Inhibition of Phytoplankton Production by UV-B Radiation in Lake Tahoe Pirjo S. Huovinen and Charles R. Goldman Dept. Biological and Environmental Science, University of Jyvaskyla, Finland Tahoe Research Group, University of California, Davis      Ozone depletion and the associated increase in biologically harmful ultraviolet radiation (UV-B) is a current global problem. In particular, its effects on natural waters and aquatic organisms is of considerable interest. UV-B inhibition of phytoplankton photosynthesis has been observed to a depth of 25 m in marine water and there is concern that because of its unusually clear optical properties, Lake Tahoe phytoplankton may be subject to the same phenomenon. The impact of incident UV-B radiation on Lake Tahoe primary productivity was measured in situ using the sensitive 14C method. Experiments were performed on two separate dates during the summer growing season. Ambient lake water was used so that natural populations of algae could be assessed. Incubations were done at 7 depths (0, 2, 5, 10, 20, 30 and 50 m) in flasks that either allowed all solar radiation to penetrate including UV-B (quartz bottles) or materials, which prevented UV-B transmission (Pyrexâ bottle with a Mylarâ wrap). Results were normalized to chlorophyll biomass to account for phytoplankton changes with depth and time. The attenuation of UV radiation was theoretically calculated using the one-dimensional radiative transfer code, DISORT.      The combined data on phytoplankton production relative to chlorophyll concentration from the two July experiments showed statistically significant (p<0.01) UV-B inhibition at 2 meters (56% reduction in algal growth). Clear UV-B inhibition was also seen at the surface (40% reduction), but this was not statistically significant (p=0.086). Inhibition was also observed deeper in the water column with reductions from UV-B exposure ranging from 4% to 25%. According to these results, the inhibition of photosynthesis in clear lake Tahoe was not seen as deep as in the oceans where inhibition down to 25 m has been observed. According to theoretical calculations the depth of 10% UV-B penetration was approximately 10 m suggesting potential for UV-B inhibition below 2 m. Published in Verh. Internat. Verein. Limnol. 27: 157-169 (2000)