DIATOM PALEOLIMNOLOGY OF TWO FLUVIAL LAKES IN THE ST. LAWRENCE RIVER: A RECONSTRUCTION OF ENVIRONMENTAL CHANGES DURING THE LAST CENTURY

Water quality degradation is a serious concern for the St. Lawrence River. While some environmental data are available for the St. Lawrence ecosystem, long‐term monitoring data are generally lacking. To infer past environmental changes, we undertook a paleolimnological assessment of diatom assemblages preserved in four 210Pb‐ and 137Cs‐dated sediment cores from two fluvial lakes in the river, and used diatom transfer functions to infer past shoreline habitat characteristics. At sites in Lake Saint‐François, a fluvial lake downstream from Cornwall, water quality decreased this century in response to human impacts (e.g. macrophyte density and nutrient levels increased). These trends were apparent from an increase in epiphytic diatom taxa, followed by an increase in eutrophic planktonic taxa. Water quality, however, appears to have improved somewhat in response to rehabilitation measures during the last two decades. From a sediment core near Montréal (Lake Saint‐Louis), we also noted a large proportion of eutrophic and epiphytic taxa, but less evidence was recorded of a recent improvement in water quality. The diatom‐based inference model for habitat characteristics appeared to reconstruct environmental conditions in the St. Lawrence River during the last century. The most notable shift has been an increase in diatom taxa commonly associated with macrophyte substrates. Trends in some of the planktonic diatoms were similar to those recorded in paleolimnological investigations from Lake Ontario, but cores from the river also may be reflecting local environments. This study shows that diatom‐based paleolimnological studies are possible in large river systems, if coring sites (e.g. fluvial lakes) are carefully selected.