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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 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 ...
. It may result from mixing seawater (salt water) and fresh water together, as in estuaries, or it may occur in brackish fossil aquifers. 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 marshland to produce brackish water pools for
freshwater prawn farming A freshwater prawn farm is an aquaculture business designed to raise and produce freshwater prawns or shrimp for human consumption. Freshwater prawn farming shares many characteristics with, and many of the same problems as, marine shrimp farmin ...
. 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 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, where a river meets the sea. The
River Thames The River Thames ( ), known alternatively in parts as the River Isis, is a river that flows through southern England including London. At , it is the longest river entirely in England and the second-longest in the United Kingdom, after the R ...
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 Teddington is a suburb in south-west London in the London Borough of Richmond upon Thames. In 2021, Teddington was named as the best place to live in London by ''The Sunday Times''. Historically in Middlesex, Teddington is situated on a long me ...
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 insofar as the average salinity is very low and the fish fauna consists predominantly of freshwater species such as roach,
dace A dace is a small fish that can be one of many different species. The unmodified name is usually a reference to the common dace (''Leuciscus leuciscus''). This, like most fish called "daces", belongs to the family Cyprinidae, mostly in subfamily ...
, carp, perch, and pike. The Thames Estuary becomes brackish between Battersea and Gravesend, and the diversity of freshwater fish species present is smaller, primarily roach and dace; euryhaline marine species such as flounder, European seabass, mullet, and smelt 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, epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the ...
and includes both euryhaline and stenohaline 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 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 syste ...
is typical of river estuaries. River estuaries form important staging points during the migration of anadromous and catadromous fish species, such as salmon, shad and
eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...
s, 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 and
plaice Plaice is a common name for a group of flatfish that comprises four species: the European, American, Alaskan and scale-eye plaice. Commercially, the most important plaice is the European. The principal commercial flatfish in Europe, it is al ...
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 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 Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
such as
sea lice Sea lice (singular: sea louse) are copepods (small crustaceans) of the family Caligidae within the order Siphonostomatoida. They are marine ectoparasites (external parasites) that feed on the mucus, epidermal tissue, and blood of host fish. The ...
that escape from the pens the farmed fish are kept in.


Mangroves

Another important brackish water habitat is the mangrove swamp 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 The archerfish (spinner fish or archer fish) form a monotypic family, Toxotidae, of fish known for their habit of preying on land-based insects and other small animals by shooting them down with water droplets from their specialized mouths. The ...
, 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, halfbeaks, and
tarpon Tarpons are fish of the genus ''Megalops''. They are the only members of the family Megalopidae. Of the two species, one (''M. atlanticus'') is native to the Atlantic, and the other (''M. cyprinoides'') to the Indo-Pacific Oceans. Species a ...
spawning or maturing among them. Besides fish, numerous other animals use mangroves, including such species as the saltwater crocodile,
American crocodile The American crocodile (''Crocodylus acutus'') is a species of crocodilian found in the Neotropics. It is the most widespread of the four extant species of crocodiles from the Americas, with populations present from South Florida and the coasts ...
, proboscis monkey,
diamondback terrapin The diamondback terrapin or simply terrapin (''Malaclemys terrapin'') is a species of turtle native to the brackish coastal tidal marshes of the Northeastern and southern United States, and in Bermuda. It belongs to the monotypic genus ''Mal ...
, and the
crab-eating frog The crab-eating frog (''Fejervarya cancrivora'') is a frog native to south-eastern Asia including Taiwan, China, the Philippines and more rarely as far west as Orissa in India. It has also been introduced to Guam, and was most likely introduced f ...
, ''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
mosquito Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small flies within the family Culicidae (from the Latin ''culex'' meaning " gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish for "li ...
es 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 Sundarbans (pronounced ) is a mangrove area in the delta formed by the confluence of the Padma, Brahmaputra and Meghna Rivers in the Bay of Bengal. It spans the area from the Baleswar River in Bangladesh's division of Khulna to the Hooghly R ...
and Bhitarkanika Mangroves are two of the large mangrove forests in the world, both on the coast of the Bay of Bengal.


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 ...
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, epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the ...
. Originally the Eridanos river system prior to the
Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological 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 finally confirmed in ...
, 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 appr ...
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 Asia ...
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) and the great sturgeons, a major source of
caviar Caviar (also known as caviare; from fa, خاویار, khâvyâr, egg-bearing) is a food consisting of salt-cured roe of the family Acipenseridae. Caviar is considered a delicacy and is eaten as a garnish or a spread. Traditionally, the te ...
. Hudson Bay is a brackish
marginal sea This is a list of seas of the World Ocean, including marginal seas, areas of water, various gulfs, bights, bays, and straits. Terminology * Ocean – the four to seven largest named bodies of water in the World Ocean, all of which have "Ocea ...
of the
Arctic ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
, it remains brackish due its limited connections to the open ocean, very high levels freshwater surface runoff input from the large
Hudson Bay drainage basin The Hudson Bay drainage basin is the drainage basin in northern North America where surface water empties into Hudson Bay and adjoining waters. Spanning an area of about , the basin is almost totally in Canada (spanning parts of the Prairies, cen ...
, 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 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 Europe, Western and Southern Europe and Anatolia, on the south by North Africa ...
. 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, 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, 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, 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,
electrodialysis Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis ...
, and other filtration processes. It is also being used in different cosmetics like hair tonic.


See also

* List of brackish bodies of water * * * *


References


Further reading


Moustakas, A. & I. Karakassis. How diverse is aquatic biodiversity research?, Aquatic Ecology, 39, 367-375
{{Authority control Liquid water Aquatic ecology Coastal geography *