A Paleolimnological Assessment of Eutrophication Trends in the St. Lawrence River Area of Concern Over the Past 70 Years
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Recent surface-water surveys suggest that high nutrient concentrations and nuisance algae remain an issue in the St. Lawrence River Area of Concern (AOC) at Cornwall, Ontario, particularly in the tributaries and nearshore zones of Lake St. Francis (LSF). Although extensive remedial actions have been completed in Cornwall and the surrounding area since its designation as an AOC, activities have primarily targeted the reduction of industrial and municipal pollution to the St. Lawrence River, with much less attention given to eutrophication. It remains unclear whether remedial actions first implemented in the 1990s have resulted in decreases to algal production and compositional changes to algal assemblages in the nearshore zones of LSF. This thesis employs a paleolimnological approach to provide a historical context for the present-day nutrient concentrations in LSF, as well as to determine whether actions associated with the Remedial Action Plan developed for the AOC have influenced algal community composition and concentrations. A sediment core was collected near the north shore of LSF and was examined for changes in the fossil diatom (Bacillariophyceae) assemblages, diatom concentrations, pigment concentrations, organic matter content, and ratios of stable isotopes (δ15N and δ13C). Diatom and pigment concentrations increased in the uppermost intervals of the core, suggesting that overall production has risen in the last few decades. Diatom assemblages were dominated by benthic taxa throughout the core. One significant change to the diatom relative abundances was observed, occurring at a depth corresponding to the late-1950s. Stable isotopes suggested a steady increase in agricultural activity since the mid- to late-20th century and a large input of terrestrial organic material to the aquatic ecosystem circa 1960. It is likely that intensive flooding resulting from the construction of the Moses-Saunders Power Dam upstream of Cornwall led to an influx of organic material from the catchment in the late-1950s, co-occurring with a diatom assemblage shift. The recent increase in algal production is likely attributable to increased nutrient concentrations resulting from intensive agricultural activity in the contributing watersheds. It is unlikely that delisting goals for eutrophication in the AOC will be reached without a drastic change in current practices.