Mono Lake, nestled in California’s eastern Sierra Nevada mountains, is a unique saltwater lake. Its composition and characteristics differ significantly from the ocean. It possesses extraordinary salinity and alkalinity. This unusual chemistry supports a specialized ecosystem found almost nowhere else.
The Unique Chemistry of Mono Lake
Mono Lake’s water has a high concentration of dissolved salts. Unlike ocean water, which is primarily sodium chloride, Mono Lake contains a diverse mix of carbonates, sulfates, and chlorides of sodium, along with borates. This unique blend makes the water approximately 2.5 to 3 times saltier than the ocean, though its exact salinity fluctuates with water levels.
The lake also exhibits high alkalinity, with a pH around 9.8 to 10, comparable to household glass cleaner. This high pH is due to abundant carbonate and bicarbonate ions. This distinctive chemistry also leads to the formation of tufa towers, limestone structures composed of calcium carbonate. These towers emerge when calcium-rich freshwater springs beneath the lake react with the lake’s carbonate-rich, alkaline water, causing calcium carbonate to precipitate and accumulate.
Life Adapted to Extreme Conditions
Mono Lake’s extreme hypersaline and alkaline conditions support a highly specialized ecosystem where few species thrive in immense numbers. The Mono Lake brine shrimp (Artemia monica), found only here, is one such organism. These tiny crustaceans, numbering in the trillions during warmer months, adapt to high salinity by transporting ions and water to maintain internal balance.
Another inhabitant is the alkali fly (Ephydra hians), which spends three of its four life stages underwater. These flies possess a waxy coating that allows them to “scuba dive” by trapping a protective air bubble. Both brine shrimp and alkali flies form the base of the lake’s food web, providing a food source for millions of migratory birds. Birds like California Gulls, Eared Grebes, and phalaropes rely on this abundant supply during their journeys, making Mono Lake an important stopover site in the Western Hemisphere Shorebird Reserve Network.
How Mono Lake Became Salty
Mono Lake’s high salinity results from its geological and hydrological characteristics. It is an “endorheic basin,” a closed lake with no natural outlet to the ocean. Water flows into the lake from surrounding mountains, carrying dissolved minerals and salts. The only way for water to leave is through evaporation, which leaves salts and minerals behind. Over thousands of years, this continuous inflow and evaporation have progressively concentrated salts in the lake.
Human activities accelerated this natural process, particularly in the 20th century. Beginning in 1941, Los Angeles diverted freshwater streams feeding Mono Lake into its aqueduct system. This reduction in freshwater inflow caused the lake level to drop substantially and its salinity to nearly double, threatening its unique ecosystem. While diversions exacerbated salt concentration, the fundamental reason for Mono Lake’s saltiness is its natural geology as a terminal lake in a closed basin.