Relative Importance of Stream-borne Particulate and Dissolved Phosphorus Fractions to Lake Tahoe Phytoplankton Lorin K. Hatch, John E. Reuter and Charles R. Goldman Present address: Water Resources Center, Univ. Minnesota, St. Paul Tahoe Research Group, University of California, Davis      Algal bioassays using stream water additions from seven Lake Tahoe basin mountain creeks were used to assess short-term bioavailability of stream P to Lake Tahoe phytoplankton during the 1996 spring snowmelt. Results indicate that 75-90% of bioassay response is due to nutrients in the < 0.45 _m range. Lake Tahoe phytoplankton are primarily P limited, although N colimitation has been observed during summer. Bioassay response did not correlate well with particulate P >0.45 _m, but was highly correlated with dissolved inorganic P. Dissolved organic P may also be stimulatory to Lake Tahoe phytoplankton. Bioassay response was relatively constant for each treatment during the study, due in part to a consistent diatom dominance of the phytoplankton community. The bioassay technique utilized is not sensitive enough to detect changes in land use along a single stream. Dissolved inorganic P was determined to best represent Lake Tahoe short-term (several days) stream bioavailable P at the present time. Physical bioavailability factors such as lake thermal dynamics and biological factors such as lake microbial dynamics should be addressed in addition to the chemical factors when considering lake biologically available. Published in Canadian J. Fisheries and Aquatic Sciences 56: 2331-2339 (1999)