Assessing shifts in diatom communities in eastern Ontario recreational lakes in relation to land-use and climate changes over the past ~150 years using a top-bottom paleolimnological approach.

Eastern Ontario, Canada, is a lake-rich, rural region, located primarily over Precambrian Shield. These lakes are typically nutrient-limited with total phosphorus (TP) less than 20 µgL ^-1 , and serve as the primary tourist attraction, contributing substantially to the local economy of this region. However, in recent years, residents have become concerned about the perceived increase in nuisance algal blooms. Due to the lack of long-term water quality monitoring data little information exists on pre-impact conditions, and water quality trends of these lakes. To address this gap, a top-bottom paleolimnological approach was used to examine diatom community shifts in 35 presently nutrient-limited (TP 5–19 µgL ^-1 ) lakes across the Mississippi and Rideau River watersheds of eastern Ontario. Shifts in diatom taxon between bottom and top sediment layers align with climate change indicators, suggesting increasing temperatures, longer ice-off periods, and reduced wind speeds, across the region. A spatial analysis conducted to determine if present-day TP concentrations correlated with modern land-use patterns revealed that the percentage of croplands and wetlands in nearshore riparian buffer zones (300 m) was positively corelated with present-day TP concentrations of the lakes. An increasing trend in the relative abundance of mesotrophic diatoms with lower sinking rates was observed across our study lakes, which is consistent with lake response to climate warming and nutrient enrichment. This research demonstrates that both land-use and climate change have had impacts on lake ecosystems in eastern Ontario over ~ 150 years.