IMPACTS OF LAND USE AND CLIMATE ON LAKE CHAMPLAIN (USA-CANADA)

Lake Champlain includes large shallow embayments in which eutrophication and food web disruptions are severe, and where global warming is expected to have its greatest effects. To understand better how climate and land use might regulate trophic state and biological structure, we analyzed cores from several embayments for indicators of nutrient availability, algal productivity, geochemisty (C, N, P, biogenic Si), pigments, diatoms, cyanobacterial and chlorophyte microfossils, and C and N stable isotopes. Here we discuss the results for Missisquoi and St. Albans Bays. The 1.5-2 m long cores included sediments deposited over a 6500-8000 year period.

Sediments from the warm mid-Holocene (7000-4500 BP) and Medieval Warm Period (800-1200) were richer in weathered minerals (Fe, Mn) and organic C and N than those from intervening period. At the time of European discovery, St. Albans Bay was oligotrophic, while Missisquoi Bay was mesotrophic, possibly reflecting its larger catchment and shallower mean depth. Between 1760 and 1880, logging and land clearing for agriculture resulted in 80-90% deforestation of the surrounding catchments. Concurrently, both bays exhibited mild eutrophication, with nutrient and pigment accumulation rates increasing by two-four fold. More extensive eutrophication occurred in St. Albans Bay between 1900 and the mid-1930s, concomitant with sewer installation, but was delayed in Missisquoi Bay until the 1980s. Missisquoi Bay evolved to eutrophy coincident with a tripling of local dairy herds and increased cultivation of fertilizer-intensive crops. The overall sequence of phytoplankton species replacement was similar in the two bays despite decadal differences in timing, with initial increases involving primarily chlorophytes and cryptophytes, and elevated cyanobacterial abundance characterizing the later, most severe stages.

Efforts to restore the bays have included banning of phosphate detergent, tertiary treatment of sewage and agricultural best management plans emphasizing P control. Our cores show that annual P storage in sediments has declined since 1990. However algae are N-limited and continue to maintain pre-restoration plan biomass due to sustained high N inputs. Return to pre-20th century levels may be impossible due to atmospheric N deposition and warming climate.