(Invited Talk) Water Quality Management in Lake Tahoe (CA-NV): The Role of Atmospheric Deposition and Climate Change

Lake Tahoe is renowned for its natural beauty and blue color. However, clarity has declined by 10 m since 1968. This is attributed to the influx of phosphorus, nitrogen and fine sediment particles <16 ƒÝm. The recent Tahoe TMDL quantified pollutant loading from all major sources including atmospheric deposition. The ratio of lake area to watershed area is small at 1.6. Using an approach based on deposition buckets and modeling we found that total N load per year over the entire lake surface was ~220 metric tons (MT), representing 55% of TN inputs from all external sources. The ratio of dry:wet TN deposition was 2.5:1. Dissolved inorganic-N loading (nitrate+ammonium) was ~150 MT or 68% of TN with a dry:wet ratio of 3.6:1. This accounted for nearly 80% of DIN loading from all external sources. Dissolved organic-N comprised 28% of the TN pool with similar dry and wet annual loading values. N-deposition over the lake was half that measured at a land-based station on the shoreline. Total phosphorus deposition to the lake surface was approximately 7 MT, accounting for 15% of P-load from all external sources. Soluble reactive-P accounted for 2.3 MT or about one-third of the TP. Areal deposition rates at Lake Tahoe for N and P were in the range reported in the literature. Road sand and terrestrial dust have been shown to be the major contributors to atmospheric-P. The high ratio of N:P in atmospheric deposition is considered the cause for a shift from N-P co-limitation to a current condition of primarily P-limitation in Lake Tahoe phytoplankton. Climate change could increase internal N and P loading and significantly change the current nutrient budget.