Brackish water
"Brackish" redirects here. For the Kittie song, see Brackish (song).Brackish water is water that is
saltier than
fresh water, but not as salty as
sea water. 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 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.
| Water salinity based on dissolved salts in parts per thousand (‰) |
|---|
| Fresh water | Brackish water | Saline water | Brine |
|---|
| < 0.5 ‰ | 0.5 - 30 ‰ | 30 - 50 ‰ | > 50 ‰ |
|---|
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.
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 west 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 [
1], [
2].
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 [
3].
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 [
4].
Some seas and lakes are brackish. The
Baltic Sea is a brackish sea adjoining the North Sea. Orginally 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 [
5].
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.
Brackish seas*
Baltic Sea (the world's largest pool of brackish water)
*
Caspian Sea (world's largest lake)
Brackish water lakes*
Chilka Lake, in
Orissa state,
India*
Pangong Tso in
Ladakh,
Jammu and Kashmir state,
IndiaCoastal lagoons, marshes, and deltas*
Kaliveli Lake, near
Pondichery,
India*
Lagos Lagoon in Lagos, Nigeria
*
Lake Pontchartrain, north of
New Orleans,
Louisiana,
USA*
Pulicat Lake, north of
Chennai,
India*The
Rann of Kutch, on the border of
India and
Pakistan*Parts of the
Rhône Delta,
France: An area known as the
CamargueEstuaries*
Amazon River, empties so much freshwater into the
Atlantic Ocean that it reduces the salinity of the sea for hundreds of miles
*
Chesapeake Bay, in
Virginia and
Maryland,
USA*
The Fleet lagoon,
Dorset,
England*
Hampton Roads,
Virginia, USA
*
Port Royal Sound part of Beaufort County,
South Carolina, USA [
6]
*Lower
Hudson River, in
New York and
New Jersey,
USA*
Lingding Yang,
Guangdong, the
People's Republic of China*The
Thames Estuary in east
LondonKeeping brackish water
aquaria is a popular specialization within the fishkeeping hobby. Many species of fish traded as
freshwater species actually do better in brackish water, for example black mollies, Florida flagfish, and some
cichlids such as orange chromides. There are also several popular species traded purely as brackish water fish, including
Monodactylus spp, scats,
archerfish, and various species of
pufferfish,
goby, flatfish, and
gar. Generally aquarists need to maintain a
specific gravity of around 1.005 to 1.010 depending on the species being kept, but practically all brackish water fish tolerate variations in
salinity well, and some aquarists maintain that regularly fluctuating the salinity in the aquarium actually keeps the fish healthy and free of parasites.
*
Biosalinity*
Desalination*
Permian Sea (underwater basin and remnant of the ancient Permian Ocean) extending underground from Eastern New Mexico (USA) to West Texas
*
Slough*
Brackish Water Aquarium FAQ
