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 ni trogen (N) to phosphorus (P) limitation has been attributed primarily to at mospheric N loading. This places an increased importance on controlling wat ershed movement of P to the lake. A stream water quality monitoring data se t consisting of nine streams in the Lake Tahoe basin has been analyzed to c haracterize the spatiotemporal variation of P delivery to the lake. This da ta is from the Lake Tahoe Interagency Monitoring Program (LTIMP), which pro vides scientific data for planning and regulatory agencies to address envir onmental problems in the Lake Tahoe basin. Results indicate that P delivery (concentrations, loads) varies greatly at interannual, seasonal, and spati al scales. Annual and seasonal total P (TP) concentrations can vary up to t hree orders of magnitude in a given stream and are strongly associated with suspended sediment. Particulate P is the major form of P transported by Ta hoe streams and was strongly correlated with percent surficial geologic dep osits, which are primarily located near streams. Tahoe streams with the hig hest annual P concentrations often had the lowest annual P loads, and visa versa. P loading is greatest during the spring snowmelt (75% of annual aver age). Potential watershed parameters influencing P delivery to Lake Tahoe h ave been identified as precipitation, basin area, basin steepness, and road and human development coverage. Results also suggest that human developmen t impacts on stream P loads are most prevalent during high precipitation ye ars. Identification and quantification of stream sediment and P sources suc h as streambanks and impervious surface is necessary to aid in watershed re storation efforts.