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Brackish refers to water with a salinity intermediate to that of fresh water and sea water (the latter has a salt concentration of about 3.5%, or 35 parts per thousand). Brackish waters originate by the mixing of sea water and freshwater, and are most common near the coasts of the oceans.

Brackish waters can occur as enclosed systems such as lakes and ponds that receive occasional inputs of oceanic water during severe storms. Brackish waters also occur as coastal estuaries or salt marshes that are more frequently flooded with saline water as a result of tidal cycles. Sometimes, brackish waters can occur far inland, for example, in parts of the prairies of North America where saline ponds and wetlands have variable salt concentrations depending on the diluting effects of recent rains or snowmelt.

The salt concentration of water is highly influential on the transport of ions across cellular membranes, the availability of nutrients in soil, and for other reasons. Most species can tolerate either saltywater or freshwater, but not both. However, organisms that live in brackish habitats must be tolerant of a wide range of salt concentrations (such species are known as euryhaline). For example, the small fish known as killifish (Fundulus spp.) are common residents of brackish coastal habitats known as estuaries, where within any day the salt concentration in tidal pools and creeks can vary from that typical of freshwater to that of the open ocean. Other fish such as salmon (e.g., Salmo spp.) and eels (Anguilla spp.) move to or from marine waters during their spawning migrations, in the process moving from environments characterized by the salt concentration of full seawater, through brackish, to freshwater. Animals that live in or move through estuaries must be tolerant of the physiological stresses associated with such large and rapid changes in salinity, as must the plants of those habitats, such as the aquatic eelgrass (Zostera spp.) and the cord grass of salt marshes (Spartina spp.).

The environmental conditions of brackish waters are highly stressful for organisms that cannot tolerate such wide swings of salinity. However, for those relatively few species that are tolerant of such difficult environmental conditions, the environmental conditions of brackish habitats represent a relatively uncompetitive, ecological opportunity to be exploited as a livelihood.

See also Saltwater.

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