A diatom-based assessment of multiple stressors over the past ~220 years on shallow Cranberry Lake in the Rideau Canal (Ontario, Canada)

Cranberry Lake (Ontario, Canada), the shallowest lake in the Rideau Canal system, has experienced multiple stressors over its ~220-year history including flooding, settlement, invasive species, and anthropogenic climate change. It was formed through hydrological manipulations starting with the inundation of surrounding marshlands to build two mill dams (1803-1816) followed by extensive flooding (1831) during canal construction. We track the inception and environmental history of this mesotrophic, polymictic lake using diatom changes preserved in a ²¹⁰Pb-dated sediment core. Diatoms first accumulated in the record ca. 1800 CE, signalling the transition from a marshland to an emergent lake. A shift in dominance from an epiphytic assemblage to open-water taxa ca. 1830 CE marked the establishment of present-day Cranberry Lake following canal flooding. Despite this substantial ecosystem alteration, the most pronounced change in the diatom record occurred ca. 1980 CE, with increases in planktonic taxa. Regional temperatures accelerated over this time, particularly since the 21^st century, and nuisance algal bloom reports began in 2012. Monitoring data available from 2014 to 2023 indicated stable lake nutrient concentrations, suggesting recent diatom shifts correspond with limnological changes associated with longer, warmer ice-free periods. Warming-related alterations may also be driving blooms, as reported from other regional lakes.