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.
While nutrient availability plays a key role, our paleolimnological
inferences suggest that climate-driven changes in fundamental lake
physicochemical properties may be contributing to, or even triggering,
recent cyanobacterial blooms in these lakes.