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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 sea or ocean, or as a line that forms the boundary between the land and the ocean The ocean (also the sea or the world ocean) is the body ...
body of
brackish water Brackish water, also sometimes termed brack water, is water occurring in a natural environment having more salinity File:IAPSO Standard Seawater.jpg, upInternational Association for the Physical Sciences of the Oceans (IAPSO) standard seawater. ...
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 and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. F ...

sediment
, making estuaries among the most productive natural habitats in the world. Most existing estuaries formed during the
Holocene The Holocene ( ) is the current Geologic time scale, geological epoch. It began approximately 11,650 radiocarbon calibration, cal years Before Present, before present, after the last glacial period, which concluded with the Holocene glacial re ...
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 Shale is a fine-grained, clastic sedimentary rock Sedimentary rocks are types of Rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at the Earth's surface ...

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 ...
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 Engli ...

harbor
s,
lagoon Garabogaz-Göl lagoon in Turkmenistan A lagoon is a shallow body of water separated from a larger body of water by reefs, barrier islands, or a barrier peninsula. Lagoons are commonly divided into coastal lagoons and atoll lagoons. They h ...

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 Physics (from grc, φυσική (ἐπιστήμη), phy ...

inlet
s, 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'' by the br ...
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 Social degeneration was a widely influential concept at the interface of the social and biological sciences in the 18th and 19th centuries. During the 18th century, scientific thinkers including George Louis Leclerc, comte de Buffon, Johann Fried ...
from a variety of factors including
soil erosion Soil erosion is the displacement of the upper layer of soil Soil (often stylized as SOiL) is an American rock band that was formed in Chicago (''City in a Garden''); I Will , image_map = , map_caption = Interactive ...

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 ...
,
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 from a body of water at a rate that the species cannot replenish, resulting in those species becoming underpopulated in that area. Overfishing can occur in water bodies of any sizes, such as ponds, r ...
and the filling of wetlands.
Eutrophication Eutrophication (from Greek ''eutrophos'', "well-nourished") is a limnological term for the process by which a body of water becomes progressively enriched with mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth" ...

Eutrophication
may lead to excessive nutrients from sewage and animal wastes; pollutants including
heavy metals Heavy metals are generally defined as metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and con ...
,
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 an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by ...
s and
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's a ...
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, Further reading). Flood relief methods are used to reduce the effects of flood waters or ...
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 File:IAPSO Standard Seawater.jpg, upInternational Association for the Physical Sciences of the Oceans (IAPSO) standard seawater. ...
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 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 process ...

fjord
s,
lagoon Garabogaz-Göl lagoon in Turkmenistan A lagoon is a shallow body of water separated from a larger body of water by reefs, barrier islands, or a barrier peninsula. Lagoons are commonly divided into coastal lagoons and atoll lagoons. They h ...

lagoon
s,
river mouth A river mouth is the part of 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 ground and becomes dry at the end of its course wi ...

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 ...
having a connection to the open sea through which the
sea water 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, 599 mM). This means that every kilogram (roughly one liter by volume) of seawater has approxim ...
enters with the rhythm of the
tide (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 level ...

tide
s. The seawater entering the estuary is diluted by the
fresh water Fresh water (or freshwater) is any naturally occurring water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is ...

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 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 shortha ...

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 t ...

Hudson River
,
Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the E ...
, 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 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 inf ...
along the
Gulf Coast The Gulf Coast of the United States is the coastline along the Southern United States The southern United States, also known as the American South, the southern states, or simply the South, is a geographic and cultural List of regions of th ...

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 fe ...
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 rose by . More precise data gathered from satellite radar measurements reveal an a ...

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 sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment ...

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 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 process ...

Fjord
-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#Glaciology, ablation over many years, often Century, centuries. Glaciers slowl ...

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), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice she ...

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; Yup'ik The Yup'ik or Yupiaq (sg & pl) and Yupiit or Yupiat (pl), also Central Alaskan Yup'ik, Central Yup'ik, Alaskan Yup'ik ( own name ''Yup'ik'' sg ''Yupiik'' dual ''Yupiit'' pl; rus ...
, the
Puget Sound Puget Sound () is a sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physic ...
region of western
Washington state Washington (), officially the State of Washington, is a state in the Pacific Northwest The Pacific Northwest (PNW), sometimes referred to as Cascadia, is a geographic region in western North America North America is a contine ...
,
British Columbia ( en, Splendour without diminishment) , image_map = British Columbia in Canada 2.svg , Label_map = yes , coordinates = , official_lang = None , Slogan = Beautiful British Co ...
, eastern Canada,
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an Autonomous administrative division, autonomous territory* * * within the Danish Realm and the List of islands by area, world's largest island, located between the Arctic Ocean, Arcti ...

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#No ...

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 ar ...

volcano
es, and
landslide The term landslide or, less frequently, landslip refers to several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a vari ...

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 Geologic time scale, geological epoch. It began approximately 11,650 radiocarbon calibration, cal years Before Present, before present, after the last glacial period, which concluded with the Holocene glacial re ...
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 in the US state of California. It is surrounded by a contiguous region known as the San Francisco Bay Area (often simply "the Bay Area"), and is dominated by the large cities of San Francisco, San Jose, ...

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 List of longest rivers of the United States (by main stem), second-longest river and chief river of the second-largest Drainage system (geomorphology), drainage system on the North American continent, second only to ...

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 estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the E ...
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, the gradient of a scalar-valued function, scalar-valued differentiable function of Function of several variables, several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the Vec ...

gradient
. 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. 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 central New Jersey in the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily located ...
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 ...
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 File:IAPSO Standard Seawater.jpg, upInternational Association for the Physical Sciences of the Oceans (IAPSO) standard seawater. Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (se ...

salinity
and
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. F ...

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 photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cel ...
, 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 File:IAPSO Standard Seawater.jpg, upInternational Association for the Physical Sciences of the Oceans (IAPSO) standard seawater. Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (se ...

salinity
and
sedimentation Sedimentation is the tendency for particle (ecology), particles in Suspension (chemistry), suspension to settle out of the fluid in which they are Entrainment (engineering), entrained and come to rest against a barrier. This is due to their motio ...
. Many 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, and Chondrichthyes, cartilaginous and bony fish as we ...

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 subphylum vertebrate, Vertebrata. Familiar example ...
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 construct a space suitable for habitation or temporary refuge, or as a byproduct of Animal locomotion, locomotion. Burrows pro ...
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 (self-feeding) components of the plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a current (or wi ...

Phytoplankton
are key primary producers in estuaries. They move with the water bodies and can be flushed in and out with the
tide (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 level ...

tide
s. Their productivity is largely dependent upon the
turbidity Turbidity is the cloudiness or haze, haziness of a fluid caused by large numbers of individual Particle (ecology), particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of wat ...

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, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms mak ...

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, ...
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 a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a Bacte ...
, 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 ...
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 chemical substances or energy, such as noise, heat, or light. Pollutants, the components of pollution, can be ...

pollution
and
overfishing Overfishing is the removal of a species of fish from a body of water at a rate that the species cannot replenish, resulting in those species becoming underpopulated in that area. Overfishing can occur in water bodies of any sizes, such as ponds, r ...
. 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 sy ...

plastics
,
pesticide Pesticides are substances that are meant to control pests. The term pesticide includes all of the following: herbicide, insecticides (which may include insect growth regulators, termiticides, etc.) nematicide, molluscicide, piscicide, a ...
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 organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to ...

phenol
s and
heavy metals Heavy metals are generally defined as metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and con ...
. 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 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 of the Earth.
environments, such as estuaries and bay muds: a geological record of human activities of the last century. The elemental composition of
biofilm A biofilm comprises any syntrophic consortium of microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any indiv ...

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 The Amur (russian: река́ Аму́р, ), or Heilong Jiang (, "Black Dragon A dragon is a large, snake, serpentine, legendary creature that appears in the folklore of many cultures worldwide. Beliefs about dragons vary considerably th ...

Amur River
and damaged its estuary soil. Estuaries tend to be naturally
eutrophic Eutrophication (from Greek ''eutrophos'', "well-nourished") is a limnological term for the process by which a body of water ( Lysefjord) in Norway Norway ( nb, ; nn, ; se, Norga; smj, Vuodna; sma, Nöörje), officially the Ki ...
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 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 (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the E ...
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 ...

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 The Orange River (from Afrikaans/Dutch language, Dutch: ''Oranjerivier'') is a river in Southern Africa. It is the longest river within the borders of South Africa and the Orange River Basin extends extensively from Lesotho into South Africa a ...
Estuary * Lake St Lucia Estuary


Asia

*
Gulf of Ob The Gulf of Ob (also known as ''Ob Bay'') (russian: О́бская губа́, ''Obskaya guba'') is a bay of the Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximate ...

Gulf of Ob
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 Kara Sea (russian: Ка́рское мо́ре, ''Karskoye more'') is part ...
Estuary * * Hàn River Estuary * Kraburi River Estuary * Waeru River Estuary of
Chanthaburi Province Chanthaburi ( th, จันทบุรี, ; Chong language, Chong: จันกะบูย, ''chankabui'',องค์ บรรจุน. ''สยามหลากเผ่าหลายพันธุ์.'' กรุงเทพฯ: มต ...
* Dawei River Estuary *
Naf River The Naf River ( bn, নাফ নদী ''Naf Nodi'' ; my, နတ်မြစ် ; rki, ) is an international river marking the border Borders are geographic boundaries of political entities or legal jurisdiction Jurisdiction (from La ...
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, Me ...

Meghna River
Estuary


Europe

* The
Gironde Gironde ( US usually, , ; oc, Gironda, ) is a largest department Department may refer to: * Departmentalization, division of a larger organization into parts with specific responsibility Government and military *Department (country subdivis ...

Gironde
*
Golden Horn 300px, The Golden Horn as seen from Galata Bridge 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 F ...

Golden Horn
* The
Humber The Humber is a large tidal estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone betw ...
* Severn Estuary *
Shannon Estuary The Shannon Estuary ( gle, Inbhear na Sionainne) is a large estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form ...
*
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 Western This limit of the estuary has been defined in two main ways: * The narrow estuary is strongly tidal and is know ...
*
The Wash The Wash is a rectangular bay and estuary at the north-west corner of East Anglia East Anglia is a geographical area in the East of England. The area included has varied but the legally defined NUTS 2 statistical unit comprises the coun ...

The Wash
*
Unterelbe The Unterelbe or, in English usually the Lower Elbe, refers to the lower reaches of the river Elbe , german: Elbe, Low German: ''Ilv'' or ''Elv'' , name_etymology = , image = Labe_udoli.jpg , image_size = , ...

Unterelbe
*
Western Scheldt The Western Scheldt ( nl, Westerschelde) in the province of Zeeland in the southwestern Netherlands The Netherlands ( nl, Nederland ), informally referred to as Holland, is a country primarily located in Western Europe and partly in the Dut ...
* Tagus Estuary


North America

*
Albemarle Sound Albemarle Sound () is a large estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between ...

Albemarle Sound
including
Outer Banks The Outer Banks (frequently abbreviated OBX) are a string of barrier islands and Spit (landform), 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 coastl ...
of
North Carolina North Carolina () is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. North Carolina is the List of U.S. states and territories by area, 28th largest and List of states and territories of the Un ...

North Carolina
*
Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the E ...
including
Hampton Roads Hampton Roads is the name of both a body of water that serves as a wide channel for the James River (Virginia), James, Nansemond River, Nansemond and Elizabeth River (Virginia), Elizabeth rivers between Old Point Comfort and Sewell's Point where ...
*
Columbia River Estuary The Columbia River Estuary is situated on the Oregon–Washington (state), Washington border and the West Coast of the United States, Pacific Coast of the United States. It was traditionally inhabited by the Chinookan peoples, Chinook Native Am ...
*
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 ...
* Drake's Estero *
East River The East River is a salt water Estuary, 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 o ...
*
Estuary of Saint Lawrence The estuary of the Saint Lawrence in Quebec ) , image_map = Quebec in Canada 2.svg , Label_map = yes , coordinates = , official_lang = Quebec French, French , capital = ...
*
Fraser River The Fraser River is the longest river within British Columbia, Canada, rising at Fraser Pass near Blackrock Mountain (Canada), Blackrock Mountain in the Rocky Mountains and flowing for , into the Strait of Georgia at the city of Vancouver. It ...

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 *
Indian River Lagoon The Indian River Lagoon is a grouping of three lagoons: the Mosquito Lagoon, the Banana River, and the Indian River (Florida), Indian River, on the Atlantic Coast of Florida; one of the most biodiverse estuaries in the Northern Hemisphere and is h ...

Indian River Lagoon
* *
Lake Borgne Lake Borgne (french: Lac 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 from the French languag ...
*
Lake Merritt Lake Merritt is a large tidal lagoon in the center of Oakland, California Oakland is the largest city and the county seat A county seat is an administrative center, seat of government, or capital city of a county or Parish (administrative divi ...
*
Lake Pontchartrain near Mandeville, Louisiana, in 2004 Lake Pontchartrain ( ; french: Lac Pontchartrain, ; es, Lago Pontchartrain) is a brackish water, brackish estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers ...

Lake Pontchartrain
*
Long Island Sound Long Island Sound is a tidal estuary of the Atlantic Ocean, lying predominantly between the U.S. state of Connecticut Connecticut () is the southernmost state in the New England region of the United States. As of the 2010 Census, it has the h ...

Long Island Sound
*
Mobile Bay photo with Mobile Bay in the center 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 (Alabama), Fort Morgan Peninsula on the eastern side and D ...
*
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 * New York-New Jersey Harbor *
Coos Bay Coos Bay is an S-shaped 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 Physics (from grc, φυσ ...

Coos Bay
*
Puget Sound Puget Sound () is a sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physic ...
*
Pamlico Sound Pamlico Sound ( ) in North Carolina in the US is the largest lagoon along the North American East Coast of the United States, East Coast, extending long and 15 to 20 miles (24 to 32 km) wide. It is part of a large, interconnected network of ...
including the
Outer Banks The Outer Banks (frequently abbreviated OBX) are a string of barrier islands and Spit (landform), 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 coastl ...
of
North Carolina North Carolina () is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. North Carolina is the List of U.S. states and territories by area, 28th largest and List of states and territories of the Un ...

North Carolina
*
San Francisco Bay San Francisco Bay is a shallow estuary in the US state of California. It is surrounded by a contiguous region known as the San Francisco Bay Area (often simply "the Bay Area"), and is dominated by the large cities of San Francisco, San Jose, ...

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 inf ...


Oceania

*
Gippsland Lakes The Gippsland Lakes are a network of lakes, marshes and lagoons in east Gippsland, Victoria (Australia), Victoria, Australia covering an area of about . The largest of the lakes are Lake Wellington (Gunai language: ''Murla''), Lake King (Gunai ...

Gippsland Lakes
*
Port Jackson Port Jackson, consisting of the waters of Sydney Harbour, Middle Harbour Middle Harbour (or ''Warring-Ga''), a semi–mature tide (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple ...

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 The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the List of rivers by discharge, largest river by Discharge (hydrology), discharge volume of water in the world, and the disputed List of rivers by length, longest ...
* Iguape-Cananéia-Paranaguá estuary lagoon complex *
Lagoa dos Patos Lagoa dos Patos (, , ; English: ''Ducks' Lagoon'') is the largest lagoon Garabogaz-Göl lagoon in Turkmenistan A lagoon is a shallow body of water separated from a larger body of water by reefs, barrier islands, or a barrier peninsula. La ...

Lagoa dos Patos
and
Lagoon Mirim Image:LagoaMirim.jpg, 200px, Fishermen at Lagoon Mirim Lagoon Mirim (Portuguese, ) or Merín (Spanish, ) is a large estuary, estuarine lagoon which extends from southern Rio Grande do Sul state in Brazil into eastern Uruguay. Lagoa Mirim is separate ...
*
Rio de la Plata Rio or Río is the Portuguese, Spanish, Italian, and Maltese word for "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 ...

Rio de la Plata


See also

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


References


External links


Animated documentary on Chesapeake Bay
NOAA The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency within the United States Department of Commerce The United States Department of Commerce is an executive department The United ...

NOAA
. *
The Estuary Guide (Based on experience and R&D within the UK)
{{Authority control Geodesy Coastal geography Fishing and the environment Coastal and oceanic landforms
Bodies of water {{See also, Limnology Compare to :Landforms and :Wetlands Bodies of water may exist within land areas or within other bodies of water and may be natural, man-made or a combination. Water Physical geography Hydrography Natural resources, +Water A ...
Aquatic ecology