Phosphorus has surpassed nitrogen as the most imminent threat to Lake Tahoe's internationally famed water clarity, which has been declining an average of one foot a year for the past 25 years. This was one of the findings from the UC Davis Tahoe Research Group presented at a recent meeting focused on new scientific findings and long-term trends. The direct culprits of murkier water are algae, whose growth is limited by the nutrient in shortest supply. Once, nitrogen was the least available nutrient, but air pollution from the urban Bay Area and wind-blown dust from the agricultural Central Valley, swept into California's Sierra Nevada mountains by prevailing winds and deposited into the waters by rain and snow, tipped the scales. Now, studies suggest, the key limiting nutrient is phosphorus, which comes mainly from sediment and soil associated with land erosion and land disturbance. "Our goal is to enable agencies to plan in a way that will protect the quality of the lake," says John Reuter, a UC Davis researcher and director of the Lake Tahoe Interagency Monitoring Program.