Mono Lake

One of the most thoroughly studied saline lakes is Mono Lake, which lies on the western edge of the North American Great Basin just east of the Sierra Nevada. With recent salinities in the range from 70 to 90 g l_1, a pH of about 10 and very high concentrations of bicarbonate and carbonate, it is an alkaline, saline lake. As is often typical of saline lakes, Mono Lake is productive: rapidly growing algae support a simple food web that includes very abundant brine shrimp, Artemia monica, and an alkali fly, Ephydra hians, which in turn feed thousands of birds. No fish occur in the lake. The lake is a major breeding site for the California gull (Larus californicus), and is a critical stop-over for migrating phalaropes (Phalaropus spp.) and eared grebes (Podiceps nigricollis). The streams that flow into Mono Lake from the Sierra Nevada are a plentiful source of freshwater that were tapped by the City of Los Angeles by a complex diversion scheme initially implemented in 1941. Largely as a consequence of this interbasin transfer of water, the lake's level had fallen about 14 m and its salinity doubled from 1941 to 1982. Laboratory experiments indicated that further increases in salinity were likely to have profound impacts on the ecology as photosynthesis was found to decline about 10% for each 10% increase in salinity, and survival and reproduction ofthe brine shrimp was found to be increasingly impaired to the point where cyst hatching would cease if salinities were to increase by about 50% from their 1980 values. If diversions by Los Angeles were to have continued unabated, this salinity would have been reached within several decades. The end result in the mid-1990s of almost two decades of litigation and environmental assessment was modifications to the water rights of the City of Los Angeles, which led to higher lake levels. In contrast to the dismal conditions at a number of saline lakes, such as the Aral Sea, and continuing declines in level at other lakes, such as Walker Lake (Nevada), the resolution of the contest at Mono Lake is a good example of how scientific expertise can contribute in a positive way to solutions of environmental problems.

As was observed in eastern African soda lakes, climatic variations as well as diversions have significant influences on Mono Lake. In the early 1980s, California experienced substantially above-average snow and rainfall resulting in a large rise in lake level and chemical stratification that blocked the complete vertical mixing that usually occurred during the winter. Ammonium, which would have been replenished in the upper lake, accumulated in the deep water, but remained very low in the euphotic zone. Since Mono Lake is a nitrogen-limited lake, phytoplankton abundance and productivity declined. The combination of resumed diversions and drought conditions led to sufficient evaporative concentration to weaken the chemical stratification and permit wind and cooling to turn over the lake in the late 1980s, entrain ammonium-rich water, and restore higher algal biomass and productivity. After a series of years with winter mixing and average productivity in the early 1990s, diversions were curtailed in the mid-1990s, as ordered by the revised water rights agreement, and California experienced above average precipitation. Mono Lake became meromictic again with subsequent reductions in productivity. Multiyear records of annual primary productivity by phytoplankton have conspicuous differences as a function of meromictic or monomictic conditions. During meromixis, the development of persistent anoxia below the chemocline alters other chemical conditions with biological consequences. Methane and dissolved sulfide accumulate, and bacterial communities adapted to metabolize reduced forms of elements become active.

Artemia monica is the only macrozooplankter in Mono Lake. Each year a first generation hatches from overwintering cysts, matures, and produces a second generation via release of live nauplii. A small third generation sometimes occurs, but very few animals survive through the winter. Besides exerting strong grazing pressure on the phytoplankton, Artemia regenerate ammonium that supports algal growth. Artemia are an important food for large numbers of gulls breeding at the lake in the spring and for as many as one million grebes in the autumn. Some life-history characteristics of Artemia are indicative of differences in algal abundance and primary productivity. Although large numbers of eggs are produced in all years, on average, fewer cysts and live nauplii are produced during meromictic years, and maturation of the first generation can be slowed and fecundity and body size reduced as compared to nonmeromictic years.

Changes in the Artemia populations translate to influences on the birds feeding at the lake. The fledging rate per pair of California gulls reflects their clutch size and prefledging survival, both of which should be influenced by the adult food supply. In fact, fledging success was low immediately following the onset of meromixis and remained low during the subsequent 3 years of meromixis in the 1990s.

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