Lake Tahoe Phytoplankton Biomass and Community Composition Debbie Hunter Environmental Science and Policy University of California, Davis      Phytoplankton growth, biomass and community composition are central to lake biology. As the primary producers, the quantity and quality of phytoplankton determine the productivity of higher organisms. Additionally, phytoplankton physiology affects lake chemistry and clarity. A clear understanding of phytoplankon dynamics leads to more accurate models of lake nutrient budgets and clarity.      These small plants are sensitive to changes in the aquatic environment and can be useful as biological indicators. Tracking phytoplankton community composition over long periods can lend supporting evidence for changes that have already occurred and predictive patterns for long-term changes.      Figure 1 represents phytoplankton biomass and community composition over ten years (1992- 2001). These monthly samples have been collected at the middle of the lake. They are composites from the euphotic zone. Over the 10-year span it is easy to spot the annual season cycles of the phytoplankton biomass. Some years have higher overall biomass than other years but notably there is not a significant upward trend over time. The flora is variable from year to year with some predictable patterns among the phytoplankton taxonomic groups. Typically the highest biomass occurs in the spring. Diatoms continue to dominate the phytoplankton assemblage, especially during periods of peak biomass. However, the predominant diatom species from one year to the next is not predictable. Dominance is sometimes shared among several diatom species. At other times dominance is claimed by one species for most of the year. One of four species is generally a candidate for dominance, Stephanodiscus alpina, Cyclotella ocellata, Asterionella formosa, or Synedra radians.      Cryptomonads and Chrysophytes are present in the assemblage every month of the year. The same can be said for Dinoflagellates and Chlorophytes, but to a lesser degree. Cryptophytes are secondarily important within the phytoplankton assemblage. However, the Cryptophytes are typically found in the deep chlorophyll maximum for six months of the year. Chrysophyte biomass trails behind the Cryptophytes but their vertical distribution is more ubiquitous. In 1994 and 1998 there were blooms of the Chrysophyte Uroglena americana that elevated the relative importance of that phytoplankton group.      Ongoing data analysis will compare phytoplankton biomass and composition from the 1970Ős and 1980Ős. Changes in the community assemblages will render evidence of nutrient limitation shifts as well as changes in the vertical distributions.