Saline Lakes

Saline lakes narrowly fit into the biome concept because of their heterogeneous origin and fluctuating water regime; instead, they are azonal ecosystems, meaning that their ecological features are not predictable from the general climatic zone in which they occur. Despite their stepping-stone geographic distribution, the global volume of saline lake water (24,950 cubic miles, or 104,000 cubic kilometers) is almost as great as that of the world's freshwater (29,989 cubic miles, or 125,000 cubic kilometers). Saline lakes are common landscape features on every continent, except in warm deserts, as this biome is in general too dry to allow their presence.

However, as river catchments become somewhat less arid, there are depressions containing saltwater for at least some time after episodic rain events. Asian steppes and neighboring semiarid regions located southward are spattered with many of the world's saline lakes, livening up the ancient Mongols' home turf. Some are large and permanent lakes, such as the Caspian and Aral Seas, and Lakes Balkhash, Issyk-Kul, Chany, Alakul, and Tengiz. Others are small and temporary, like some Central Anatolian gölü (Turkey) and many nuur of Mongolian steppes.

Out of Asia, the wealth of saline lakes increases with the many potholes of the Canadian prairies, the playas of the United States and Mexico, the high-Andean salares, the lacke of southeast Austria, the Hungarian sodic szék, north African and Middle East chotts and sabkhas, the Ethiopian Rift Valley salt lakes, the South African pans, the lagunas of the Mancha Húmeda Biosphere Reserve and Andalusian campiñas, and the saladas of the Monegros region (Spain). Saline lakes are common even in some regions of Antarctica, like the Vestfold, Bunger, and Larsemann Hills, the McMurdo Dry Valley, and west of the Antarctic Peninsula.

How Saline Lakes Form and Their Function

Saline waters have a brackish taste. Their salinity is more than 10 percent that of seawater, which has a fairly constant sodium chloride composition. However, inland saline waters show a variable ionic composition because their dissolved salt content does not come from either dilution or concentration of seawater. Many saline lakes are like bathtubs without a drain, from which water can only get out one way, through evaporation. This usually occurs when the lake basin is the bottom of a large catchment, like Great Salt Lake (United States) or Lake Eyre (Australia). On the other hand, saline lakes may be fed with salts from either saline groundwater discharges (e.g., the Davsnii Lake in Mongolia), or the washing out of other salt lakes, evaporite rocks like gypsum, or some volcanic rocks (e.g., Mono Lake in the United States). All of these sources of salts combine unevenly in all saline lakes. Thus, terminal depressions at the end of large catchments used to be tectonic, where associated fractures act as pipes through which groundwater discharges to the lake bed, like the Sidi El Hani Sebkha in Tunisia.

Nonetheless, salinity of groundwater discharging into a lake may be the result of geochemical evolution of water in its flow along regional paths within large aquifers; strikingly, this might be composed of rocks with a low salt content, like quartz sandstone; this is the case of Laguna de las Torres in Spain. The diversity in the origin of salt lake basins and in their water inputs is a forcing function of the sharp patterns in spatial heterogeneity and fluctuations through time, controlling saline lake ecosystems. Thus, the water regime of endorheic saline lakes with large catchments are most sensitive to one or more global circulation phenomena, such as the El Niño Southern Oscillation (ENSO), the North Atlantic Oscillation, or monsoons; the higher the number of global phenomena involved, the sharper the fluctuations. Accumulated effects of positive and/or negative anomalies (wet and dry periods) provide complex fluctuation scenarios at different timescales.

This is the case, for example, of Mar Chiquita (province of Córdoba, Argentina), where three different climates affect its huge catchment, which are controlled by ENSO, the South Atlantic anticyclone, and the Amazonian tropical cell. Its flooded surface shifts from about 772 square miles (2,000 square kilometers) in dry periods to more than 3,861 square miles (10,000 square kilometers) during wet periods, making it the fifth-largest salt lake in the world. Moreover, fluctuations of local scope, rather than global, are more likely to affect saline lakes with small catchments, such as those of Mediterranean zones. These are narrow strips of land in the west coasts of the continents and around the Mediterranean Sea, with short-river, densely drained watersheds.

Summer drought and late-summer to early-autumn heavy storms feature in the Mediterranean climate, but their occurrence is very irregular through time and space. For example, the Laguna de Salicor (Mancha Húmeda Biosphere Reserve, Spain) suffered a severe thunderstorm (super cell) between May 20 and 26, 2007, that poured 9.4 inches (240 millimeters) of rain in 24 hours; as a result, the water column depth shifted from less than 6.3 inches to 7.9 feet (16 to 240 centimeters) between April and June 2007, and water salinity decreased from 172 to 2.38 g l-1.

Life on Mars

Saline lakes are like different planets for Earth biota. Oceans contain plenty of life, despite their salinity. Nonetheless, environmental gradients are extreme in saline lakes, where marine organisms would feel like a fish out of water. Saline water biota must avoid dehydration because of the difference in osmotic pressure between the cells and the environment. Many microbials do this by accumulating “friendly” solutes in their cells. However, osmotic concentration rockets during desiccation in temporary salt lakes, and water salinity increases beyond that of seawater (hypersaline waters). Whether saline lakes are always or eventually hypersaline, extremophiles are then the dominant characters; they do not just tolerate hypersaline waters, but actually require them for growth. Ecosystems of permanently flooded, relatively deeper saline lakes are also commonly controlled by a strong, yet vertical gradient, called chemocline. This forms when a water layer lies over a denser one in a lake water column because of their differences in chemistry, that is, salinity (meromixis). Lakes Lyons and Mahoney (Canada), Big Soda (United States), Chaunaca and El Molino palaeo-lakes (Bolivia), the Salada de Chiprana (Spain), Lakes Van (Turkey), Shala (Ethiopia), Qinghai Hu (China), Panggong Tso (Indian Tibet), Gnotuk (Australia), and Bonney (Antarctica) are classic examples of meromictic salt lakes.

Like thermal stratification, meromixis avoids deepwater deficient in oxygen (O₂) mixing with upper, aerated water layers. In low, dark water layers, sulfate reduction is an alternative to photosynthesis and O₂-consuming decomposition to obtain energy. Sulfate-reducing bacteria can do that by “breathing” the large amounts of sulfur present in salt lakes as either organic matter or dissolved sulfate (SO^2-₄). This form of anaerobic respiration gives off hydrogen sulfide (H₂S) as a waste; it is toxic for O₂ respiring organisms, and its rotten-egg odor is a marker of this process in saline lakes. Most H₂S reacts with metal ions in the water to produce metal sulfides, such as ferrous sulfide (FeS); these are dark colored and not soluble in water, leading to the typical black or brown mud of salt lakes.

Still, some H₂S escapes upward and supports surprisingly dense life crowds at the chemocline interface. Phototrophic green and purple sulfur bacteria bunch there, taking advantage of both the sunlight from above and the H₂S below, as an electron donor in photosynthesis, instead of H₂O. Accumulations of phototrophic bacteria are usually accompanied by similar layers of other organisms, either slightly above or below them. This microbial loop replaces or just couples to the conventional big-fish-eats-small-fish food chains in saline lakes; it provides the means for cycling significant amounts of energy and matter under such harsh conditions for life. The resulting layered structure is analogous to that of microbial mats, which are typical of some saline aquatic habitats where sunlight reaches the sediment–water interface, but spanning centimeters instead of millimeters, respectively.

Knowledge about the microbial component led to scientists disregarding former misconceptions of extremely saline lakes as sterile, like the Dead Sea. Actually, bacterial photosynthesis often exceeds algal production during the winter months; for example, up to 85.5 percent of total production in Deadmoose Lake (Canada), or 17.1 percent of the total annual planktonic primary production. Regarding maximum daily rates, gross primary productivity may vary between 7.67 and 7.34 g C m-2 d-1 in macrophyte- and microbial mats-dominated saline lakes of the Mancha Húmeda Biosphere Reserve (Spain), and 10.15 g C m-2 d-1 in phytoplankton-dominated salt ponds in Israel. These figures indicate that maximum productivity rates are similar, whatever the functional group of primary producer and the lake salinity. These are comparable to or higher than productivity rates of other types of lakes and wetlands that are highly productive ecosystems. On the other hand, respiration rates may also be highest in saline lakes because of increased needs for energy to cope with extreme salinities. As a result, the range of net primary productivity rates in saline lakes is very wide, even in the same site across time.

The subsequent pulses in primary production of saline lakes may result in such contrasted trophic patterns along time that a single site may behave as two distinct ecosystems in different moments. This occurs in both permanently and temporarily flooded saline lakes. For example, in the Laguna de Mar Chiquita (Argentina), food webs may involve combinations of either microbial-planktonic-waterfowl communities that are usual in saline lakes, which are dominated by the Andean flamingo, or planktonic-fish communities dominated by the Argentinian silverside. Similarly, primary production in the Laguna de Salicor (Mancha Húmeda Biosphere Reserve, Spain) regularly supports a waterfowl population that is hardly higher than 600 individuals per month; however, in the breeding seasons 2007–09, a few thousand birds were recorded during several months; species richness also increased from 7 to 21 species, respectively.

Although biodiversity patterns are rarely documented under such contrasted trophic patterns, it may be logically induced that saline lake fluctuations increase their biodiversity, whether the changes involve primary productivity, hydrology, and/or hydrochemistry. Nevertheless, the negative correlation observed between species richness and salinity, over a broad range of salinities, underlay once upon a time the general assumption that salinity was an important determinant of saline lakes biota. Salinity is negatively correlated with species richness and community composition when salinity is lower than 10 g l-1, but not at intermediate or high ranges of salinity (i.e., 10–30 and 100–200 g l-1, respectively), according to more recent and precise studies. In other words, salinity is not the only, or perhaps not the most important factor, which determines the occurrence of a particular taxon in a saline lake.

Saline lakes are select attributes of certain landscapes in all continents; they are usually found in groups of saline water islands surrounded by land. At the scale of the continents, these groups are linked by biogeographical relationships. For example, saline lakes of the Mediterranean basin, east-central Europe, the Middle East, and central Asia were refuges for biota during glaciations; genetic exchange between them was then very active, but it decreased during subsequent interglacial periods, such as in the present time. As a result, new species appeared, and endemisms and vicariant distributions became increasingly common. It is the case of the submerged herbs of the genus Althenia; although its complete distribution is not known, it extends from the Atlantic coast of France and Morocco through the Mediterranean to Asia, with isolated records from Turkey, Iran, and south-central Siberia.

Similarly, the presence of large branchiopod crustaceans up to 0.86 inch (22 millimeters) long in the Mancha Húmeda Biosphere Reserve was thought to be unique among Spanish salt lakes, and reveals unsuspected affinities with those of central Asia. These communities appear in temporarily flooded habitats, unlike their relative (the brine shrimp Artemia), which is common in permanent hypersaline waters. Branchiopods are ancient crustaceans; they are too large in size to escape predators; as a result, their distribution is restricted to extreme environments that are saline, temporarily flooded, turbid, and/or cold. The three former attributes feature many saline lakes, like those of the Mancha Húmeda.

Ecologist Ramón Margalef did field observations of these communities; he found out how the islands of water that saline lakes remain in are presently connected. One of the most typical shoreline birds of saline lakes is called Charadrius alexandrinus (Kentish plover). Birds and flying insects exploit saline lake resources in an alternating way; they move around looking for available resting, refuge, food, and/or breeding sites. Usually, these resources are not available in all sites of a saline lake district at the same time, which increases overall biodiversity. For this reason, the biota of saline lakes usually follow a metapopulation pattern, that is, a great population (saline lake district) consisting of small populations (sites). Trip distance is extreme in the case of intercontinental seasonal migrations, which are reserved to waterfowl.

Insect migrations are not limited to the exchange between locations within a saline lake district. Soil beetles of the carabid group, for example, are able to fly and disperse locally. However, their populations also move both horizontally and vertically within a single lake, depending on the temporal variations in soil salinity and water content. Actually, carabids are major cyclers of matter and energy in the shores of saline lakes, thanks to their diverse feeding strategies, including omnivores, predators, carrion feeders, and granivores. The outermost belt of saline lake biota is featured by the presence of plant communities that support a varying degree of occasional flooding and soil salinity. These include many glassworts of the Salicornia genus and Limonium sea lavenders, and Micronecmum coralloides, another example of east–west Mediterranean disjunct species. Some are considered halonitrophilous because they indicate relative soil enrichment in nitrogen after decomposition of the high amounts of organic matter accumulated in their shores.

Habitats on a Human Scale

Saline lakes are not marginal habitats for humans. One of the most common species found in saline lakes is an introduced one, cattle skeletons. The present inability to adapt to saline lake environments, in which large-scale economy is poor, plays a crucial role in the evolution of human attitudes toward saline lakes. As a result, saline lakes are frequently used for spilling solid urban waste and sewage water. Adaptation to marginal habitats may be limited by consequences of demographic characteristics of marginal populations. Salt lakes are not found in the most densely populated parts of the planet, but even so, 500 million people live in regions where saline lakes are a common feature of the landscape. Genetic, developmental, and functional constraints also limit the adaptation to marginal habitats. However, functions, values, services, and products of saline lakes are abundant and valuable. Many have market value. Saline lakes are important sources of minerals such as uranium, lithium, zeolites, and sodium chloride. The brine shrimp Artemia and its cysts are a base for the aquaculture industry, the common green algae Dunaliella is a very important source of carotene and glycerol, and the cyanobacteria Spirulina is used for the food industry.

Salt lakes and their muds also have a great interest for salt and mud therapy. Important spa resorts were born in saline lakes, like in Mar Chiquita (Argentina) and Laguna de la Hijosa (Mancha Húmeda Biosphere Reserve, Spain), and some of them remain active as locally strategic economic alternatives, like the Playas de La Mancha (Spain). Occasionally flooded salt flats are aesthetic icons that are frequently used by the publicity and film industries, or even for land speed records. This is the case of the Bonneville Salt Flats (United States), where portions of the movies Pirates of the Caribbean: At World's End, Independence Day, and many others have been filmed. Desiccation polygons of salt flats throughout the world are extensively used as symbols of drought.

However, the cultural values of salt lakes are not just a matter of Hollywood and the mass media. Salt caravans are common in all continents, like that going from Salinas Grandes to Buenos Aires (Argentina) between the 17th and 18th centuries, or the 2,000-year azalai of Tuaregs between Bilma and Agadez (Niger). Cities, transport ways, territorial networks, and cultures were born at both sides of salt caravans. If populations of saline lakes were marginal, they would be sparse, fragmented, prone to isolation and civilization collapse, or they would be demographic sinks subject to immigration from highly populated areas. These and many more functions, values, services, and products of saline lakes are only possible thanks to their particular traits, including the flatness, desiccation, salinity, physiological adaptations to osmotic potential, protection against excessive sunlight, accumulation of proteins, extremophilous fauna, landscape uniqueness, and chromatic richness.

Saline lakes have been conserved in a relatively good condition until recent times. Irrigation agriculture was not feasible in their surroundings or with their water. However, technology has allowed intensive exploitation of groundwater and great diversions of surface waters, which were the main inputs to some saline lakes. As a result, the level of the Aral Sea, formerly the fourth-largest saline lake in the world, has fallen some 66 feet (20 meters) since 1960, increasing salinity and affecting its characteristic processes and communities. Similar impacts occur elsewhere in the world. On the contrary, the excess of water also affects many saline lakes.

Usually, it is because of the input of wastewater, after having passed (or not) through a water treatment plant. In this case, the impact is double, involving a change in hydrology and a decrease in water quality. Even worse is the deliberate filling of a salt lake from these water sources, allegedly for “restoration ecology” purposes; these projects aim at getting huge populations of waterfowl in a single site, whatever its hydrological and trophic functioning was. To make things even worse, the excess of organic matter, lack of recycling, and pollution frequently result in large waterfowl kills because of avian botulism outbreaks.

In 2022, included in an extensive government funding package was an appropriation of $1.25 million dollars for the US Geological Society to establish a regional Integrated Water Availability Assessment research program in the Great Basin in the western United States. A goal of the program was to preserve saline lakes for migratory birds that depend on them for resting, feeding, and breeding.