Importance of Climate on Boreal Lakes from Northwest Ontario Over Millennia: A Paleolimnological Perspective
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The lack of long-term data on the response of boreal lakes to climate change has been seen as an impediment to the assessment of the vulnerability and risks that northwest Ontario faces in light of future climate change. The overall objective of this thesis was to provide a centennial-to-multi-millennial perspective on the impacts of past climate change on boreal lakes in northwest Ontario. Second chapter has resulted in the development and application of a paleoecological model, based on the modern-day distributions of chironomid assemblages to lake depth, in a small boreal lake. Changes in the chironomid assemblages provided significant and strong support for the diatom-based inference techniques that estimated lower water levels, and consequently drought-like conditions, throughout northwest Ontario during the Medieval Climate Anomaly. Third chapter provides evidence that supports an early-to-mid-Holocene period of aridity, with reduced water levels across the boreal region in northwest Ontario. This conclusion was based on changes in diatom assemblages in well-dated sediment cores from three lakes spanning a distance of ~200 km across the boreal forest region. During the early-to-mid-Holocene, benthic diatom taxa predominate in all cores, suggesting lower lake levels by ~2-5 m, when compared to modern-day diatom assemblages from these lakes. Increases in planktonic diatom taxa to greater than 50% of the entire assemblage, occurs beginning ~4500 to 4000 cal yr BP, suggesting positive water balance over the last 4000 years in comparison to the mid-Holocene period. Fourth chapter provides evidence of enhanced primary production and higher abundances of cyanobacteria in three northwest Ontario lakes during the warmer early-to-mid-Holocene in northwest Ontario. This later study was based on changes in lake-water primary production from fossil pigments, spectrally-inferred chlorophyll a, and diatom assemblages in well dated cores, taken from the main central basin of the three study lakes. The majority of indicators show a coherent pattern of enhanced primary production during the early-to-mid-Holocene period, with elevated concentrations of cyanobacterial pigments in two of the three lakes. If conditions become warmer and drier in the future in northwest Ontario, challenges associated to water quantity and quality should be expected.