Mono mania: you can't drink the water, but brine shrimp and alkali flies prosper in a mineral-rich California lake

Natural History, June, 2004 by Robert H. Mohlenbrock

As my wife Beverly and I drove down U.S. Highway 395, heading south toward Lee Vining, California, we crossed over the crest of a pass and found a large, silvery lake spread before us. Covering sixty-six square miles, the waters of Mono Lake fill a deep depression in the center of a basin-shaped landscape. Beyond the lake we saw gray-topped mountains. Known as the Mono Craters, these mountains are volcanic in origin, and the gray we were seeing was not snow, but ash and pumice from eruptions that took place as recently as 600 years ago.

Approaching the lakeshore, we could see that the water was bordered by grotesque gray formations; others pierced the lake surface from below. All were so-called tufa towers, formations of calcium carbonate deposited by the mineral-rich lake water, which is two and a half times saltier than seawater and eighty times more alkaline.

The federal government has designated the lake and its surroundings the Mono Basin National Scenic Area, and the natural attractions are under the care and management of the Inyo National Forest and the Mono Lake Tufa State Reserve. A modern visitor center overlooks the west side of the lake.

The Mono Basin was formed 3 million or 4 million years ago, as the Sierra Nevada was uplifted to the west. Water runoff from the mountains created Mono Lake, because the surrounding basin has no natural outlet. When the last ice age ended, about 12,000 years ago, water from the melting glaciers enlarged the lake until it was 900 feet deep and covered 358 square miles. Since then, however, Mono Lake has been evaporating and shrinking. And as that process continues, the salts and minerals washed into it by the mountain streams become increasingly concentrated.

On average, the region gets only ten inches of precipitation a year, mostly as snow: far too little to counter the drying trend. In addition, in 1941 the city of Los Angeles began to divert the water from four of the five major streams that enter Mono Lake. Within four decades the lake dropped forty feet and its surface area shrank from eighty-six to sixty square miles. Then, after much debate between officials from Los Angeles and people concerned about the future of Mono Lake, the State Water Resources Control Board issued a decision in 1994 that protects the lake and the streams that feed it.

The peculiar tufa towers owe their existence to calcium in freshwater springs entering beneath the lake and mixing with the lake water. The result is the formation of solid structures of a porous calcium carbonate rock also known as travertine. The tufa towers originally formed below the surface around the mouths of springs, but some of them have become partly exposed by the falling water level, and others are now totally stranded on the shore. The ones in the readily accessible South Tufa Area are between 200 and 900 years old.

On the north shore of Mono Lake lies Black Point, a 576-foot hill originally created underwater by volcanic eruptions. As its top cooled and contracted, it developed narrow crevices more than fifty feet deep. Today you can walk inside the crevices and view tufa towers thought to be about 10,000 years old.

Although sometimes referred to as a dead sea, Mono Lake supports ample life. Quite recently, in fact, Richard B. Hoover and Elena V. Pikuta, both microbiologists at NASA's National Space Science and Technology Center in Huntsville, Alabama, discovered some previously unknown bacterial species that five in the lake's salty, alkaline, oxygenless mud. (These investigators are on the prowl for the kinds of organisms that might survive in extreme extraterrestrial environments.) More conspicuous species are green algae, brine shrimp, and alkali flies.

In winter, when the single-celled algae reproduce unhindered, the lake turns a pea-soup green. At other times of the year, the algae become food for the brine shrimp and the larval and adult flies. The shrimp, which grow to about half an inch long, are active from April to October, laying eggs that overwinter on the lake bottom. Alkali flies can walk into the lake in an air bubble and lay their eggs on tufa. After hatching, the larvae grow for three or four weeks and then pupate. (The Paiute Indians who lived in the area used the pupae for food.) The adult flies begin to emerge and mate in the late spring. Most live for just a few weeks, but some adults, as well as some eggs, survive the winter to begin the cycle anew.

The shrimp and flies are major food sources for eighty species of migrating birds. An estimated 1.5 million eared grebes, 50,000 Wilson's phalaropes, 50,000 California gulls, and 200 snowy plovers visit Mono Lake each spring and summer. Red-necked phalaropes stop off on their way to their winter home in South America.

HABITATS

Alkaline flat Trees are absent in the salty flats that surround the lake, but two shrubs, greasewood and rabbit-brush, are common. Saltgrass forms mats, above which protrude prickly Russian thistle, smotherweed (also called bassia), sweetscent (better called stinkweed!), and western tansy mustard. The attractive alkali buttercup also grows here.