Brackish water, sometimes termedbrack water,[1][2] is water occurring in a natural environment that has moresalinity thanfreshwater, but not as much asseawater. It may result from mixing seawater (salt water) and fresh water together, as inestuaries, or it may occur in brackishfossil aquifers. The word comes from the Middle Dutch rootbrak. Certain human activities can produce brackish water, in particularcivil engineering projects such as dikes and the flooding of coastalmarshland to produce brackish water pools forfreshwater prawn farming. Brackish water is also the primary waste product of thesalinity gradient power process. Because brackish water is hostile to the growth of most terrestrial plant species, without appropriate management it can be damaging to the environment (see article onshrimp farms).
Technically, brackish water contains between 0.5 and 30 grams ofsalt per litre—more often expressed as 0.5 to 30parts per thousand (‰), which is aspecific gravity of between 1.0004 and 1.0226. Thus,brackish covers a range ofsalinity regimes and is not considered a precisely defined condition. It is characteristic of many brackish surface waters that theirsalinity can vary considerably over space or time. Water with asalt concentration greater than 30‰ is consideredsaline.
Brackish water condition commonly occurs when fresh water meets seawater. In fact, the most extensive brackish water habitats worldwide areestuaries, where a river meets the sea.
TheRiver Thames flowing throughLondon is a classic river estuary. The town ofTeddington a few miles west of London marks the boundary between thetidal and non-tidal parts of the Thames, although it is still considered a freshwater river about as far east asBattersea insofar as the average salinity is very low and the fish fauna consists predominantly of freshwater species such asroach,dace,carp,perch, andpike. TheThames Estuary becomes brackish between Battersea andGravesend, and the diversity of freshwater fish species present is smaller, primarily roach and dace;euryhaline marine species such asflounder,European seabass,mullet, andsmelt 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 adjacentNorth Sea and includes both euryhaline andstenohaline marine species. A similar pattern of replacement can be observed with the aquatic plants and invertebrates living in the river.[3][4]
This type ofecological succession from freshwater to marineecosystem is typical of river estuaries. River estuaries form important staging points during the migration ofanadromous and catadromous fish species, such assalmon,shad andeels, 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 andplaice are two commercially important species that use the Thames Estuary for this purpose.
Estuaries are also commonly used asfishing grounds and as places for fish farming or ranching.[5] 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 externalparasites such assea lice that escape from the pens the farmed fish are kept in.[6]
Another important brackish water habitat is themangrove swamp ormangal. Many, though not all, mangrove swamps fringe estuaries andlagoons where the salinity changes with each tide. Among the most specialised residents ofmangrove forests aremudskippers, fish that forage for food on land, andarcherfish, 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 assnappers,halfbeaks, andtarpon spawning or maturing among them. Besides fish, numerous other animals use mangroves, including such species as thesaltwater crocodile,American crocodile,proboscis monkey,diamondback terrapin, and thecrab-eating frog,Fejervarya cancrivora (formerlyRana cancrivora). Mangroves represent important nesting sites for numerous birds groups such asherons,storks,spoonbills,ibises,kingfishers,shorebirds andseabirds.
Although often plagued withmosquitoes 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.[7]
Some seas and lakes are brackish. TheBaltic Sea is a brackish sea adjoining theNorth Sea. Originally theEridanos river system prior to thePleistocene, 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. Asseawater 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, whilepike are confined to the less saline surface waters.
TheCaspian 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 (theCaspian seal) and the greatsturgeons, a major source ofcaviar.
Hudson Bay is a brackishmarginal sea of theArctic Ocean, it remains brackish due its limited connections to the open ocean, very high levels freshwatersurface runoff input from the largeHudson Bay drainage basin, and low rate of evaporation due to being completely covered in ice for over half the year.
In theBlack Sea the surface water is brackish with an average salinity of about 17–18 parts per thousand compared to 30 to 40 for the oceans.[8] The deep,anoxic water of the Black Sea originates from warm, salty water of theMediterranean.
Lake Texoma, a reservoir on the border between the U.S. states ofTexas andOklahoma, is a rare example of a brackish lake that is neither part of anendorheic basin 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 theRed 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 thatstriped bass, a fish normally found only in salt water, has self-sustaining populations in the lake.[9][10]
Brackish water is 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. Oncedesalinated it can also be used for agriculture, livestock, and municipal uses.[11] Brackish water can be treated usingreverse osmosis,electrodialysis, and other filtration processes.[12]
^Klaassen, K.; Bormann, H.; Klenke, T.; Liebezeit, G. (2008). "The impact of hydrodynamics and texture on the infiltration of rain and marine waters into sand bank island sediments — Aspects of infiltration and groundwater dynamics".Senckenbergiana Maritima.38 (2). Springer Science and Business Media LLC:163–171.doi:10.1007/bf03055293.ISSN0080-889X.S2CID6229273.
