A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride or other mineral salts, with saline levels surpassing that of ocean water (3.5%, i.e. 35 grams per litre or 0.29 pounds per US gallon). Specific microbial and crustacean species thrive in these high salinity environments that are otherwise inhospitable to most lifeforms. Some of these species attain dormancy when they are desiccated and organisms of certain species have been shown to survive for over 250 million years.
The most saline lake in the world is the Don Juan Pond, located in the McMurdo Dry Valleys in Antarctica. The Don Juan Pond has a salinity level of over 44%, (i.e. 12 times saltier than the ocean water). Its high salinity prevents the Don Juan from freezing even when temperatures are below −50 °C (−58 °F). There are other hypersaline lakes in the McMurdo Dry Valleys, but most of them are covered year round with ice, which contributed to the relatively late discovery of their existence.
The most saline lake outside of Antarctica is Lake Assal, in Djibouti, which has a salinity level of 34.8%. But probably the best known examples of hypersaline lakes are the Dead Sea (33.7% salinity) and the Great Salt Lake (5–27% variable salinity). The Dead Sea, dividing Israel and the West Bank from Jordan, is the world's deepest hypersaline lake. The Great Salt Lake, located in Utah, while having nearly three times the surface area of the Dead Sea, is shallower and experiences much greater fluctuations in level than the Dead Sea. Because of this, the Great Salt Lake's degree of salinity also fluctuates greatly. At its lowest recorded levels, it approaches 7.7 times the salinity of ocean water, but when its levels are high, its salinity drops to only slightly higher than the ocean.
Other hypersaline lakes can be found in western North America (besides the Great Salt Lake, Mono Lake), Africa (besides Lake Assal, the seasonal Makgadikgadi Pan), and on the French Polynesian island of Niau, there is a swampy landlocked hypersaline lagoon.
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