A PALEOLIMNOLOGICAL ASSESSMENT OF THE TROPHIC STATUS OF LAKE CHAMPLAIN (USA AND CANADA) USING DIATOM MICROFOSSILS AND GEOCHEMICAL INDICATORS

Cultural eutrophication is a well-documented phenomenon occurring in lakes throughout the world. Anthropogenic activities in the basin surrounding a lake are often the main cause for the nutrient enrichment that leads to eutrophication. To assess changes in a northeastern United States lake, several sediment cores were collected from Lake Champlain (USA and Canada). Each site chosen has its unique relationship to anthropogenic impacts. In this study, which presents a subset of all the cores we have analyzed, we used a multiproxy approach involving: diatoms, organic matter content (%C), carbon/nitrogen ratios (C/N), and bulk organic matter stable carbon isotopes. All proxies were analyzed at 1 cm resolution.

In the most recent sediments, diatom microfossils document a shift to more eutrophic conditions. This is indicated by the increased abundance of Fragilaria crotonensis and Aulacoseira ambigua. The %C and C/N values remain stable until approximately 1900, when organic matter content starts increasing and C/N values begin a declining trend. In the top interval of the cores, C isotope values decrease concurrently with the observed decrease in C/N ratios. All three geochemical proxies (%C, C/N ratios, and C isotopes) suggest that the organic matter input to the sediments is predominantly of algal origin. Thus, together, they also indicate an increase in trophic status in Lake Champlain. The recent change in trophic status can be attributed to anthropogenic activities, which include shoreline development as well as other disturbances that lead to increased nutrient runoff from the lake's large watershed.