Saline Lakes and Saltpans

Abstract

There are several reasons why lakes are (or become) saline. Evidence lies in lagoons along open oceans where seawater may percolate through sandy barriers and lakes become saline or brackish, occasionally transforming their character. Further inland, saline lakes accumulate salt content by freshwater inflow and river discharge that carries minimal amounts of salt, and as a consequence salt concentrations increase in a basin (and a lake) through evaporation. Particularly if a basin is endorheic (without outflow), salinity can range from a few to over 300 % per mil with a tendency to increase concentration as the basin ages, allowing for the many different elements found in salt or brine. With the exception of the extraordinary ancient Dead Sea (the deepest hypersaline lake in the world exceeding 300 m in depth), concentrations of salinity in deep lakes is generally lower than in shallow lakes, as vertical circulation needs more time in deeper waters. In lakes comprising of various depths, salinity can vary from section to section clearly demonstrated in the Caspian Sea where the north is shallow and saline, the south is deep and almost fresh in character. Certain saline lakes no longer appear as lakes; a stable salt crust (over a brine) may cover its surface several metres thick such as the Salar de Uyuni in southwest Bolivia, the world’s largest salt flat at 10,600 km2. With climate changing over time to more arid conditions, a salt lake often transforms into saltpans with a salt crust intercalated with silt and clay, and becomes the uppermost strata of a sediment archive. Saline lakes are specific ecosystems and as salinity increases the number of species decreases toward negligible. In addition, several lakes became saline (or more saline) by anthropogenic influence over water input; the Aral Sea catastrophe has receded up to 90 % since humans diverted the two rivers that feed it for irrigation purposes. Saltpans in particular exhibit many additional aspects clearly discernable from satellite imagery: intensive colours (alternating with seasons), significant variations between shallow and deeper waters associated with evaporation deposits, and internal patterns left on salt crust such as networks or polygons formed by salt expansion and shrinkage. The combination of Halobacteria and Cyanophytes containing beta-carotene an organic compound, results in the red to orange colours of saline lakes and saltpans, and particularly the relationship between the genus Dunaiella salina and Halobacterium cutirubrum regulates the intensity of the colour.