A hypersaline lake is a lacustrine body of water which exhibits levels of dissolved salts that exceed seawater (e.g. greater than 35 grams per litre). A large percentage of the Earth's hypersaline lakes occur in endorheic basins, viz. a topographic depression that has no surface hydraulic outlet. Typically the lake waters or feed waters are in contact with rock formations high in soluble salts. While most hypersaline lakes are rich in sodium and chloride ions, certain lakes have very high levels of other ions, such as magnesium, calcium and bromine.
The evolution of a hypersaline lake often involves high levels of evaporation, engendered by pervasive warm temperatures and a large percentage of full sunlight days, engendering maximum water loss, thus concentrating mineral salt concentrations.
Occurrence and formation
Hypersaline lakes are found in disparate areas of the globe, and several major formation mechanisms are evident in their hydrogeological evolution. Generally thsese mechanisms can be grouped into (a) a source of mineral salts for dissolution, or: (b) an evaporation rate that is sufficiently high relative to brackish or freshwater inputs, such that cumulative increases in salinity occur.
The source of mineral salts may be the lake bed itself, underlying bedrock, rocks exposed in the drainage basin or subsurface rock in the catchment basin, which can be intercepted by subsurface flow. Perhaps the most unusual mineral feed is at Lake Assal in Djibouti, where sea water flows by gravity through a subterranean channel, dissolving additional salts from the subsurface rock channel, before discharging these feedwaters to the bottom of Lake Assal.
A specialized formation is seen on certain atolls, such as Niau in French Polynesia. In the case of Niau, most of the atoll area is covered by a central depression holding a swampy hypersaline lagoon. This formation is presumably derived from isolation of a marine pool, with successive evaporative concentration of salts, much like a giant tidepool; an alternative theory is that the lagoon was originally a freshwater depression, subsequently invaded by subterranean channels carrying ocean water into the lagoon.
While the high levels of dissolved mineral salts typically limit the biodiversity of higher level organisms found in hypersaline lakes, there are generally a profusion of bacteria, archaea and algae present in these water bodies. In many cases such as Lake Urmia in Iran and Mono Lake in California, there are also lifeforms such as brine shrimp; correspondingly the presence of brine shrimp often engenders feeding use of hypersaline lakes by large numbers of migratory birds. In many cases there are important higher lifeforms at the fringing marshes to hypersaline lakes; for example, the peripheral marshes of Lake Urmia support a number of amphibians and other higher lifeforms, notably including the endangered Lake Urmia newt.
In some cases such as the Makgadikgadi Pan in southern Africa, the lake completely dries out in the dry season. In this locale many of the lifeforms are able to survive in a desiccated form until the lake is replenished by rainwater and runoff during the next precipitation cycle.
There are occurrences of hypersaline lakes on every continent. The following provide an assortment of examples in different world regions:
Don Juan Pond, Antarctica
Don Juan Pond in Antarctica is the most extreme example of hypersalinity on Earth; in fact, this small lake has such high mineral concentrations that it never freezes, even though ambient termperatures are commonly in the range of minus 30 degrees Celsius.
This hypersaline lake is located east of Ross Island in Wright Valley in the region of the McMurdo Dry Valleys, in a cold desert biome.
Makgadikgadi Pan, Botswana
The Makgadikgadi Pan in Botswana is an expansive set of salt pan depressions in southern Africa. This ephemeral lake formation blooms with microbial life, brine shrimp and hundreds of thousands of flamingos in the rainy season. Originally it was an enormous permanent prehistoric lake formed by a late Pleistocene uplift blocking the southerly flow of a now vanished African river. This region is part of the earliest habitat sites in the prehistory of man, where significant advances in stone tool technology developed prior to migration northward of Homo sapiens.
Lake Urmia, Iran
Lake Urmia is a shallow landlocked lake in northwest Iran, whose surface area and volume have been shrinking dramatically over the last several decades.
The drainage basin consists chiefly of arid and semi-arid lands including part of the Dasht-e Kavir of the Iranian Plateau.
No species of fish are presently sustained; however, at least seven amphibian taxa occur in Lake Urmia fringing marshes and its tributaries, notably including the Lake Urmia newt (Neurergus crocatus), a vulnerable species inhabiting rivers draining to the lake.
Dead Sea, Israel and Jordan
The Dead Sea, forming a portion of the border between Israel and Jordan, is the deepest hypersaline lake on Earth. This water body has only one principal source, the Jordan River.
The deep basin was formed from the continental rifting induced by plate separation at the margin of the Arabian Plate severing from the African continent. This water surface represents the lowest point in absolute elevation on Earth.
The Dead Sea's chief salts are magnesium and potassium anions paired with chloride cations, at 52 and 37 percent of the total salinity respectively. Sodium occurs at quite low levels. Bromide cations are also present.
Mono Lake, USA
Mono Lake in California is situated at the southwest edge of the Great Basin. With elevated pH level and a rich assortment of dissolved minerals, Mono Lake comprises a highly productive aquatic ecosystem, albeit of limited biodiversity. As a well defined stable basin, Mono Lake is arguably one ot the oldest lakes in North America.
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