Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more
salinity than
freshwater, but not as much as
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...
. It may result from mixing seawater (salt water) and fresh water together, as in
estuaries
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...
, or it may occur in brackish fossil
aquifer
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteris ...
s. The word comes from the Middle Dutch root ''
brak''. Certain human activities can produce brackish water, in particular
civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads, bridges, canals, dams, airports, sewa ...
projects such as dikes and the flooding of coastal
marsh
A marsh is a wetland that is dominated by herbaceous rather than woody plant species.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p Marshes can often be found a ...
land to produce brackish water pools for
freshwater prawn farming
A freshwater prawn farm is an aquaculture business designed to raise and produce freshwater prawns or shrimp for human consumption. Freshwater prawn farming shares many characteristics with, and many of the same problems as, marine shrimp farmin ...
. Brackish water is also the primary waste product of the
salinity gradient power
Osmotic power, salinity gradient power or blue energy is the energy available from the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electrodialysis (RED) and
pressure retarde ...
process. 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
Shrimp farming is an aquaculture business that exists in either a marine or freshwater environment, producing shrimp or prawns (crustaceans of the groups Caridea or Dendrobranchiata) for human consumption.
Marine
Commercial marine shrimp farm ...
).
Technically, brackish water contains between 0.5 and 30 grams of
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
per litre—more often expressed as 0.5 to 30
parts per thousand
In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...
(‰), which is a
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
of between 1.0004 and 1.0226. 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 or time. Water with a salt concentration greater than 30‰ is considered
saline. See
the Salinity Table from the
Wikipedia Salinity Article.
Brackish water habitats
Estuaries
![Monodactylus argenteus](https://upload.wikimedia.org/wikipedia/commons/d/d7/Monodactylus_argenteus.JPG)
Brackish water condition commonly occurs when fresh water meets seawater. In fact, the most extensive brackish water habitats worldwide are
estuaries
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...
, where a river meets the sea.
The
River Thames
The River Thames ( ), known alternatively in parts as the River Isis, is a river that flows through southern England including London. At , it is the longest river entirely in England and the second-longest in the United Kingdom, after the R ...
flowing through
London
London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
is a classic river estuary. The town of
Teddington
Teddington is a suburb in south-west London in the London Borough of Richmond upon Thames. In 2021, Teddington was named as the best place to live in London by ''The Sunday Times''. Historically in Middlesex, Teddington is situated on a long me ...
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
Roach may refer to:
Animals
* Cockroach, various insect species of the order Blattodea
* Common roach (''Rutilus rutilus''), a fresh and brackish water fish of the family Cyprinidae
** ''Rutilus'' or roaches, a genus of fishes
* California roach ...
,
dace
A dace is a small fish that can be one of many different species. The unmodified name is usually a reference to the common dace (''Leuciscus leuciscus''). This, like most fish called "daces", belongs to the family Cyprinidae, mostly in subfamily ...
,
carp,
perch
Perch is a common name for fish of the genus ''Perca'', freshwater gamefish belonging to the family Percidae. The perch, of which three species occur in different geographical areas, lend their name to a large order of vertebrates: the Per ...
, and
pike
Pike, Pikes or The Pike may refer to:
Fish
* Blue pike or blue walleye, an extinct color morph of the yellow walleye ''Sander vitreus''
* Ctenoluciidae, the "pike characins", some species of which are commonly known as pikes
* ''Esox'', genus of ...
. The
Thames Estuary
The Thames Estuary is where the River Thames meets the waters of the North Sea, in the south-east of Great Britain.
Limits
An estuary can be defined according to different criteria (e.g. tidal, geographical, navigational or in terms of salini ...
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
The European bass (''Dicentrarchus labrax'') is a primarily ocean-going fish native to the waters off Europe's western and southern and Africa's northern coasts, though it can also be found in shallow coastal waters and river mouths during the su ...
,
mullet, and
smelt
Smelt may refer to:
* Smelting, chemical process
* The common name of various fish:
** Smelt (fish), a family of small fish, Osmeridae
** Australian smelt in the family Retropinnidae and species ''Retropinna semoni''
** Big-scale sand smelt ''Ath ...
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
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea, epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the ...
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 a freshwater to marine
ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
is typical of river estuaries. River estuaries form important staging points during the migration of
anadromous and catadromous fish species, such as
salmon
Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus ''Salmo'') and North Pacific (genus '' Oncorhy ...
,
shad and
eel
Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...
s, 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
Plaice is a common name for a group of flatfish that comprises four species: the European, American, Alaskan and scale-eye plaice.
Commercially, the most important plaice is the European. The principal commercial flatfish in Europe, it is al ...
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
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
such as
sea lice
Sea lice (singular: sea louse) are copepods (small crustaceans) of the family Caligidae within the order Siphonostomatoida. They are marine ectoparasites (external parasites) that feed on the mucus, epidermal tissue, and blood of host fish. The ...
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
Mudskippers are any of the 23 extant species of amphibious fish from the subfamily Oxudercinae of the goby family Oxudercidae. They are known for their unusual body shapes, preferences for semiaquatic habitats, limited terrestrial locomotion and ...
, fish that forage for food on land, and
archer fish
The archerfish (spinner fish or archer fish) form a monotypic family, Toxotidae, of fish known for their habit of preying on land-based insects and other small animals by shooting them down with water droplets from their specialized mouths. The ...
, 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,
halfbeak
Hemiramphidae is a family of fishes that are commonly called halfbeaks, spipe fish or spipefish. They are a geographically widespread and numerically abundant family of epipelagic fish inhabiting warm waters around the world. The halfbeaks are ...
s, and
tarpon
Tarpons are fish of the genus ''Megalops''. They are the only members of the family Megalopidae. Of the two species, one (''M. atlanticus'') is native to the Atlantic, and the other (''M. cyprinoides'') to the Indo-Pacific Oceans.
Species a ...
spawning or maturing among them. Besides fish, numerous other animals use mangroves, including such species as the
saltwater crocodile
The saltwater crocodile (''Crocodylus porosus'') is a crocodilian native to saltwater habitats and brackish wetlands from India's east coast across Southeast Asia and the Sundaic region to northern Australia and Micronesia. It has been list ...
,
American crocodile
The American crocodile (''Crocodylus acutus'') is a species of crocodilian found in the Neotropics. It is the most widespread of the four extant species of crocodiles from the Americas, with populations present from South Florida and the coasts ...
,
proboscis monkey,
diamondback terrapin
The diamondback terrapin or simply terrapin (''Malaclemys terrapin'') is a species of turtle native to the brackish coastal tidal marshes of the Northeastern and southern United States, and in Bermuda. It belongs to the monotypic genus ''Mal ...
, and the
crab-eating frog
The crab-eating frog (''Fejervarya cancrivora'') is a frog native to south-eastern Asia including Taiwan, China, the Philippines and more rarely as far west as Orissa in India. It has also been introduced to Guam, and was most likely introduced f ...
, ''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
mosquito
Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small flies within the family Culicidae (from the Latin ''culex'' meaning " gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish for "li ...
es 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
Sundarbans (pronounced ) is a mangrove area in the delta formed by the confluence of the Padma, Brahmaputra and Meghna Rivers in the Bay of Bengal. It spans the area from the Baleswar River in Bangladesh's division of Khulna to the Hooghly R ...
and
Bhitarkanika Mangroves
Bhitarkanika Mangroves is a mangrove wetland in Odisha, India, covering an area of in the Brahmani River and Baitarani River deltas.
History
The Bhitarkanika Mangroves were zamindari forests until 1952, when the government of Odisha abolish ...
are two of the large mangrove forests in the world, both on the coast of the
Bay of Bengal
The Bay of Bengal is the northeastern part of the Indian Ocean, bounded on the west and northwest by India, on the north by Bangladesh, and on the east by Myanmar and the Andaman and Nicobar Islands of India. Its southern limit is a line betwee ...
.
Brackish seas and lakes
Some seas and lakes are brackish. The
Baltic Sea
The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain.
The sea stretches from 53°N to 66°N latitude and ...
is a brackish sea adjoining the
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea, epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the ...
. Originally the
Eridanos river system prior to the
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...
, 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
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...
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
Cod is the common name for the demersal fish genus '' Gadus'', belonging to the family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gadus'' is commonly not call ...
are an example of a species only found in deep water in the Baltic, while
pike
Pike, Pikes or The Pike may refer to:
Fish
* Blue pike or blue walleye, an extinct color morph of the yellow walleye ''Sander vitreus''
* Ctenoluciidae, the "pike characins", some species of which are commonly known as pikes
* ''Esox'', genus of ...
are confined to the less saline surface waters.
The
Caspian Sea
The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central Asia ...
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
The Caspian seal (''Pusa caspica'', syn. ''Phoca caspica'') is one of the smallest members of the earless seal family and unique in that it is found exclusively in the brackish Caspian Sea. It lives along the shorelines, but also on the many roc ...
) and the great
sturgeons, a major source of
caviar
Caviar (also known as caviare; from fa, خاویار, khâvyâr, egg-bearing) is a food consisting of salt-cured roe of the family Acipenseridae. Caviar is considered a delicacy and is eaten as a garnish or a spread. Traditionally, the te ...
.
Hudson Bay is a brackish
marginal sea
This is a list of seas of the World Ocean, including marginal seas, areas of water, various gulfs, bights, bays, and straits.
Terminology
* Ocean – the four to seven largest named bodies of water in the World Ocean, all of which have "Ocea ...
of the
Arctic ocean
The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
, 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
The Hudson Bay drainage basin is the drainage basin in northern North America where surface water empties into Hudson Bay and adjoining waters. Spanning an area of about , the basin is almost totally in Canada (spanning parts of the Prairies, cen ...
, and low rate of evaporation due to being completely covered in ice for over half the year.
In the
Black Sea
The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...
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
The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:
* Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
water of the Black Sea originates from warm, salty water of the
Mediterranean
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western Europe, Western and Southern Europe and Anatolia, on the south by North Africa ...
.
Lake Texoma
Lake Texoma is one of the largest reservoirs in the United States, the 12th largest US Army Corps of Engineers' (USACE) lake, and the largest in USACE Tulsa District. Lake Texoma is formed by Denison Dam on the Red River in Bryan County, Oklaho ...
, a reservoir on the border between the U.S. states of
Texas
Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...
and
Oklahoma
Oklahoma (; Choctaw language, Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by Texas on the south and west, Kansas on the nor ...
, is a rare example of a brackish lake that is neither part of an
endorheic basin
An endorheic basin (; also spelled endoreic basin or endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water, such as rivers or oceans, but drainage converges instead into lakes ...
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
The striped bass (''Morone saxatilis''), also called the Atlantic striped bass, striper, linesider, rock, or rockfish, is an anadromous perciform fish of the family Moronidae found primarily along the Atlantic coast of North America. It has al ...
, a fish normally found only in salt water, has self-sustaining populations in the lake.
Brackish marsh
Other brackish bodies of water
*
Human uses
Brackish water is being 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. Once
desalinated it can also be used for agriculture, livestock, and municipal uses. Brackish water can be treated using
reverse osmosis
Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pre ...
,
electrodialysis
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis ...
, and other filtration processes. It is also being used in different cosmetics like hair tonic.
See also
*
List of brackish bodies of water
*
*
*
*
References
Further reading
Moustakas, A. & I. Karakassis. How diverse is aquatic biodiversity research?, Aquatic Ecology, 39, 367-375
{{Authority control
Liquid water
Aquatic ecology
Coastal geography
*