Before scientists can recommend how to rescue Lake Tahoe, they must first learn more about the different particles that invade and reduce transparency in the deep Sierra lake, say UC Davis environmental researchers. For example, the researchers are trying to understand the different clarity problems created by sediments and by the growth of phytoplanktons, a group of microscopic, algae-like plants, says Alan Jassby, a research ecologist with the environmental science and policy department, in a paper co-authored by scientists Charles Goldman, John Reuter and Robert Richards, all of the Tahoe Research Group. In addition, researchers need to know more about the size and density distribution of particles that enter the lake before erosion control efforts can be most effective. Knowing the clarity problems created by different particles would help inform the resource-management decisions to be undertaken in the Tahoe Basin -- knowledge that is critical in deciding how to spend the millions of federal and state dollars earmarked to restore and preserve the lake's famous clarity, Jassby says. "It is important to find this out because you'd take different measures to mitigate the problem. Through erosion control measures, perhaps you could stem 99 percent of the particles from erosion, but it wouldn't necessarily make a difference if the problem is with tiny clay particles or with the nutrients for the phytoplankton," Jassby said. "We now understand the mechanisms for seasonal and interannual variability, and we can judge whether a shift in transparency is a short-term phenomenon, or a longer-term change that requires a management response. In deep lakes such as Tahoe, it is not enough to institute erosion control measures that target total suspended sediment discharge if the relevant-sized particles get through unhampered," Jassby and the others say in the paper published in the journal Limnological Oceanography. "Indeed the larger, less important particles are the most likely to be removed by watershed management practices and the resulting improvements to the lake may be far less than anticipated," the authors conclude. The water quality in Lake Tahoe has steadily declined since Goldman began studying the lake in the late 1950s; algal growth has led to the loss of a third of the lake's transparency, harming its long-term health. Yet, he and other scientists believe that with appropriate action, the degradation can be reversed, but they caution, the work must begin promptly. Otherwise, within 10 to 12 years, irremediable damage will occur. Data collection for the researchers' work was supported by contracts from the state of California, the Tahoe Regional Planning Agency and the U.S. Geological Survey through the Lake Tahoe Interagency Monitoring Program. A U.S. Environmental Protection Agency grant supported data analysis for the journal article.