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 flowing through
London 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 insofar 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 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 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 freshwater to marine
ecosystem 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
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 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 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
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
archerfish, 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 spawning or maturing among them. Besides fish, numerous other animals use mangroves, including such species as the
saltwater crocodile,
American crocodile,
proboscis monkey,
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 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 is a brackish sea adjoining the
North Sea. Originally the
Eridanos river system prior to the
Pleistocene, 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 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 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.
Hudson Bay is a brackish
marginal sea of the
Arctic Ocean, 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, and low rate of evaporation due to being completely covered in ice for over half the year. In the
Black 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. The deep,
anoxic water of the Black Sea originates from warm, salty water of the
Mediterranean.
Lake Texoma, a reservoir on the border between the U.S. states of
Texas and
Oklahoma, is a rare example of a brackish lake that is neither part of an
endorheic 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 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 ==