Mono Lake is an alkaline and hypersaline lake in Mono County, California that is a critical nesting habitat for several bird species and is an unusually productive ecosystem.

Geology

Mono Lake is believed to have formed at least 760,000 years ago, dating back to the Long Valley eruption. Sediments located below the ash layer hint that Mono Lake could be a remnant of a larger and older lake that once covered a large part of Nevada and Utah, making it among the oldest lakes in North America.

Mono Lake is a terminal lake in a watershed fed from melting runoff with no outlet. Dissolved salts in the runoff thus remain in the lake and raise the pH and the salt concentration.

Mono Lake is in a geologically active area at the north end of the Mono-Inyo Crater volcanic chain of the Long Valley Caldera. The geological activity is caused by faulting at the base of the Sierra Nevada, and is associated with the crustal stretching of the Basin and Range Province.

Volcanic activity continues in the Mono Lake vicinity: the most recent eruption occurred 350 years ago at Paoha Island in Mono Lake. Panum Crater (on the south shore of the lake) is an excellent example of a combined rhyolite dome and cinder cone.

Limnology

The lake contains approximately 280 million tons of dissolved salts, with the salinity varying on the amount of water in the lake at any given time. Before 1941, the salinity was approximately 50 grams per liter (g/l) (compared to a value of 31.5 g/l for the world's oceans). In January 1982, when the lake reached its lowest level of 6,372 feet (1,942 m), the salinity had nearly doubled to 99 g/l. In 2002, it was measured at 78 g/l and is expected to stabilize at an average 69 g/l as the lake replenishes over the next 20 years.

An important aspect of the end of water diversions was an onset of "meromixis" in Mono Lake. Prior to the end of water diversions, Mono Lake was typically "monomictic," which means that at least once each year the deeper waters and the shallower waters of the lake mixed thoroughly, thus bring oxygen and other nutrients to the deeper waters. In meromictic lakes, the deeper waters do not undergo this mixing. In meromictic lakes, the deeper layers are more saline than the water near the surface, and are typically nearly devoid of oxygen. As a result, meromixis greatly changes a lake's ecology.

Mono Lake has undergone meromictic periods prior to the one commencing in 1994, and this most recent meromictic period brought on by the end of water diversions ended by 2004.

Ecology

The hypersalinity and high alkalinity (pH=10 or equivalent to 2.5 grams of NaOH per liter of water) of the lake, means that no fish are native to the lake. An attempt by the California Department of Fish and Game to stock the lake failed. The lake is famous for the Mono Lake brine shrimp, Artemia monica, a tiny species of brine shrimp, no bigger than a thumbnail, that are found nowhere else on earth. During the warmer summer months, an estimated 4-6 trillion brine shrimp inhabit the lake. The brine shrimp feed on microscopic planktonic algae which reproduce rapidly during winter and early spring after winter runoff brings nutrients to the surface layer of water. By March the lake is "as green as pea soup" with photosynthesizing algae. Brine shrimp has no food value for humans, but is a staple for birds of the region. Also an important food source, alkali flies (Ephydra hians) live along the shores of the lake and walk underwater encased in small air bubbles to graze and to lay eggs. The whole food chain of the lake is based on the high population of single-celled algae present in the warm shallow waters.

Mono Lake is a vital resting and eating stop for migratory shorebirds and has been recognized as a site of international importance by the Western Hemisphere Shorebird Reserve Network. Nearly 2,000,000 waterbirds, including 35 species of shorebirds, use Mono Lake to rest and eat for at least part of the year. Some shorebirds that depend on the resources of Mono Lake include American Avocets, Killdeer and sandpipers. Over 1.5 million Eared Grebes and phalaropes use Mono Lake during their long migrations.

Late every summer tens of thousands of Wilson's Phalaropes and Red-necked Phalaropes arrive from their nesting grounds, and feed until they continue their migration to South America or the tropical oceans respectively.

In addition to migratory birds, a few species spend several months to nest at Mono Lake. Mono Lake is the second largest nesting population of California Gulls, second only to the Great Salt Lake in Utah. After abandoning the landbridged Negit Island in the late 1970s, California gulls have moved to some nearby islets and have established new, if less protected nesting sites. Cornell University and Point Reyes Bird Observatory have continued the study of nesting populations on Mono Lake that was begun over 20 years ago. Snowy Plovers also arrive at Mono Lake each spring to nest along the remote eastern shores.

credited to wikipedia and flickr: Bill Wight CA