Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more
salinity
Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
than
freshwater
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...
, but not as much as
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
. It may result from mixing seawater (salt water) and fresh water together, as in
estuaries
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environme ...
, or it may occur in brackish fossil
aquifer
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characte ...
s. The word comes from the Middle Dutch root '' brak''. Certain human activities can produce brackish water, in particular
civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads, bridges, canals, dams, airports, sewa ...
projects such as dikes and the flooding of coastal
marsh
A marsh is a wetland that is dominated by herbaceous rather than woody plant species.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p Marshes can often be found ...
land to produce brackish water pools for freshwater prawn farming. Brackish water is also the primary waste product of the salinity gradient power process. Because brackish water is hostile to the growth of most terrestrial plant species, without appropriate management it is damaging to the environment (see article on shrimp farms).
Technically, brackish water contains between 0.5 and 30 grams of
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
per litre—more often expressed as 0.5 to 30 parts per thousand (‰), which is a
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its dens ...
of between 1.0004 and 1.0226. Thus, ''brackish'' covers a range of salinity regimes and is not considered a precisely defined condition. It is characteristic of many brackish surface waters that their salinity can vary considerably over space or time. Water with a salt concentration greater than 30‰ is considered saline. See the Salinity Table from the Wikipedia Salinity Article.
Brackish water habitats
Estuaries
Brackish water condition commonly occurs when fresh water meets seawater. In fact, the most extensive brackish water habitats worldwide are
estuaries
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environme ...
, where a river meets the sea.
The
River Thames
The River Thames ( ), known alternatively in parts as the The Isis, River Isis, is a river that flows through southern England including London. At , it is the longest river entirely in England and the Longest rivers of the United Kingdom, se ...
flowing through
London
London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
is a classic river estuary. The town of Teddington a few miles west of London marks the boundary between the tidal and non-tidal parts of the Thames, although it is still considered a freshwater river about as far east as
Battersea
Battersea is a large district in south London, part of the London Borough of Wandsworth, England. It is centred southwest of Charing Cross and extends along the south bank of the River Thames. It includes the Battersea Park.
History
Batt ...
insofar as the average salinity is very low and the fish fauna consists predominantly of freshwater species such as
roach
Roach may refer to:
Animals
* Cockroach, various insect species of the order Blattodea
* Common roach (''Rutilus rutilus''), a fresh and brackish water fish of the family Cyprinidae
** ''Rutilus'' or roaches, a genus of fishes
* California roa ...
carp
Carp are various species of oily freshwater fish from the family Cyprinidae, a very large group of fish native to Europe and Asia. While carp is consumed in many parts of the world, they are generally considered an invasive species in parts of ...
,
perch
Perch is a common name for fish of the genus ''Perca'', freshwater gamefish belonging to the family Percidae. The perch, of which three species occur in different geographical areas, lend their name to a large order of vertebrates: the Per ...
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 ...
becomes brackish between Battersea and
Gravesend
Gravesend is a town in northwest Kent, England, situated 21 miles (35 km) east-southeast of Charing Cross (central London) on the south bank of the River Thames and opposite Tilbury in Essex. Located in the diocese of Rochester, it is ...
, and the diversity of freshwater fish species present is smaller, primarily roach and dace;
euryhaline
Euryhaline organisms are able to adapt to a wide range of salinities. An example of a euryhaline fish is the molly (''Poecilia sphenops'') which can live in fresh water, brackish water, or salt water.
The green crab ('' Carcinus maenas'') is an ...
marine species such as
flounder
Flounders are a group of flatfish species. They are demersal fish, found at the bottom of oceans around the world; some species will also enter estuaries.
Taxonomy
The name "flounder" is used for several only distantly related species, thou ...
,
European seabass
The European bass (''Dicentrarchus labrax'') is a primarily ocean-going fish native to the waters off Europe's western and southern and Africa's northern coasts, though it can also be found in shallow coastal waters and river mouths during the s ...
smelt
Smelt may refer to:
* Smelting, chemical process
* The common name of various fish:
** Smelt (fish), a family of small fish, Osmeridae
** Australian smelt in the family Retropinnidae and species ''Retropinna semoni''
** Big-scale sand smelt ''A ...
become much more common. Further east, the salinity increases and the freshwater fish species are completely replaced by euryhaline marine ones, until the river reaches Gravesend, at which point conditions become fully marine and the fish fauna resembles that of the adjacent
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian ...
and includes both euryhaline and
stenohaline
Stenohaline describes an organism, usually fish, that cannot tolerate a wide fluctuation in the salinity of water. Stenohaline is derived from the words: "''steno''" meaning narrow, and "''haline''" meaning salt. Many fresh water fish, such as g ...
marine species. A similar pattern of replacement can be observed with the aquatic plants and invertebrates living in the river.
This type of
ecological succession
Ecological succession is the process of change in the species structure of an ecological community over time. The time scale can be decades (for example, after a wildfire) or more or less.
Bacteria allows for the cycling of nutrients such as car ...
from a freshwater to marine
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 syst ...
is typical of river estuaries. River estuaries form important staging points during the migration of anadromous and catadromous fish species, such as
salmon
Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus '' Salmo'') and North Pacific (genus '' Onco ...
,
shad
The Alosinae, or the shads,Alosinae eels, giving them time to form social groups and to adjust to the changes in salinity. Salmon are anadromous, meaning they live in the sea but ascend rivers to spawn; eels are catadromous, living in rivers and streams, but returning to the sea to breed. Besides the species that migrate through estuaries, there are many other fish that use them as "nursery grounds" for spawning or as places young fish can feed and grow before moving elsewhere.
Herring
Herring are forage fish, mostly belonging to the family of Clupeidae.
Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Ocean ...
and plaice are two commercially important species that use the Thames Estuary for this purpose.
Estuaries are also commonly used as fishing grounds, and as places for fish farming or ranching. For example,
Atlantic salmon
The Atlantic salmon (''Salmo salar'') is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlantic salmon are ...
farms are often located in estuaries, although this has caused controversy, because in doing so, fish farmers expose migrating wild fish to large numbers of external parasites such as sea lice that escape from the pens the farmed fish are kept in.
Mangroves
Another important brackish water habitat is the
mangrove swamp
Mangrove forests, also called mangrove swamps, mangrove thickets or mangals, are productive wetlands that occur in coastal intertidal zones. Mangrove forests grow mainly at tropical and subtropical latitudes because mangroves cannot withstand fre ...
or mangal. Many, though not all, mangrove swamps fringe estuaries and lagoons where the salinity changes with each tide. Among the most specialised residents of mangrove forests are mudskippers, fish that forage for food on land, and archer fish, perch-like fish that "spit" at insects and other small animals living in the trees, knocking them into the water where they can be eaten. Like estuaries, mangrove swamps are extremely important breeding grounds for many fish, with species such as snappers,
halfbeak
Hemiramphidae is a family of fishes that are commonly called halfbeaks, spipe fish or spipefish. They are a geographically widespread and numerically abundant family of epipelagic fish inhabiting warm waters around the world. The halfbeaks ar ...
s, and tarpon spawning or maturing among them. Besides fish, numerous other animals use mangroves, including such species as the
saltwater crocodile
The saltwater crocodile (''Crocodylus porosus'') is a crocodilian native to saltwater habitats and brackish wetlands from India's east coast across Southeast Asia and the Sundaic region to northern Australia and Micronesia. It has been l ...
proboscis monkey
The proboscis monkey (''Nasalis larvatus'') or long-nosed monkey is an arboreal Old World monkey with an unusually large nose, a reddish-brown skin color and a long tail. It is endemic to the southeast Asian island of Borneo and is found mostl ...
, diamondback terrapin, and the crab-eating frog, ''Fejervarya cancrivora'' (formerly ''Rana cancrivora''). Mangroves represent important nesting sites for numerous birds groups such as herons, storks, spoonbills, ibises, kingfishers, shorebirds and seabirds.
Although often plagued with mosquitoes and other insects that make them unpleasant for humans, mangrove swamps are very important buffer zones between land and sea, and are a natural defense against hurricane and tsunami damage in particular.
The Sundarbans and
Bhitarkanika Mangroves
Bhitarkanika Mangroves is a mangrove wetland in Odisha, India, covering an area of in the Brahmani River and Baitarani River deltas.
History
The Bhitarkanika Mangroves were zamindari forests until 1952, when the government of Odisha abolishe ...
are two of the large mangrove forests in the world, both on the coast of the
Bay of Bengal
The Bay of Bengal is the northeastern part of the Indian Ocean, bounded on the west and northwest by India, on the north by Bangladesh, and on the east by Myanmar and the Andaman and Nicobar Islands of India. Its southern limit is a line bet ...
.
Brackish seas and lakes
Some seas and lakes are brackish. The
Baltic Sea
The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain.
The sea stretches from 53°N to 66°N latitude and from ...
is a brackish sea adjoining the
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian ...
. Originally the Eridanos river system prior to the
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
, since then it has been flooded by the North Sea but still receives so much freshwater from the adjacent lands that the water is brackish. As
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
is denser, the water in the Baltic is stratified, with seawater at the bottom and freshwater at the top. Limited mixing occurs because of the lack of tides and storms, with the result that the fish fauna at the surface is freshwater in composition while that lower down is more marine. Cod are an example of a species only found in deep water in the Baltic, while pike are confined to the less saline surface waters.
The
Caspian Sea
The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central A ...
is the world's largest lake and contains brackish water with a salinity about one-third that of normal seawater. The Caspian is famous for its peculiar animal fauna, including one of the few non-marine seals (the
Caspian seal
The Caspian seal (''Pusa caspica'', syn. ''Phoca caspica'') is one of the smallest members of the earless seal family and unique in that it is found exclusively in the brackish Caspian Sea. It lives along the shorelines, but also on the many roc ...
) and the great
sturgeon
Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the Late Cretaceous, and are descended from other, earlier acipenseriform fish, which date back to the Early ...
Arctic ocean
The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
, it remains brackish due its limited connections to the open ocean, very high levels freshwater
surface 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 t ...
input from the large Hudson Bay drainage basin, and low rate of evaporation due to being completely covered in ice for over half the year.
In the
Black Sea
The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...
the surface water is brackish with an average salinity of about 17-18 parts per thousand compared to 30 to 40 for the oceans. The deep,
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 ...
water of the Black Sea originates from warm, salty water of the
Mediterranean
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on ...
.
Lake Texoma, a reservoir on the border between the U.S. states of
Texas
Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...
and
Oklahoma
Oklahoma (; Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a state in the South Central region of the United States, bordered by Texas on the south and west, Kansas on the north, Missouri on the northeast, Arkansas on the east, New ...
, is a rare example of a brackish lake that is neither part of an
endorheic basin
An endorheic basin (; also spelled endoreic basin or endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water, such as rivers or oceans, but drainage converges instead into lakes ...
nor a direct arm of the ocean, though its salinity is considerably lower than that of the other bodies of water mentioned here. The reservoir was created by the damming of the
Red River of the South
The Red River, or sometimes the Red River of the South, is a major river in the Southern United States. It was named for its reddish water color from passing through red-bed country in its watershed. It is one of several rivers with that name. ...
, which (along with several of its tributaries) receives large amounts of salt from natural seepage from buried deposits in the upstream region. The salinity is high enough that
striped bass
The striped bass (''Morone saxatilis''), also called the Atlantic striped bass, striper, linesider, rock, or rockfish, is an anadromous perciform fish of the family Moronidae found primarily along the Atlantic coast of North America. It has ...
, a fish normally found only in salt water, has self-sustaining populations in the lake.
Brackish marsh
Other brackish bodies of water
*
Human uses
Brackish water is being used by humans in many different sectors. It is commonly used as cooling water for power generation and in a variety of ways in the mining, oil, and gas industries. Once desalinated it can also be used for agriculture, livestock, and municipal uses. Brackish water can be treated using
reverse osmosis
Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic ...
, electrodialysis, and other filtration processes. It is also being used in different cosmetics like hair tonic.
See also
*
List of brackish bodies of water
Bodies of water of brackish nature are found around the world in a wide variety of settings, shapes and sizes. The following is a list of notable bodies of brackish water.
Brackish seas
* Baltic Sea (the world’s largest inland brackish sea)
...