Cyanobacterial blooms in a warming climate: Paleolimnological assessments of three Boreal Shield lakes in central Ontario, Canada

Paleolimnological techniques were used to assess long-term water quality changes in three cyanobacterial bloom-impacted lakes in Algoma, Ontario, Canada. Since the 2000s, frequent cyanobacterial blooms have been reported in these lakes despite stable nutrient levels, allowing investigation of recent climate warming as a possible environmental driver of the blooms. While diatom and chironomid community changes varied among the lakes, accelerated warming (~1990) led to near-synchronous shifts in paleolimnological indicators, including increased sedimentary chlorophyll a, planktonic diatoms (Discostella stelligera, elongate taxa), and chrysophyte scales. Diatom-inferred total phosphorus and chironomid-inferred hypolimnetic oxygen models indicate oligo-mesotrophic conditions prior to ~1950, with slightly higher late-summer oxygen concentrations in Desbarats and Bright lakes. Our results suggest a shift towards enhanced thermal stability due to regional warming, supported by instrumental records documenting rising air temperature, reduced wind speed, and a longer ice-free period observed in the last half century. Our paleolimnological inferences indicate that climate-driven changes in fundamental lake physicochemical properties and internal nutrient loading may be contributing to, or even triggering, recent cyanobacterial blooms in these lakes.