Assessing Diatom Shifts in Embayments and Lakes in and Around Georgian Bay (Lake Huron) Using Paleolimnology

Abstract

Freshwater ecosystems in Ontario are affected by a growing number of multiple stressors, with climate change often acting as a “threat multiplier”. Diatom-based paleolimnological approaches are used to provide the much needed long-term ecological information for embayments and lakes in and around Georgian Bay. In Chapter 2, the relative importance of nutrients and warming on the recent increase in sedimentary Asterionella formosa relative abundances in George Lake by comparing regional air temperature trends, 20 years of annually measured lake water nutrients (total nitrogen, total inorganic nitrogen, and total phosphorus), and long-term regional total inorganic nitrogen deposition data. Recent increases in A. formosa occur during the warmest period on record, even in the absence of nutrient enrichment. In Chapter 3, sedimentary diatom assemblages from two lakes in Killarney Provincial Park (north of Georgian Bay) are shown to clearly track the acidification of regional lakes around the turn of the 20th century in response to smelting activities in Sudbury, Ontario. Although the Killarney lakes showed signs of chemical recovery (increase in pH), the recent (post-1980s) diatom assemblages are not similar to either pre-acidification or acidification assemblages suggesting factors such as climate warming are partly driving recent diatom assemblage changes, rather than simply chemical recovery. In Chapter 4, diatom responses to shoreline development and climate warming in three embayments from southeastern Georgian Bay, two with shoreline development (impact) and one without (unimpacted). In the early 20th century, compositional changes in diatoms at the two impacted embayments are indicative of a modest increase in nutrient conditions. In the mid to late-20th and early 21st centuries, pronounced shifts towards higher abundances of lightly silicified pennate (Asterionella formosa and Fragilaria crotonensis) and/or small centric (Cyclotella ocellata, C. comensis, and C. gordonensis) planktonic diatoms at all three embayments were consistent with a response to warming and associated changes in thermal and other physical properties. Collectively, my studies show that diatom assemblages are tracking long-term changes in limnological variables, but with the prevalence of multiple environmental stressors in a warming climate, diatom assemblages are on a new trajectory and therefore unlikely to return to pre-industrial scenarios.