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"Brackish" redirects here. For the Kittie song, see Brackish (song). Brackish water (less commonly brack water) is water that is saltier than fresh water, but not as salty as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur as in brackish fossil aquifers. Certain human activities can produce brackish water, in particular certain civil engineering projects such as dikes and the flooding of coastal marshland to produce brackish water pools for freshwater prawn farming. Brackish water is also the primary waste product of the blue energy process. Because brackish water is hostile to the growth of most terrestrial plant species, without appropriate management it is very damaging to the environment (see article on shrimp farms). Technically, brackish water contains between 0.5 and 30 grams of salt per litre—more often expressed as 0.5 to 30 parts per thousand (ppt or ‰). 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 and/or time.
Etymology The term brackish water derives from the Low German word Brack, which is a small lake created when a storm tide breaks a dike and floods land behind the dike. Estuaries The most important brackish water habitats are estuaries, where a river meet the sea. The River Thames flowing through London is one of the most familiar of all river estuaries. The town of Teddington a few miles west of London marks the limit of the tidal part of the Thames, although it is still a freshwater river about as far east as Battersea. In this section, the fish fauna consists predominantly of freshwater species such as roach, dace, carp, perch, and pike. The Thames becomes brackish between Battersea and Gravesend, and the fish fauna comprises a limited selection of freshwater species, particularly roach and dace, as well as euryhaline marine species such as flounder, European seabass, mullet, and smelt. Further east, the salinity increases and the freshwater fish species are completely replaced by 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. 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 is typical of river estuaries. River estuaries form important staging points during the migration of anadromous and catadromus fish species, such as salmon and 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 and plaice are two commercially important species that use the Thames Estuary for this purpose. Estuaries are also used as fishing grounds and as places for fish farming or ranching. Atlantic salmon farms are often located in estuaries, for example, though 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 or mangal. Many, though not all, mangrove swamps fringe estuaries and lagoon 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, magrove swamps are extremely important breeding grounds for many fish, with species such as snappers, halfbeaks, and tarpon spawning or maturing among them. Besides fish, numerous other animals use mangroves, including such specialists as the American crocodile, proboscis monkey, diamondback terrapin, and the crab-eating frog, Rana cancrivora. Although often plagued with mosquitoes and other insects that make them unpleasant places to visit, mangrove swamps are very important buffer zones between land and sea, and are a natural defense against hurricane and tsunami damage in particular *. Brackish Seas & Lakes Some seas and lakes are brackish. The Baltic Sea is a brackish sea adjoining the North Sea. Originally the confluence of two major river systems prior to the Pleistocene, since that it has been flooded by the North Sea but still receives so much freshwater from the adjacent lands that the water is brackish. Because the salt water coming in from the sea is more dense than freshwater, the water in the Baltic is stratified, with salt water 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 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 caviare. Notable brackish bodies of water Brackish seas Brackish water aquaria
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