Secondary salinization of aquatic ecosystems occurs when the salt-to-water ratio in an inland water body increases due to antrhopogenic factors. This process occurs in both freshwater and saltwater environments, and can have equally dramatic effects on either. Alhough natural, or primary, salinization does occur, anthropogenically-induced secondary salinization poses a much greater and ever-increasing problem. The specific impacts vary widely from system to system; however, the threat it poses to both human and non-human use of major bodies of water is becoming increasingly recognized throughout the world.
Why salinization occurs
Generally, salinization of inland waters occurs in one of three ways: salt input increases, freshwater input decreases, or freshwater extraction increases. Under some conditions, all of these processes occur naturally, and in this case are known collectively as primary salinization. For instance, salinity of inland waters will increase under drought conditions, when decreased precipitation results in decreased freshwater input while evaporation removes freshwater from the system. Lakes fed by runoff without natural outlets will likewise experience naturally increasing salinity. For example, the Salton Sea in California, which was created in the early 20th century when floods overran an existing dyke on the Colorado River, is now fed by precipitation and runoff and has no natural discharge for accumulated salts. Since this area of the country is naturally arid, the lake has been gradually decreasing in volume and increasing in salinity through evaporation since its creation. Currently, salinity in the sea is increasing at 1% per year. Consequences have included the extirpation of plankton-eating fish, and several waterbird die-offs have been linked to avian diseases that thrive in hypersaline conditions.
However, natural systems will generally maintain salinity equilibria unless perturbed by human alterations, and anthropogenic use has contributed to increasing salinity in many bodies of water throughout the world. Each of the three mechanisms for salinization increases rapidly under human use, particularly in arid regions and in agricultural systems. To distinguish these processes from natural (primary) salinity increases, anthropogenically-influenced salinization is also known as secondary salinization.
Increased salt input can result from fertilizer runoff, herbicide contamination, brine discharge from mines, and the common agricultural phenomenon of salinization of surrounding soils. When plants are removed, the decreased demand for groundwater causes the water table to rise to the soil surface. The excess water evaporates, depositing salts that had previously been dissolved in the groundwater on the soil surface. When this occurs in lake basins, increased soil salinity results in increasingly saline runoff and increased lake water salinity. This process is a major threat to the stability of both salt and freshwater systems, particularly in arid agricultural regions such as Western Australia.
Anthropogenic use is often responsible for changing the inflow and outflow regimes of inland bodies of water. Freshwater input is often decreased by human diversion of feeder streams for agricultural, industrial, or other consumptive purposes. On the other hand, extraction of freshwater from inland bodies is a common agricultural and industrial practice, providing water for plant operation, irrigation, and livestock, but also reducing the lake area and further concentrating salts.
Additionally, climate change has the potential to accelerate salinization in certain regions by altering storm recharge or evaporative conditions. Regions that become more arid will naturally experience increased salinization. Conversely, regions that are currently arid may see increased precipitation that may either mitigate existing salinization problems through dilution or instead accentuate salinization by increasing the surface area of the lake and, thus, its rate of freshwater evaporation. Though the role of climate change in salinization will vary both regionally and temporally, there is no doubt that it will play an important part in years to come.
Impacts to biota
In both saline and freshwater lakes, increased salinity has tremendous potential to disturb the organisms that inhabit these systems. Zooplankton are often particularly vulnerable and display major shifts in survival and community structure in the presence of small changes in salt levels. At an evolutionary scale, extremophiles are organisms that have adapted to extreme changes in salinity or other abiotic factors. In some cases salinization can be driven to an extent of forming a hypersaline environment, where fish and certain other aquatic species can no longer thrive.
In California’s Mono Lake, a runoff-fed system with no discharge, salinity has increased rapidly over the past half-century due to evaporation and freshwater diversion. If levels continued to increase at this rate, the salinity of the water would extirpate the lake’s primary inhabitant, a unique brine shrimp (Artemia monica), by preventing its eggs from hatching. Diatom species diversity and richness is also known to decrease rapidly with increasing aquatic salt content. Further up the food chain, shorebirds, whose habitat in the American West and other arid agricultural regions is particularly threatened by secondary salinization, demonstrate dehydration, altered behavior, weight loss, and low reproductive success under saline conditions.
Strategies for mitigating salinization must account for all of its complex contributing processes. Lake Kinneret in Israel has instituted a Salinity Diversion Channel, designed to draw off the lake’s most saline inputs and make the water more suitable for use. Mitigation recommendations for China’s Bosten Lake include, not only making more water available to the lake through changes in patterns of agricultural and industrial use, but also reforestation and the creation of artificial reed beds. On Lake Toolibin in Western Australia, a comprehensive mitigation plan is in place, including groundwater pumping in the lake basin, the creation of buffer strips containing highly evaporative vegetation, and engineering work to improve outflows of salt. Although such mitigating measures may provide hope for compromised ecosystems, many salinity increases are irreversible, and the best hope for the future of aquatic ecosystems is undoubtedly an increased awareness of salinization and a concerted effort toward prevention.
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This article was partially researched by a student at the University of Massachusetts, Amherst participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.