- Brackish water
Brackish water (less commonly brack water) is water that has more
salinitythan fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutchroot "brak," meaning "salty." Certain human activities can produce brackish water, in particular certain civil engineeringprojects 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 salinity gradient powerprocess. 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 to 30 grams of
saltper 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.
Brackish water habitats
Brackish is a mixture of sea water and fresh water, and an
estuaryis a body of water where fresh and salt water mix. The most extensive brackish water habitats worldwide are estuaries, where a river meets the sea.
Thamesflowing through Londonis a classic river estuary. The town of Teddingtona 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 Batterseainsofar as the average salinity is very low and the fish fauna consists predominantly of freshwater species such as roach, dace, carp, perch, and pike. The Thames Estuarybecomes truly brackish between Battersea and Gravesend, and the diversity of freshwater fish species present is smaller, primarily roach and dace, euryhalinemarine species such as flounder, European seabass, mullet, and smeltbecome 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 Seaand includes both euryhaline and stenohalinemarine species. A similar pattern of replacement can be observed with the aquatic plants and invertebrates living in the river [http://www.the-river-thames.co.uk/thames.htm] , [http://www.the-river-thames.co.uk/wildlife.htm] .
This type of
ecological successionfrom a freshwater to marine ecosystemis typical of river estuaries. River estuaries form important staging points during the migration of anadromous and catadromus fish species, such as salmonand 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. Herringand plaiceare 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 salmonfarms 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 parasitessuch as sea licethat escape from the pens the farmed fish are kept in [http://www.saveourseatrout.com/] .
Another important brackish water habitat is the
mangrove swampor 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, halfbeaks, and tarponspawning 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, "Fejervarya cancrivora formerly Rana cancrivora". Although often plagued with mosquitoesand 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 [http://www.scidev.net/News/index.cfm?fuseaction=readNews&itemid=1823&language=1] .
Brackish seas and lakes
Some seas and lakes are brackish. The
Baltic Seais a brackish sea adjoining the North Sea. Originally the confluenceof 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. Codare an example of a species only found in deep water in the Baltic, while pike are confined to the less saline surface waters [http://www.internat.naturvardsverket.se/index.php3?main=/documents/nature/nacatego/marine/marine.htm] .
Caspian Seais 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.
Black seathe surface water is brackish with an average salinity of about 18 parts per thousand compared to 30 to 40 for the oceans. The deep, anoxic water of Black sea originates from warm, salty water of the Mediterranean
Notable brackish bodies of water (by type, in alphabetical order)
Baltic Sea(the world’s largest pool of brackish water)
Caspian Sea(world’s largest lake)
Brackish water lakes
*Lake Charles in Lake Charles,
Chilka Lake, in Orissastate, India
Lake Maracaibo, in Zuliastate, Venezuela
*Lake Monroe in
Pangong Tsoin Ladakh, Jammu and Kashmirstate, India
Coastal lagoons, marshes, and deltas
Burgas Lakesnear the Bulgarian Black Sea Coast
lagoon, Dorset, England
Kaliveli Lake, near Pondichery, India
Kerala Backwaters, Series of lagoons and lakes in Kerala
Lagos Lagoonin Lagos, Nigeria
Lake Pontchartrain, north of New Orleans, Louisiana, U.S.
Pulicat Lake, north of Chennai, India
Rann of Kutch, on the border of Indiaand Pakistan
*Parts of the Rhône Delta,
France: An area known as the Camargue
Widewater, and land-locked lagoonnear Lancing, England
Amazon River, empties so much freshwater into the Atlantic Oceanthat it reduces the salinity of the sea for hundreds of miles
Chesapeake Bay, in Maryland, U.S.
Delaware Bay, an extension of the Delaware Riverin New Jerseyand Delaware, USA
Hampton Roads, Virginia, USA
Hudson River, in New Yorkand New Jersey, U.S.
East River, New York, USA
Lingding Yang, Guangdong, the People's Republic of China
Port Royal Soundpart of Beaufort County, South Carolina, USA [http://www.lowcountryestuarium.org]
*Saint Lawrence and Saguenay Rivers, the part downstream from Québec and Saguenay respectively
San Francisco Bayand San Pablo Bayadjacent to San Franciscoin California, U.S.
Thames Estuaryin South East England
* [http://www.springerlink.com/content/p2q719335u606034/fulltext.pdf Moustakas, A. & I. Karakassis. How diverse is aquatic biodiversity research?, Aquatic Ecology, 39, 367-375]
Brackish water aquarium
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