In an optimistic outlook for the health of Lake Tahoe, the °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis Tahoe Research Group has found a second straight year of slight improvement in the Sierra lake's clarity. The average transparency for the year 1999, as determined from the average of about 35 measurements taken during the best viewing conditions, was 69 feet, or 21.04 meters. This is the second year in a row that there has been an improvement in transparency. In 1998, average transparency was 66 feet. In 1997, lake transparency reached a record low of 64 feet. "It would be comforting to conclude that we are gradually winning the eutrophication battle at Tahoe," said Charles Goldman, °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis limnologist and director of the research group. "Unfortunately, since 1968 we have observed that although we may have improved transparency for as many as five years in a row, which occurred following the 1983 El Nino, the trend of declining transparency in Lake Tahoe continues." Goldman said the variation that appears in the long-term trend depends upon a combination of environmental factors. The most significant impact comes from the extent of spring mixing of the lake's deep waters, which provides plant nutrients such as nitrogen and phosphorus to support algal growth. Only during the colder winters, where February and early March storms have enough energy to stir the lake at or near the bottom, does this algae-feeding mix occur. Such winters typically occur about every four years. These years of deep mixing provide the highest years of algal growth, and since they often are also accompanied by heavy snow pack and spring runoff, the inorganic contribution of sediment eroded from the streams and watershed also adds to the loss of transparency from enhanced algal growth. The transparency measurements are made from aboard the °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis research vessel the John Le Conte using the Secchi disk, which resembles a white dinner plate. These measurements have determined a pattern of average decline in transparency of about 5 percent a year. Without serious and effective efforts to control nutrient and sediment input into the lake, this declining trend in lake transparency will gradually result in converting Tahoe's famous cobalt blue waters to green. Projections show this trend of significant transparency loss would occur gradually over the next 30 years. State and federal agencies, together with the University of California, Davis, the University of Nevada, Reno, and the Desert Research Institute, have completed agreements to coordinate their efforts to repair the basin and stem the eutrophication and transparency loss of this California treasure. On Feb. 16, a historic pact was signed between the institutions to form the Tahoe Environmental Science System, to integrate research facilities and efforts in this common cause of lake restoration.