Lakes with salinities above 3 g l_1 are usually considered saline, though this value is somewhat arbitrary. Salinity is defined as the sum of total ions by weight and usually includes the major cations (sodium, potassium, calcium, and magnesium) and anions (bicarbonate plus carbonate, chloride, and sulfate). Natural waters can attain salinities of several hundred grams per liter and vary considerably in their chemical composition. The ionic composition of saline lakes depends on the ionic ratios in the inflows and extent of evaporative concentration. As the saturation of specific salts is exceeded, they precipitate and can lead to the formation of large evaporite deposits. Typically, calcium and magnesium carbonates are the first minerals to precipitate. If sufficient calcium remains in solution, calcium sulfate often precipitates next. In the most concentrated waters, chloride is the dominant anion and sodium is usually the dominant cation; Great Salt Lake in
Utah (USA) is such an example. In rare cases, other combinations of ions can occur in highly concentrated waters such as the sodium-magnesium-chloride waters of the Dead Sea, the sodium-chloride-sulfate brine in Lake Mahega, Uganda (Figure 1), or the exceptional calcium chloride brine in Don Juan Pond (Antarctica). Lakes of intermediate salinities include the sodium carbonate or soda lakes of eastern Africa and the triple salt waters (sodium carbonate-chloride-sulfate) of Mono Lake, California, USA.
A considerable diversity of halophilic microorganisms with representatives from the three domains of life, the Archaea, Bacteria, and Eukarya, inhabit saline lakes. Only recently have modern molecular techniques, such as gene sequencing, been applied to natural communities of microbes and much remains to be learned. At especially high salt concentrations, microbes lack grazers and can attain very high abundances that can color saline lakes bright red or orange. Only a very few metabolic processes have not been observed at high salinities and these include halophilic methanogenic bacteria able to use acetate or hydrogen plus carbon dioxide and halophilic nitrifying bacteria.
As salinity increases in inland waters, biodiversity tends to decrease, but in the mid-range of salinities other factors cause considerable variation in species diversity. The strongest relationship between species richness of plants, algae, and animals occurs, generally, below a salinity of about 10 g l-1. An investigation by William D. Williams, an Australian professor who pioneered studies of saline lakes, found that species richness of macroinvertebrates in Australian lakes highly correlated with salinity over a salinity from 0.3 to 343 g l-1 but nonsignificant over intermediate ranges of salinity. Many taxa had broad tolerances of salinity at the intermediate values. Instead, a variety of other factors, including dissolved oxygen concentrations, ionic composition, pH, and biological interactions, appear to influence species richness and composition.
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