Clear Lake, Calif., is a shallow, 40,000-acre body of fresh water that lies about 100 miles north of San Francisco. For centuries, it was home for a large colony of Western grebes, lovely birds that swim with the stately grace of swans and dive as skillfully as loons. But 15 years ago, in an environmental tragedy unwittingly perpetrated by man, large numbers of grebes began dying off, and the once-clear waters of the lake turned murky and green. Now, by introducing a new ecological cycle, scientists have saved Clear Lake's grebes and even clarified its water.
The grebe's problems began in the late '40s when the local mosquito abatement district sprayed thousands of pounds of DDD, a chlorinated hydrocarbon pesticide, on Clear Lake to rid the area of swarms of buzzing black gnats. The chemical, a close cousin of DDT, worked so well that developers previously put off by the gnats began building houses around the lake.
Unhappily, after an absence of several years, the gnats returned. In 1954 and again in 1957, stronger doses of DDD killed them off. About the same time, the lake's population of grebes began to decrease, dropping from 1,000 pairs to only 20 within one year. The baffling change was explained in 1962 by Rachel Carson, in Silent Spring. Grebes, she wrote, feed mainly on fish. The fish, in turn, eat insect larvae and zooplankton, and these foods had become saturated with the DDD dumped into Clear Lake. Thus, over a long period, the grebes accumulated lethal amounts of the long-lasting pesticide in their tissues and died by the hundreds. Even worse, because of the DDD in their eggs, thousands of grebes never hatched. Between 1958 and 1963 only one young bird was seen at Clear Lake.
Meanwhile, the proliferation of houses around the lake was having another, equally unforeseen effect. Household wastes, laden with nutrients, seeped into the water and fertilized algae. By 1961, the lake and its beaches were covered with green slime.
Planting Fish. What could be done to clean up the mess? The mosquito abatement district switched from the persistent DDD to methyl parathion, a chemical that is effective against gnats but that deteriorates and becomes harmless in a short time. At the same time, the district hired a team of scientists from the University of California at Davis to find a way to control the gnats biologically. Led by Entomologist Sherburne F. Cook Jr., the team decided that a small fresh-water smelt, the Mississippi silverside, might find the gnats appetizing. In 1967 they "planted" 3,000 fingerlings in the lake.
The silversides have multiplied prodigiously. They not only eat the gnats but also compete for the nutrients that stimulate algae growth. As a result, the algae are disappearing, and the lake has regained 80% of its original clarity. No longer troubled by DDD, the grebes are making a comeback. This year 82 young birds were born in the area, four times as many as last year. But an ecological balance is not easily restored; large game fish now have to be imported to feed on the wildly proliferating silversides. Once the game fish are established, Californians have to learn that the fishing is good at Clear Lake, and then . . .