the sedimentation.


Human impact

Of the thirty-two largest cities in the world in the early 1990s, twenty-two were located on estuaries. As ecosystems, estuaries are under threat from human activities such as
pollution Pollution is the introduction of s into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). s, the components of po ...

pollution
and
overfishing Overfishing is the removal of a species of 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, an ...
. They are also threatened by sewage, coastal settlement, land clearance and much more. Estuaries are affected by events far upstream, and concentrate materials such as pollutants and sediments. Land run-off and industrial, agricultural, and domestic waste enter rivers and are discharged into estuaries. Contaminants can be introduced which do not disintegrate rapidly in the marine environment, such as
plastics Plastics are a wide range of syntheticA synthetic is an artificial material produced by organic chemistry, organic chemical synthesis. Synthetic may also refer to: In the sense of both "combination" and "artificial" * Synthetic chemical or s ...

plastics
,
pesticide Pesticides are substances that are meant to control pest (organism), pests. The term pesticide includes all of the following: herbicide, insecticides (which may include insect growth regulators, termiticides, etc.) nematicide, molluscicide, pi ...
s,
furan Furan is a heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). Heterocyclic chemist ...

furan
s,
dioxins Dioxin may refer to: *1,2-Dioxin or 1,4-Dioxin, two unsaturated Heterocyclic#6-Membered rings, heterocyclic 6-membered rings where two carbon atoms have been replaced by oxygen atoms, giving the chemical formula#Molecular formula, molecular formul ...
,
phenol Phenol (also called carbolic acid) is an aromatic forms of benzene (top) combine to produce an average structure (bottom) In chemistry, aromaticity is a property of cyclic compound, cyclic (ring (chemistry), ring-shaped), plane (geometry), p ...

phenol
s and
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 m ...
. Such toxins can accumulate in the tissues of many species of aquatic life in a process called
bioaccumulationBioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance at a rate faster than that at which the substance is lost or eliminated by ...
. They also accumulate in
benthic The benthic zone is the ecological region at the lowest level of a body of water ( Lysefjord) in Norway Norway ( nb, ; nn, ; se, Norga; smj, Vuodna; sma, Nöörje), officially the Kingdom of Norway, is a Nordic countries, Nordic ...
environments, such as estuaries and bay muds: a geological record of human activities of the last century. The elemental composition of biofilm reflect areas of the estuary impacted by human activities, and over time may shift the basic composition of the ecosystem, and the reversible or irreversible changes in the abiotic and biotic parts of the systems from the bottom up. For example, Chinese and Russian industrial pollution, such as phenols and heavy metals, has devastated fish stocks in the Amur River and damaged its estuary soil. Estuaries tend to be naturally eutrophic because land runoff discharges nutrients into estuaries. With human activities, land run-off also now includes the many chemicals used as fertilizers in agriculture as well as waste from livestock and humans. Excess oxygen-depleting chemicals in the water can lead to Hypoxia (environmental), hypoxia and the creation of dead zone (ecology), dead zones. This can result in reductions in water quality, fish, and other animal populations. Overfishing also occurs.
Chesapeake Bay The Chesapeake Bay ( ) is the largest in the United States. The Bay is located in the and is primarily separated from the Atlantic Ocean by the (including the parts: the / and the state of ) with its mouth of the Bay at the south end located ...
once had a flourishing oyster population that has been almost wiped out by overfishing. Oysters filter these pollutants, and either eat them or shape them into small packets that are deposited on the bottom where they are harmless. Historically the oysters filtered the estuary's entire water volume of excess nutrients every three or four days. Today that process takes almost a year, and sediment, nutrients, and algae can cause problems in local waters.


Examples


Africa

* Orange River Estuary * Lake St Lucia Estuary


Asia

* Gulf of Ob Estuary * Yenisei Gulf Estuary * Puerto Princesa Subterranean River National Park, Puerto Princesa Underground River * Hàn River (Vietnam), Hàn River Estuary * Kraburi River Estuary * Waeru River Estuary of Chanthaburi Province * Dawei River Estuary * Naf River Estuary * Meghna River Estuary


Europe

* The Gironde estuary, Gironde * Golden Horn * The Humber *
Severn Estuary , Gloucestershire Gloucestershire ( abbreviated Glos) is a Counties of England, county in South West England. The county comprises part of the Cotswold Hills, part of the flat fertile valley of the River Severn, and the entire Forest of Dea ...

Severn Estuary
* Shannon Estuary * Thames Estuary * The Wash * Unterelbe * Western Scheldt * Tagus Estuary


North America

* Albemarle Sound including Outer Banks of North Carolina *
Chesapeake Bay The Chesapeake Bay ( ) is the largest in the United States. The Bay is located in the and is primarily separated from the Atlantic Ocean by the (including the parts: the / and the state of ) with its mouth of the Bay at the south end located ...
including Hampton Roads * Columbia River Estuary *
Delaware Bay Delaware Bay is the estuary outlet of the Delaware River on the northeast seaboard of the United States. Approximately in area, the bay's fresh water mixes for many miles with the salt water of the Atlantic Ocean. The bay is bordered inland by t ...
* Drakes Estero, Drake's Estero * East River * Estuary of Saint Lawrence * Fraser River *
Galveston Bay Galveston Bay ( ) is a bay in the western Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North America North America is a continent e ...

Galveston Bay
* Great Bay (New Hampshire), Great Bay * Indian River Lagoon * Laguna Madre * Lake Borgne * Lake Merritt * Lake Pontchartrain * Long Island Sound * Mobile Bay *
Narragansett Bay Narragansett Bay is a bay and estuary on the north side of Rhode Island Sound covering , of which is in Rhode Island. The bay forms New England's largest estuary, which functions as an expansive natural harbor and includes a small archipelago. Smal ...

Narragansett Bay
* Newport Back Bay * Geography of New York-New Jersey Harbor Estuary, New York-New Jersey Harbor * Coos Bay *
Puget Sound Puget Sound () is a of the , an inlet of the , and part of the . It is located along the northwestern coast of the of . It is a complex system of interconnected marine waterways and basins, with one major and two minor connections to the ope ...
* Pamlico Sound including the Outer Banks of North Carolina *
San Francisco Bay San Francisco Bay is a shallow estuary An estuary is a partially enclosed Coast, 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 ...

San Francisco Bay
*
Tampa Bay Tampa Bay is a large natural harbor A harbor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the U ...


Oceania

* Gippsland Lakes * Port Jackson (Sydney Harbour) *
Spencer Gulf The Spencer Gulf is the westernmost and larger of two large inlets (the other being Gulf St Vincent) on the southern coast of Australia, in the state of South Australia, facing the Great Australian Bight. It spans from the Cape Catastrophe and ...

Spencer Gulf


South America

* Amazon River * Iguape-Cananéia-Paranaguá estuary lagoon complex * Lagoa dos Patos and Lagoon Mirim * Rio de la Plata


See also

* * * * * * * * * * * * * * * * *


References


External links


Animated documentary on Chesapeake Bay
NOAA. *
The Estuary Guide (Based on experience and R&D within the UK)
{{Authority control Estuaries, Geodesy Coastal geography Fishing and the environment Coastal and oceanic landforms Bodies of water Aquatic ecology