In some parts of the world, saline lakes are common features. Take, for instance, the image below, from our January 2017 article about fires in Argentina. But saline lakes are an environment unto themselves.
Lakes cover about 4 percent of the Earth’s land surface. Many of the largest ones (by area) are salty: Utah’s Great Salt Lake, the Caspian Sea (arguably the world’s biggest lake), Iran’s Lake Urmia, and the Dead Sea. Unlike marine and brackish waters, saline lakes typically form inland, and do not connect to the ocean. They tend to be ephemeral, filling with water in periods of increased rainfall, and drying out under the Sun.
In general, the saltier the lake, the fewer animals that can tolerate it. Yet a number of invertebrates call saline lakes home. Brine shrimp, for instance, have evolved to live in the salty, low-oxygen environment of the Great Salt Lake. The shrimp—also called “sea monkeys”—can survive even as water recedes. They are so plentiful, in fact, that fishermen corrale them using oil booms in an annual harvest.
Just how salty are these lakes? That depends on location and season. The Dead Sea has a salinity of 34 percent, while the Great Salt Lake varies between 10 and 30 percent; the same is true of Lake Urmia. (For comparison, open ocean waters average a 3.5 percent salt content.)
Australia’s scorching hot weather and scant rainfall make it a hotbed for saline lakes—thousands of them. In her story on the colorful salt lakes Down Under, my colleague Kathryn Hansen describes how they formed:
Millions of years ago, declines in rainfall caused river flows to ebb and river valleys to fill in with sediment. Wind then sculpted the loose sediment to form the lake basins that remain today. (The wind also sculpted some of the lighter sediments into parallel dunes that fringe each lake downwind to the east-southeast.) Some of the lakes now fill with runoff directly from the Stirling Range; others are controlled primarily by groundwater.
The lakes below in Western Australia range from pea soup-brown to pinkish in hue. Their color changes based on sediments, aquatic and terrestrial plant growth, water chemistry, algae, and hydrology.
At Urmia, the rise and fall of lake also has an effect on water color:
The color changes have become common in the spring and early summer due to seasonal precipitation and climate patterns. Spring is the wettest season in northwestern Iran, with rainfall usually peaking in April. Snow on nearby mountains within the watershed also melts in the spring. The combination of rain and snowmelt sends a surge of fresh water into Lake Urmia in April and May. By July, the influx of fresh water has tapered off and lake levels begin to drop.
The fresh water in the spring drives salinity levels down, but the lake generally becomes saltier as summer heat and dryness take hold. That’s when the microorganisms show their colors, too.
While many salt lakes vary in size according to rainfall, some like Lake Urmia, have been shrinking in recent years.
Hot, sunny days help create saline lakes by evaporating massive amounts of water, but salt lakes can also occur in cold climes. For instance, Don Juan Pond sits in Antarctica’s McMurdo Valley, where winter temperatures can drop to -50 degrees Celsius (-58 degrees Fahrenheit). Don Juan is so salty that waters rarely freeze. Its extreme environment resembles that of Mars. While the lake is far too salty and cold for even salt-loving brine shrimp, it does house microorganisms, Brown University geologist, Jay Dickson, told the NASA Earth Observatory.
“There is certainly biology in the vicinity of the pond and some evidence for biologic activity in the pond itself, but this activity could be explained by abiotic processes,” Dickson said. “Mars has a lot of salt and used to have a lot of water.”