That Clear Lake can remain so fecund after enduring its share of human-caused stresses over the years is surprising. During the 1940s and '50s, when the pesticide DDD was used to eliminate a prevalent gnat, populations of grebes and other birds declined because the pesticide poisoned them. The grebe devastation that marked the initial discovery of the impacts of pesticides on bird populations merited a mention in Rachel Carson's 1962 book, Silent Spring, which is credited by many as launching the modern environmental movement. In addition, home and commercial development along the lake has contributed to lakeside erosion and eliminated much of the wetlands, further limiting birds and other wildlife and allowing additional nutrients to flow into the lake. Algae blooms that at times have covered vast areas of the lake have been exacerbated by sediments and nutrients draining off the shore into the lake. And then, in the 1970s, a diligent state fisheries biologist named Larry Week discovered that the catfish and bass in Clear Lake had elevated mercury levels. "Clear Lake was Larry's lake, and he had been reading about mercury poisoning in Japan. He knew about Sulphur Bank Mercury Mine, and he wondered if it was having an impact on fish here," says Terry Knight, a retired engineer and national environmental/outdoors writer living in Lakeport, the county seat of Lake County. The mercury pronouncement grabbed headlines. Knight says the lake gained its greatest notoriety when, in 1986, the popular summer tourist destination was one of the first major sports-fishing lakes in the state to be posted with a warning that levels of mercury in the fish were unhealthy. The state media broadcast Clear Lake's dilemma far and wide, Knight says. Just as the country headed into a recession, the lake's tourist economy took a dive. Knight believes Clear Lake regained its popularity as an outdoor resort only as fish in other Northern California lakes, rivers and even the San Francisco Bay Delta were found to carry similarly high mercury levels. Today, the state fish and game department warns people not to consume in one month more than a pound of Clear Lake largemouth bass that measure over 15 inches and no more than three pounds of Clear Lake catfish. Women who are pregnant or may soon become pregnant, nursing mothers and children under the age of 6 are told to not eat any fish from Clear Lake -- nor those caught in Lake Berryessa at the apex of Solano, Yolo and Napa counties. When UC Davis first became involved with the project, the idea that mercury had been seeping into Clear Lake through the porous dam of crushed waste rock bulldozed between the former mercury mine called Herman Pit and the lake was not, however, a foregone conclusion, says Tom Suchanek, director of the UC Davis Clear Lake Environmental Research Center. "We were hypothesizing that the mercury was coming from the mine, but others said the mercury was coming from natural geothermal springs on the lake bottom. We went out and tested springs all over the lake and found no evidence that the springs were a major source of mercury -- but the mine was a significant contributor." Others thought the mercury was washing into the lake through the Coast Range watershed. That idea was also tested and rejected. The research team's investigations showed that pit water that seeped through the dam into the lake actually increased in acidity, from the pit's pH of 3 to pH levels of 2.6 and 2.8. The change in acidity indicated that the water was picking up minerals along the way, Suchanek says. The researchers' hypothesis that the piles of exposed waste rock were a major contributing factor was reinforced in 1992 when the Environmental Protection Agency recontoured the dam. The bank, which had been built from mine tailings graded into a 40 percent incline, was given a more moderate slope, covered with topsoil and seeded with grass to stop runoff from the rains. The resulting drop in mercury levels in sediments next to the mine was dramatic, Suchanek says. But still, the team had no absolute proof that the site was the culprit. The big breakthrough came in 1995, just as the research team was about to wrap up its studies and hand over the findings to the EPA Superfund administrator. Heavy flooding that year caused the Herman Pit, which is 13 feet above the elevation of the lake, to overflow its banks and dump quantities of acid pit water into the lake. "That was when we first observed large clouds of white flocculent material in the lake, right where the pit had overflowed," Suchanek reports. When the pit's acidic water combined with the lake's alkali water, a clay precipitate, composed of aluminum, mercury, silicates and a quantity of iron, eventually created a yogurt-like coagulate several inches thick on the bottom of the lake. The researchers monitored the particles over the summer. What they found proved to be a key to the mercury mystery. As the sun warmed the shallow lake over the summer, the researchers saw that the conditions in the floc enhanced the concentration of toxic methyl mercury, likely through the activities of microorganisms. The acid mine drainage also contained large amounts of sulfate used by sulfate-reducing bacteria thought by the researchers to be responsible for at least a large part of the methyl mercury production. Through a series of experiments conducted by microbiologist Doug Nelson and his students, the role of the microorganisms and floc in producing methyl mercury became clearer. It appears that they transform the inorganic -- and thus non-absorbable -- mercury into a highly charged methyl mercury. Methyl mercury is what scientists call bioavailable -- a form that animals can absorb. This more toxic form also accumulates through the food web so that the biggest animals -- the bass and catfish -- have the highest concentration, according to Nelson, who is co-director of the Clear Lake Environmental Research Center and co-principal investigator with Suchanek on the mercury studies. Meanwhile, UC Davis environmental engineer Geoff Schladow charted the lake currents, discovering that prevailing northwest winds created a top current that, when it hit the eastern shore near the mine, turned into a westerly undertow, transporting the methyl mercury-laden floc across the bottom of the lake to the west side's farthest arms. "The lake is a methyl-mercury generating engine," Suchanek concludes. "The concentration of methyl mercury in the waters flowing out of Clear Lake is five times higher than that entering into the lake from the watershed." In the past two years, the Superfund team has concentrated on documenting how the entire system functions. Schladow has been tracing the movement of fluids from the pit to the lake. Nelson has been trying to find exactly where the acid mine drainage is entering the lake by looking along the lakeshore for hot spots -- areas with high concentrations of sulfates and very low pH levels. He has also been looking at the bacteria that are transforming the inorganic mercury into a bioavailable form. Soil scientist Vic Claussen has been learning how to revegetate the highly acid soil to keep erosion down and minimize the rain percolation. Geologist Ken Verosub is looking at the magnetic signature in the sediments to see if it correlates to the changing land use. Fish biologist Joe Cech is figuring out how fish take up mercury -- past the gills or by ingesting organic materials in the water. Former civil and environmental engineering graduate student Andy Bale put together a model of the accumulation of methyl mercury in the food chain. Meanwhile, bird ecologist and ecotoxicologist Dan Anderson has been studying the osprey and grebes to see if the mercury has had any impacts on the bird life. "The mercury in the fish doesn't seem to have an effect on these birds," Anderson reports. "Maybe the excess young are filling in the gaps. But we are finding one thing: These birds are depositing a lot of mercury in their feathers, which may be a mechanism to get rid of it as the birds molt." The UC Davis findings were handed over this winter to EPA Remedial Project Manager Ellen Manges. Still to be answered is how to fix the problem, Manges says; she will use the UC Davis studies to put together a set of findings that, after public hearings in Lake County, will result in a remediation plan. She expects the process to take at least another year. In the meantime, she's mindful of the lessons the Superfund site may bring to the Bay-Delta water-quality problems. Addressing the problem of the mercury-laden fish is considered just a small part of a bigger commitment to the Clear Lake environment, say those who live there. Lake County residents have become fierce environmentalists, raising funds so that the Lake County Land Trust can buy private land and rehabilitate wetlands for wildlife. Lake County Supervisor Karen Mackey expects some additional environmental challenges as the county attracts industries and more people to this land of paradise. "The key is the people who live here," Mackey says. "They will help drive that positive vision of the lake. People want to be where it is beautiful and where they can be committed to the land, lake, air and mountains."