Brackish water is a unique aquatic environment where fresh water and sea water meet and mix, creating a transitional zone with a fluctuating salt content. This intermediate habitat is neither a purely freshwater nor a fully marine system, resulting in distinct physical and biological characteristics.
Defining Brackish Water by Salinity
The technical definition of brackish water is determined by its salinity, which is the concentration of dissolved salts measured in parts per thousand (ppt). Seawater is consistently around 35 ppt, while freshwater contains less than 0.5 ppt of dissolved salts. Brackish water occupies the broad range between these two extremes, typically falling between 0.5 ppt and 30 ppt.
This wide range means that a body of water is considered brackish even if it is only slightly saltier than a river or nearly as salty as the open ocean. The variable salinity is created by the physical mixing of the two water sources, which can change hourly with the tides and seasonally with precipitation and river flow. Because the precise salt content is constantly in flux, brackish water is not considered a single, precisely defined condition but rather a dynamic salinity regime.
Principal Locations of Brackish Water
The most extensive and well-known brackish habitats worldwide are estuaries, which are semi-enclosed coastal bodies of water where rivers connect with the sea. The mixing process begins when the denser, saltier ocean water pushes inland beneath the less dense, lighter river water, forming a structure known as a salt wedge. This tidal influence pushes the salt front upstream, while the river’s outflow maintains a seaward flow of fresh water.
Other coastal areas also feature brackish conditions, including tidal marshes, river deltas, coastal lagoons, and mangrove swamps. The Baltic Sea is a notable example of a large brackish sea, where limited exchange with the highly saline Atlantic Ocean and massive freshwater input from surrounding rivers result in a low overall salinity, averaging only around 10 ppt in its surface waters.
Biological Adaptations in Brackish Environments
Life in a brackish environment presents a severe challenge because organisms must constantly manage the rapid and unpredictable shifts in salinity. Organisms that can tolerate this wide range of salt concentrations are classified as euryhaline, and their survival depends on a biological process called osmoregulation. Osmoregulation is the active process of regulating the water and electrolyte balance within an organism’s body to maintain internal stability.
In animals, this often involves specialized organs like the kidneys or gills that actively pump salt ions in or out to keep the internal chemistry stable. For example, fish like the striped bass or the bull shark are euryhaline and can adjust their osmoregulatory mechanisms to move between fresh and salt water. Plants in these zones, such as smooth cordgrass (Spartina alterniflora), employ a different strategy, using specialized glands on their leaves to excrete excess salt that has been absorbed by the roots.