Salinity is the concentration of dissolved salts in water, a measurement expressed in parts per thousand (ppt). One ppt means there is one gram of salt for every thousand grams of water. The world’s oceans have an average salinity of about 35 ppt. Low-salinity water, often called brackish water, is less salty than seawater but contains more salt than freshwater. This condition occurs where freshwater and saltwater mix, with a salinity range generally from 0.5 to 30 ppt.
Formation of Low-Salinity Environments
Low-salinity environments are formed where large volumes of freshwater dilute seawater. This dilution primarily comes from the discharge of rivers, but direct rainfall and the melting of ice can also contribute. These conditions are characteristic of specific coastal and transitional zones found across the globe.
Examples of low-salinity environments include estuaries, deltas, fjords, and brackish marshes. In these locations, the influx of freshwater from land meets the tidal influence of the ocean, establishing a salinity gradient. Salinity is lowest near the freshwater source, like a river mouth, and increases as one moves toward the open sea. The extent of this gradient can fluctuate based on tides and seasons.
Seasonal patterns of rainfall and evaporation can cause shifts in the salinity of these zones. For instance, heavy rainfall will lower the overall salinity. Conversely, during hotter, drier months, reduced freshwater input and higher evaporation rates can cause salinity levels to rise. This dynamic nature means organisms living in these environments must be adapted to frequent changes.
Biological Effects on Organisms
The primary challenge for organisms in low-salinity water is managing osmosis. Osmosis is the natural movement of water across a semipermeable membrane, such as a cell wall, from an area of lower salt concentration to an area of higher salt concentration. For marine life accustomed to the high salinity of the ocean, being in a less salty environment creates osmotic stress, as water continuously enters their bodies, diluting their internal fluids.
To survive, organisms have developed distinct strategies. Many species, including salmon and crabs, are osmoregulators. These animals actively manage their internal salt balance by expending energy to pump excess water out of their bodies while retaining necessary salts. This process allows them to move between freshwater and saltwater environments.
Other organisms, like jellyfish and certain mollusks, are osmoconformers. Their internal salt concentration matches that of their external environment. This strategy requires less energy but restricts these organisms to specific salinity ranges, as they cannot survive significant or rapid changes in the water’s salt content. This difference in adaptation leads to the classification of organisms as either euryhaline, meaning they can tolerate a wide range of salinities, or stenohaline, meaning they are limited to a narrow salinity range.
Ecological Significance
Low-salinity zones, particularly estuaries and coastal marshes, are important to the marine ecosystem. They are recognized as nursery grounds for a vast number of species, many of which are important for commercial fishing industries. The brackish waters provide juvenile fish, shrimp, and crabs with an environment rich in nutrients and with fewer large predators than the open ocean.
These transition zones support a high level of biodiversity. The mix of freshwater and saltwater allows for a unique assembly of species, including some adapted specifically to brackish conditions, as well as tolerant species from both freshwater and marine environments. This convergence of life forms creates a complex and productive food web.
These environments can also function as a natural filter or barrier. The specific salinity conditions can prevent the spread of certain marine pests and diseases. Organisms not adapted to tolerate salinity fluctuations find it difficult to move through these zones, which helps to isolate different marine populations.
Human Interactions and Applications
Human activities can have a profound impact on low-salinity environments. The construction of dams and water diversion for agriculture or urban use can reduce the amount of freshwater that reaches estuaries. This reduction in flow leads to an increase in salinity, altering the ecosystem and harming the species that depend on the brackish conditions.
There are also practical applications that leverage the properties of low-salinity water, particularly in aquaculture. A common practice for treating fish for external parasites is a “low salinity dip.” Fish are temporarily placed in water with a much lower salt concentration than their normal habitat. While the fish can tolerate this change for a short period, the parasites attached to their skin and gills cannot survive the osmotic shock, providing a non-chemical method for parasite control.