Stream phosphorus transport in the Lake Tahoe basin, 1989-1996

Lake Tahoe is undergoing the initial stages of cultural eutrophication due to human alteration of the airshed and watershed. The lake's switch from nitrogen (N) to phosphorus (P) limitation has been attributed primarily to atmospheric N loading. This places an increased importance on controlling watershed movement of P to the lake. A stream water quality monitoring data set consisting of nine streams in the Lake Tahoe basin has been analyzed to characterize the spatiotemporal variation of P delivery to the lake. This data is from the Lake Tahoe Interagency Monitoring Program (LTIMP), which provides scientific data for planning and regulatory agencies to address environmental problems in the Lake Tahoe basin. Results indicate that P delivery (concentrations, loads) varies greatly at interannual, seasonal, and spatial scales. Annual and seasonal total P (TP) concentrations can vary up to three orders of magnitude in a given stream and are strongly associated with suspended sediment. Particulate P is the major form of P transported by Tahoe streams and was strongly correlated with percent surficial geologic deposits, which are primarily located near streams. Tahoe streams with the highest annual P concentrations often had the lowest annual P loads, and visa versa. P loading is greatest during the spring snowmelt (75% of annual average). Potential watershed parameters influencing P delivery to Lake Tahoe have been identified as precipitation, basin area, basin steepness, and road and human development coverage. Results also suggest that human development impacts on stream P loads are most prevalent during high precipitation years. Identification and quantification of stream sediment and P sources such as streambanks and impervious surface is necessary to aid in watershed restoration efforts.