A MULTIPROXY PALEOLIMNOLOGICAL STUDY OF LAKE CHAMPLAIN, USA-CANADA

Changing land use and human population growth in the Lake Champlain basin from 1760 to present have had significant impacts on the trophic status of the lake. Due to the lake's large size and complex morphometry, however, not all regions of the lake have been equally affected. We collected 7 sediment cores from widely spaced locations throughout the lake to assess site-specific trajectories of anthropogenic enrichment and its consequences. Four additional cores will be collected in summer 2007. Cores were analyzed for a large array of indicators of trophic state, including diatom composition, soft algae microfossils, paleopigments, C and N stable isotopes, organic content, phosphorus fractions, total N, and biogenic silica. All assays suggest that Lake Champlain was borderline oligotrophic-mesotrophic during the 17th century. With land clearing in the 18th century, high inputs of organic matter and sediment occurred in some lake regions, and with them a small amount of eutrophication. Most lake eutrophication has been post-1950, however, and thus probably is related to fertilizer spreading, phosphate detergents and shoreline development. Indicators of eutrophication include rising concentrations of photosynthetic pigments, organic carbon, available and total phosphorus, nitrogen, and biogenic silica; falling concentrations of UV-protective pigments (as water transparency declined); decreasing C:N ratios, and increasingly negative delta 13C values. All major algal groups have been present in the lake throughout its history and have become more abundant as nutrient availability has increased. However, paleopigments indicate that the main algal beneficiaries of eutrophication have been cryptophytes, diatoms and bloom-forming cyanobacteria.