Investigating Long-term Environmental Trends in Central Ontario Lakes Impacted by Cyanobacterial Blooms
blue-green algae , paleolimnology , climate change , aquatic ecology
Cyanobacterial blooms degrade water quality by increasing turbidity, causing taste and odour problems, depleting deep-water oxygen concentrations, and producing toxins – all of which can alter aquatic food webs, and depreciate the social and economic value of waterbodies. A common driver of blooms is nutrient enrichment; however, climate-related factors, like surface water temperature, intensity and duration of thermal stratification, precipitation, and wind speed, are also important predictors. Consequently, climate change is expected to increase the spatial extent, severity, frequency, and duration of cyanobacterial blooms. Reports of blooms have increased in recent decades in Canadian lakes, but a lack of long-term monitoring hinders attempts to identify the causes. This thesis examined environmental indicators preserved in lake sediment cores to reveal multi-century trends in water quality and investigate drivers for these recent cyanobacterial blooms. In a remote oligotrophic lake, marked increases in cyanobacterial microfossils in surficial sediments and an increasing trend in primary production since ~1930 CE, in the absence of nutrient enrichment, suggest a climatic driver for recent unprecedented Dolichospermum blooms. In three rural northeastern Ontario lakes, eutrophication in ~1930 CE potentially associated with forest removal and settlement was tracked in diatom assemblages in two of the lakes. However, bloom occurrence in these lakes over a half-century later was associated with distinct diatom species shifts, indicative of enhanced thermal stratification. In Callander Bay, Lake Nipissing, a climate-mediated shift from polymictic conditions to sustained summertime stratification, and increased bottom water anoxia and internal nutrient loading since ~2000 CE, are linked to recent cyanobacterial prevalence. At eight additional sites across Lake Nipissing, diatoms in modern and pre-industrial era sediments revealed strikingly similar assemblage shifts indicative of lake-wide enhanced thermal stratification, indicating more favourable conditions for blooms. Estimates of baseline nutrient and hypolimnetic oxygen concentrations derived in this study can be used to guide management targets. Collectively, increasing primary production tracked over the last several decades (without parallel increases in nutrient enrichment) across all study lakes has likely occurred due to regional warming and a longer ice-free growing season, and invokes climate change as an important driver of cyanobacterial blooms in temperate lakes.