An estuary is a partially enclosed
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 ...
al body of
brackish water
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 estuari ...
with one or more rivers or streams flowing into it, and with a free connection to the open
sea
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 Sea, ...
.
Estuaries form a transition zone between
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 w ...
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,
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 (band) ...
, and the influx of
saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing 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 appr ...
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 and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
, 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. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...
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 features or to water-circulation patterns. They can have many different names, such as
bays,
harbor
A harbor (American English), harbour (British English; see spelling differences), or haven is a sheltered body of water where ships, boats, and barges can be docked. The term ''harbor'' is often used interchangeably with ''port'', which is a ...
s,
lagoon
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') ...
s,
inlets, or
sound
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' b ...
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 from a variety of factors including
soil erosion,
deforestation
Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated ...
,
overgrazing
Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, game reserves, or nature res ...
,
overfishing and the filling of wetlands.
Eutrophication
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...
may lead to excessive nutrients from sewage and animal wastes; pollutants including
heavy metals,
polychlorinated biphenyl
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 ...
s,
radionuclides and
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
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 waters."Flood Control", MSN Encarta, 2008 (see below: Further reading). Flood relief methods are used to reduce the effects of flood waters or high water level ...
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, 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 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 is affected by tidal fluctuations, or where the fluctuations are less than a certain amount. This applies to rivers which flow into tidal bodies such as oceans, b ...
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, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Förden and East Jutland Fjorde, Germany, ...
s,
lagoon
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') ...
s,
river mouths, and
tidal creek
A tidal creek or tidal channel is a narrow inlet or estuary that is affected by the ebb and flow of ocean tides. Thus, it has variable salinity and electrical conductivity over the tidal cycle, and flushes salts from inland soils. Tidal cree ...
s. 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 ...
having a connection to the open sea through which the
sea water enters with the rhythm of the
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 effects of tides on estuaries can show
nonlinear
In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many othe ...
effects on the movement of water which can have important impacts on the ecosystem and waterflow. The seawater entering the estuary is diluted by the
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
The Severn Estuary ( cy, Aber Hafren) is the estuary of the River Severn, flowing into the Bristol Channel between South West England and South Wales. Its high tidal range, approximately , means that it has been at the centre of discussions in t ...
in the
United Kingdom
The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...
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. It originates in the Adirondack Mountains of Upstate New York and flows southward through the Hudson Valley to the New York Harbor between N ...
,
Chesapeake Bay
The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the Eastern Shore of Maryland / ...
, and
Delaware Bay
Delaware Bay is the estuary outlet of the Delaware River on the northeast seaboard of the United States. It is approximately in area, the bay's freshwater mixes for many miles with the saltwater of the Atlantic Ocean.
The bay is bordered inland ...
along the
Mid-Atlantic coast, and
Galveston Bay and
Tampa Bay
Tampa Bay is a large natural harbor and shallow estuary connected to the Gulf of Mexico on the west-central coast of Florida, comprising Hillsborough Bay, McKay Bay, Old Tampa Bay, Middle Tampa Bay, and Lower Tampa Bay. The largest freshwater ...
along the
Gulf Coast
The Gulf Coast of the United States, also known as the Gulf South, is the coast, coastline along the Southern United States where they meet the Gulf of Mexico. The list of U.S. states and territories by coastline, coastal states that have a shor ...
.
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 ...
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
Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cry ...
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 sediments (clay, silt, pebbles, sand, shingle) along a coast parallel to the shoreline, which is dependent on the angle incoming wave direction ...
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 d ...
, which have the effects of modifying the estuarine circulation.
Fjord
In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Förden and East Jutland Fjorde, Germany, ...
-type estuaries are formed in deeply eroded valleys formed by
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 as ...
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
moraines 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 ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S. ...
, the
Puget Sound
Puget Sound ( ) is a sound of the Pacific Northwest, an inlet of the Pacific Ocean, and part of the Salish Sea. It is located along the northwestern coast of the U.S. state of Washington. It is a complex estuarine system of interconnected ma ...
region of western
Washington state
Washington (), officially the State of Washington, is a state in the Pacific Northwest region of the Western United States. Named for George Washington—the first U.S. president—the state was formed from the western part of the Washington ...
,
British Columbia
British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...
, eastern Canada,
Greenland
Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland i ...
,
Iceland
Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...
, 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, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
,
volcano
A volcano is a rupture in the Crust (geology), crust of a Planet#Planetary-mass objects, planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface.
On Ear ...
es, and
landslides.
Inundation
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrolog ...
from eustatic sea-level rise during the
Holocene
The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...
Epoch
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by ...
has also contributed to the formation of these estuaries. 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 ...
produced estuaries; one example is the
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. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizonta ...
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 of the second-largest drainage system in North America, second only to the Hudson Bay drainage system. From its traditional source of Lake Itasca in northern Minnesota, it fl ...
.
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 estuary in the United States. The Bay is located in the Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the Eastern Shore of Maryland / ...
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. Sm ...
.
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, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...
. The freshwater-seawater boundary is eliminated due to the intense
turbulent mixing 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. It is approximately in area, the bay's freshwater mixes for many miles with the saltwater of the Atlantic Ocean.
The bay is bordered inland ...
and the
Raritan River
Raritan River is a major river of New Jersey. Its watershed drains much of the mountainous area of the central part of the state, emptying into the Raritan Bay on the Atlantic Ocean.
History
Geologists assert that the lower Raritan provided t ...
in
New Jersey
New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...
are examples of vertically homogeneous 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, South Australia.
[ Gostin, V. & 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, sea, sound, or bight. A cove is a small, circular bay with a na ...
to any of the other estuary types.
Physiochemical variation
The most important variable characteristics of estuary water are the concentration of dissolved oxygen,
salinity and
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
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 is affected by tidal fluctuations, or where the fluctuations are less than a certain amount. This applies to rivers which flow into tidal bodies such as oceans, b ...
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
Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal fl ...
which are extremely difficult to colonize. No points of attachment exist for
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 that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...
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
Hypoxia means a lower than normal level of oxygen, and may refer to:
Reduced or insufficient oxygen
* Hypoxia (environmental), abnormally low oxygen content of the specific environment
* Hypoxia (medical), abnormally low level of oxygen in the t ...
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 diss ...
zones can develop.
Implications of eutrophication on estuaries
Effects of eutrophication on biogeochemical cycles
![Table_of_the_Processes_in_the_Nitrogen_Cycle](https://upload.wikimedia.org/wikipedia/commons/7/77/Table_of_the_Processes_in_the_Nitrogen_Cycle.jpg)
Nitrogen is often the lead cause of
eutrophication
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...
in estuaries in temperate zones. During a eutrophication event, biogeochemical feedback decreases the amount of available
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
. These feedbacks also increase the supply of
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and phosphorus, creating conditions where harmful algal blooms can persist. Given the now off-balance
nitrogen cycle, estuaries can be driven to
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
limitation instead of nitrogen limitation. Estuaries can be severely impacted by an unbalanced phosphorus cycle, as phosphorus interacts with nitrogen and silica availability.
With an abundance of nutrients in the ecosystem, plants and algae overgrow and eventually decompose, which produce a significant amount of carbon dioxide. While releasing into the water and atmosphere, these organisms are also intaking all or nearly all of the available oxygen creating a
hypoxic
Hypoxia means a lower than normal level of oxygen, and may refer to:
Reduced or insufficient oxygen
* Hypoxia (environmental), abnormally low oxygen content of the specific environment
* Hypoxia (medical), abnormally low level of oxygen in the t ...
environment and unbalanced
oxygen cycle
Oxygen cycle refers to the movement of oxygen through the atmosphere (air), biosphere (plants and animals) and the lithosphere (the Earth’s crust). The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as wel ...
. The excess carbon in the form of can lead to low pH levels and
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 ...
, which is more harmful for vulnerable coastal regions like estuaries.
Effects of eutrophication on estuarine plants
Eutrophication
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...
has been seen to negatively impact many plant communities in estuarine
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 ...
s.
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 are a type of ecosystem in some estuaries that have been negatively impacted by eutrophication.
Cordgrass
''Spartina'' is a taxon of plants in the grass family, frequently found in coastal salt marshes. Its species are commonly known as cordgrass or cord-grass, and are native to the coasts of the Atlantic Ocean in western and southern Europe, north ...
vegetation dominates the salt marsh landscape. Excess nutrients allow the plants to grow at greater rates in above ground biomass, however less energy is allocated to the roots since nutrients is abundant.
This leads to a lower
biomass in the vegetation below ground which destabilizes the banks of the marsh causing increased rates of
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
.
A similar phenomenon occurs in
mangrove swamps, which are another potential ecosystem in estuaries.
An increase in nitrogen causes an increase in shoot growth and a decrease in root growth.
Weaker root systems cause a mangrove tree to be less resilient in seasons of drought, which can lead to the death of the mangrove.
This shift in above ground and below ground biomass caused by eutrophication could hindered plant success in these ecosystems.
Effects of eutrophication on estuarine animals
Across all biomes,
eutrophication
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...
often results in plant death but the impacts do not end there. Plant death alters the entire food web structure which can result in the death of animals within the afflicted
biome
A biome () is a biogeographical unit consisting of a biological community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. Biome is a broader ...
. Estuaries are hotspots for
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 ...
, containing a majority of commercial fish catch, making the impacts of eutrophication that much greater within estuaries. Some specific estuarine animals feel the effects of eutrophication more strongly than others. One example is the whitefish species from the
European Alps.
[Vonlanthen, P., Bittner, D., Hudson A.G., et al. (2012). Eutrophication causes speciation reversal in whitefish adaptive radiations. Nature. 482, 337-362. DOI: 10.1038/nature0824.](_blank)
/ref> Eutrophication reduced the oxygen levels in their habitats so greatly that whitefish eggs could not survive, causing local extinctions. However, some animals, such as carnivorous fish, tend to do well in nutrient poor environments and can benefit from eutrophication. This can be seen in populations of bass or pikes.
Effects of eutrophication on human activities
Eutrophication can affect many marine habitats which can lead to economic consequences. The commercial fishing industry relies upon estuaries for approximately 68 percent of their catch by value because of the great biodiversity of this ecosystem. During an algal bloom
An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompass ...
, fishermen have noticed a significant increase in the quantity of fish. A sudden increase in primary productivity causes spikes in fish populations which leads to more oxygen being utilized. It is the continued deoxygenation of the water that then causes a decline in fish populations. These effects can begin in estuaries and have a wide effect on the surrounding water bodies. In turn, this can decrease fishing industry sales in one area and across the country. Production in 2016 from recreational and commercial fishing contributes billions of dollars to the United States’ gross domestic product (GDP). A decrease in production within this industry can affect any of the 1.7 million people the fishing industry employs yearly across the United States.
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 a 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 salmonids
Salmonidae is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes . It includes salmon (both Atlantic and Pacific species), trout (both ocean-going and landlocked), chars, freshwater whit ...
and lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s use estuaries as migration corridors. Also, migratory bird populations, such as the black-tailed godwit, rely on estuaries.
Two of the main challenges of estuarine life are the variability in salinity and 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 ...
. Many species of fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
and invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
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 environme ...
s and osmoregulator
Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration of ...
s. Many animals also burrow to avoid predation
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
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 diss ...
conditions, which can be further exacerbated by limited water flow.
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 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. Their productivity is largely dependent upon 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. The main phytoplankton present are diatoms
A diatom (New Latin, 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 com ...
and dinoflagellates
The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates a ...
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 (; 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. Typically a few micrometr ...
, is 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 contaminants 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). Pollutants, the ...
and overfishing. 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, pesticides, furans, dioxins, phenol
Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...
s and heavy metals.
Such toxins can accumulate in the tissues of many species of aquatic life in a process called bioaccumulation. They also accumulate in benthic environments, such as estuaries and bay muds
Bay mud consists of thick deposits of soft, unconsolidated silty clay, which is saturated with water; these soil layers are situated at the bottom of certain estuaries, which are normally in temperate regions that have experienced cyclical glacia ...
: a geological record of human activities of the last century. The elemental composition of biofilm
A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...
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
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplan ...
because land runoff
Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when the ...
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 and the creation of 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 estuary in the United States. The Bay is located in the Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the Eastern Shore of Maryland / ...
once had a flourishing 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 ...
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
The Orange River (from Afrikaans/Dutch: ''Oranjerivier'') is a river in Southern Africa. It is the longest river in South Africa. With a total length of , the Orange River Basin extends from Lesotho into South Africa and Namibia to the north ...
Estuary
* Lake St Lucia Estuary
Asia
* Gulf of Ob
The Gulf of Ob ( (russian: Обская губа, Obskaya guba; also known as ''Bay of Ob'', russian: Обский залив, Obsky zaliv, link=no) is a bay of the Arctic Ocean, located in Northern Russia at the mouth of the Ob River. Estuary
* Yenisei Gulf
The Yenisey Gulf (russian: Енисейский залив, ''Yeniseysky zaliv'') is a large and long estuary through which the lower Yenisey flows into the Kara Sea.
The Yenisey Gulf and its islands belong to the Krasnoyarsk Krai administrative ...
Estuary
* Puerto Princesa Underground River
* Hàn River Estuary
* Kraburi River Estuary
* Waeru River Estuary of Chanthaburi Province
* Dawei River
The Dawei River or Tavoy River ( my, ထားဝယ်မြစ်) is a river of the Tanintharyi Region, Burma. Geography
It has its source in the Tenasserim Hills and flows from north to south, the northern branch of the Tenasserim River runni ...
Estuary
* Naf River
The Naf River ( bn, নাফ নদী ''Naf Nodi'' ; my, နတ်မြစ် ; rki, ) is an international river marking the border of southeastern Bangladesh and northwestern Myanmar.
Geography
The Naf River's average depth is , and ma ...
Estuary
* Meghna River
The Meghna River ( bn, মেঘনা নদী) is one of the major rivers in Bangladesh, one of the three that form the Ganges Delta, the largest delta on earth, which fans out to the Bay of Bengal. A part of the Surma-Meghna River System, ...
Estuary
Europe
* The Gironde
* Golden Horn
The Golden Horn ( tr, Altın Boynuz or ''Haliç''; grc, Χρυσόκερας, ''Chrysókeras''; la, Sinus Ceratinus) is a major urban waterway and the primary inlet of the Bosphorus in Istanbul, Turkey. As a natural estuary that connects with t ...
* The Humber
The Humber is a large tidal estuary on the east coast of Northern England. It is formed at Trent Falls, Faxfleet, by the confluence of the tidal rivers Ouse and Trent. From there to the North Sea, it forms part of the boundary between t ...
* Severn Estuary
The Severn Estuary ( cy, Aber Hafren) is the estuary of the River Severn, flowing into the Bristol Channel between South West England and South Wales. Its high tidal range, approximately , means that it has been at the centre of discussions in t ...
* Shannon Estuary
* Thames Estuary
The Thames Estuary is where the River Thames meets the waters of the North Sea, in the south-east of Great Britain.
Limits
An estuary can be defined according to different criteria (e.g. tidal, geographical, navigational or in terms of salini ...
* The Wash
The Wash is a rectangular bay and multiple estuary at the north-west corner of East Anglia on the East coast of England, where Norfolk meets Lincolnshire and both border the North Sea. One of Britain's broadest estuaries, it is fed by the riv ...
* Unterelbe
The Unterelbe or, in English usually the Lower Elbe, refers to the lower reaches of the river Elbe in Germany influenced by the tides.
It starts at kilometre 586, at the sluice of Geesthacht, where the Elbe forms the border between Lower Sa ...
* Western Scheldt
The Western Scheldt ( nl, Westerschelde) in the province of Zeeland in the southwestern Netherlands, is the estuary of the Scheldt river. This river once had several estuaries, but the others are now disconnected from the Scheldt, leaving the W ...
* Tagus Estuary
North America
* Albemarle Sound
Albemarle Sound () is a large estuary on the coast of North Carolina in the United States located at the confluence of a group of rivers, including the Chowan and Roanoke. It is separated from the Atlantic Ocean by the Currituck Banks, a bar ...
including Outer Banks
The Outer Banks (frequently abbreviated OBX) are a string of barrier islands and spits off the coast of North Carolina and southeastern Virginia, on the east coast of the United States. They line most of the North Carolina coastline, separating ...
of North Carolina
North Carolina () is a state in the Southeastern region of the United States. The state is the 28th largest and 9th-most populous of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, Georgia and ...
* Chesapeake Bay
The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the Eastern Shore of Maryland / ...
including Hampton Roads
* Columbia River Estuary
The Columbia River Estuary is situated on the Oregon–Washington border and the Pacific Coast of the United States. It was traditionally inhabited by the Chinook Native Americans and discovered by settlers in 1788. The Estuary plays host t ...
* Delaware Bay
Delaware Bay is the estuary outlet of the Delaware River on the northeast seaboard of the United States. It is approximately in area, the bay's freshwater mixes for many miles with the saltwater of the Atlantic Ocean.
The bay is bordered inland ...
* Drake's Estero
* East River
The East River is a saltwater tidal estuary in New York City. The waterway, which is actually not a river despite its name, connects Upper New York Bay on its south end to Long Island Sound on its north end. It separates the borough of Quee ...
* Estuary of Saint Lawrence
* Fraser River
* Galveston Bay
* Great Bay
* Indian River Lagoon
* Laguna Madre
The Laguna Madre is a long, shallow, hypersaline lagoon along the western coast of the Gulf of Mexico in Nueces, Kenedy, Kleberg, Willacy and Cameron Counties in Texas, United States. It is one of seven major estuaries along the Gulf Coast ...
* Lake Borgne
Lake Borgne (french: Lac Borgne, es, Lago Borgne) is a lagoon of the Gulf of Mexico in southeastern Louisiana. Although early maps show it as a lake surrounded by land, coastal erosion has made it an arm of the Gulf of Mexico. Its name comes fro ...
* Lake Merritt
Lake Merritt is a large tidal lagoon in the center of Oakland, California, just east of Downtown. It is surrounded by parkland and city neighborhoods. It is historically significant as the United States' first official wildlife refuge, designate ...
* Long Island Sound
* Mississippi River Delta
** Lake Pontchartrain
Lake Pontchartrain ( ) is an estuary located in southeastern Louisiana in the United States. It covers an area of with an average depth of . Some shipping channels are kept deeper through dredging. It is roughly oval in shape, about from wes ...
* Mobile Bay
Mobile Bay ( ) is a shallow inlet of the Gulf of Mexico, lying within the state of Alabama in the United States. Its mouth is formed by the Fort Morgan Peninsula on the eastern side and Dauphin Island, a barrier island on the western side. The ...
* 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. Sm ...
* Newport Back Bay The Back Bay is the colloquial term for the inland delta in Newport Beach, California. It connects the Upper Newport Bay with the Newport Harbor. It is a nature reserve home to several species of birds with hiking and biking trails. The bay is re ...
* New York-New Jersey Harbor
* Coos Bay
Coos Bay is an estuary where the Coos River enters the Pacific Ocean, the estuary is approximately 12 miles long and up to two miles wide. It is the largest estuary completely within Oregon state lines. The Coos Bay watershed covers an area of abou ...
* Puget Sound
Puget Sound ( ) is a sound of the Pacific Northwest, an inlet of the Pacific Ocean, and part of the Salish Sea. It is located along the northwestern coast of the U.S. state of Washington. It is a complex estuarine system of interconnected ma ...
* Pamlico Sound
Pamlico Sound ( ) is a lagoon in North Carolina which is the largest lagoon along the North American East Coast, extending long and 15 to 20 miles (24 to 32 km) wide. It is part of a large, interconnected network of lagoon estuaries that i ...
including the Outer Banks
The Outer Banks (frequently abbreviated OBX) are a string of barrier islands and spits off the coast of North Carolina and southeastern Virginia, on the east coast of the United States. They line most of the North Carolina coastline, separating ...
of North Carolina
North Carolina () is a state in the Southeastern region of the United States. The state is the 28th largest and 9th-most populous of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, Georgia and ...
* San Francisco Bay
* Tampa Bay
Tampa Bay is a large natural harbor and shallow estuary connected to the Gulf of Mexico on the west-central coast of Florida, comprising Hillsborough Bay, McKay Bay, Old Tampa Bay, Middle Tampa Bay, and Lower Tampa Bay. The largest freshwater ...
Oceania
* Avon Heathcote Estuary
The Estuary of the Heathcote and Avon Rivers / Ihutai is the largest semi-enclosed shallow estuary in Canterbury and remains one of New Zealand’s most important coastal wetlands. It is well known as an internationally important habitat for migra ...
(Christchurch, New Zealand)
* Gippsland Lakes
The Gippsland Lakes are a network of coastal lakes, marshes and lagoons in East Gippsland, Victoria, Australia covering an overall area of about between the rural towns of Lakes Entrance, Bairnsdale and Sale. The largest of the lakes are L ...
* Port Jackson
Port Jackson, consisting of the waters of Sydney Harbour, Middle Harbour, North Harbour and the Lane Cove and Parramatta Rivers, is the ria or natural harbour of Sydney, New South Wales, Australia. The harbour is an inlet of the Tasman Sea ...
(Sydney Harbour)
* Spencer Gulf
South America
* Amazon River
* Iguape-Cananéia-Paranaguá estuary lagoon complex
The Iguape-Cananéia-Paranaguá estuary lagoon complex ( pt, Complexo Estuarino Lagunar de Iguape-Cananéia-Paranaguá) is a stretch of interconnected coastal estuaries and lagoons that extends southwest from Iguape in the south of São Paulo past ...
* Lagoa dos Patos
Lagoa dos Patos (, , ; English: ''Ducks' Lagoon'') is the largest lagoon in Brazil and the largest coastal lagoon in South America. It is located in the state of Rio Grande do Sul, southern Brazil. It covers an area of , is long and has a maximu ...
and Lagoon Mirim
* Rio de la Plata
Rio or Río is the Portuguese, Spanish, Italian, and Maltese word for "river". When spoken on its own, the word often means Rio de Janeiro, a major city in Brazil.
Rio or Río may also refer to:
Geography Brazil
* Rio de Janeiro
* Rio do Sul, a ...
See also
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References
External links
Animated documentary on Chesapeake Bay
NOAA.
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The Estuary Guide (Based on experience and R&D within the UK)
{{Authority control
Geodesy
Coastal geography
Environmental impact of fishing
Coastal and oceanic landforms
Bodies of water
Aquatic ecology