Atmospheric Deposition of Phosphorus to Lake Tahoe, CA-NV Marie S. Liu, John E. Reuter, and Charles R. Goldman Tahoe Research Group University of California, Davis      Increases in nutrient inputs, especially phosphorus, into Lake Tahoe are contributing to the rapid decrease of the LakeÕs famous clarity. A significant amount of phosphorus can be input through the atmospheric deposition of particles that contain the nutrient or have phosphorus adsorbed to the surface. At Lake Tahoe, atmospheric inputs are responsible for an estimated 26% of the annual phosphorus external inputs. Atmospheric phosphorous is especially available for algal growth because it falls directly into the photic zone, increases during the dry months at Tahoe during favorable algal growth conditions, and has a higher bioavaliability portion than stream inputs. The particles also add to the fine sediment loads that additionally decrease clarity. Bulk deposition measurements along Lake TahoeÕs north shore were collected from July through September 2000 to better understand this phosphorus source. There were large spatial variations in the magnitude of total phosphorus, but the sites seem to follow similar temporal patterns. (Figure 1) During this period ca. 44-61% of the atmospheric phosphorus was biologically available. These observations may indicate that there are several large-scale processes that drive deposition with localized sources playing a smaller role. Many of the deposited particles were 0.5 - 1 microns indicating potentially far travel distances and long setting times. (Figure 2) Scanning electron microscopy and mass spectrometry can help indicate the sources of atmospheric phosphorous. Understanding these sources help focus management efforts to best control new phosphorous inputs into the lake.