Long-term Limnological Changes in Six Lakes with Differing Human Impacts from a Limestone Region in Southwestern Ontario, Canada

Abstract

Diatom-based paleolimnological techniques have been used to track limnological changes over the last ~150 years in six oligomesotrophic, primarily shallow, hardwater lakes in southwestern Ontario. Three of the six lakes are located in national parks and consequently have had little-to-no direct human impacts in their watersheds, with the exception of logging following initial European settlement (A.D. ~1850–1910). The paleolimnological indicators suggest that these activities have led to subtle-to-moderate eutrophication and possibly a reduction of macrophyte abundances. Additional subtle limnological changes have been occurring over the last century even in the undisturbed control lakes. The marked increase of Cyclotella comensis suggests that alkalinity levels may have increased ~1950–1980. The remaining three lakes experienced additional disturbances following the 19th century logging. For example, a campsite has been located on the shore of Cyprus Lake since 1968, but study results indicate this minor disturbance did not significantly alter water quality. In contrast, the last two of the six study lakes have experienced significantly more intense human impact and showed markedly different trophic histories. For example, the catchment of Chesley Lake was heavily developed for agriculture, the shoreline was densely populated with cottages and macrophytes were deliberately removed from the littoral zone. The paleolimnological data recorded a reduction of the water quality following these disturbances. Cage aquaculture has been ongoing in Lake Wolsey since 1983. Diatom changes in the lake's recent sediment are consistent with increased nutrient concentrations. Collectively, these paleolimnological data provide lake managers with important historical data from which trajectories of past limnological changes can be inferred and realistic mitigation targets can be identified.