A diatom-based paleolimnological re-assessment of previously polymictic Lake Opinicon, Ontario (Canada): crossing an ecological threshold in response to warming over the past 25 years

Long-term changes in diatom community composition provided important insights into how multiple stressors affected shallow, macrophyte-dominated Lake Opinicon, Ontario (Canada) over the past similar to 200 years. A previous paleolimnological study of a sediment core collected in 1995 found that diatom responses to numerous large-scale cultural disturbances since the early nineteenth century were moderate in comparison to pronounced diatom responses to similar disturbances in nearby deeper lakes within the Rideau Canal system. The abundance of macrophytes in shallow and previously polymictic Lake Opinicon likely played an important role in maintaining a stable, clear-water equilibrium state. We examined diatom assemblages from a sediment core collected in 2019 to re-assess whether Lake Opinicon maintained its resistance to change over the past similar to 25 years. Despite numerous, intense early nineteenth and twentieth century cultural disturbances in Lake Opinicon's catchment (complete deforestation, flooding with the construction of the Rideau Canal), the highest rate of diatom compositional change occurred only in the past similar to 25 years, when planktonic diatoms became prevalent for the first time in the lake's postglacial history. This recent shift in assemblage composition is not explained by nutrient enrichment, as total phosphorus concentrations, measured since the 1970s, have declined significantly. The first appearance of zebra mussels (similar to 1990s) and significant increases in Secchi depth broadly co-occurred with the diatom assemblage shift, but precipitous declines in mussel populations since 2013 and a continued increase in planktonic diatom taxa suggest the impact of this invasive species was modest. Instead, changes in diatom assemblage composition were strongly related to increasing regional air temperatures. Limnological monitoring indicated that, over the past few decades, previously polymictic Lake Opinicon has experienced increasingly longer and more stable periods of thermal stratification that are consistent with observed trends in regional warming and reduced wind speed. These water column changes, which accompanied reduced ice cover duration, would have provided favourable conditions for planktonic diatom growth. We conclude that the nature and high rate of diatom compositional change over the past similar to 25 years signifies that an ecological threshold was crossed in response to warming and changes in lake thermal structure.

Automated summary

Long-term changes in diatom community composition in Lake Opinicon, Ontario, Canada over the past 200 years provide important insights into how multiple stressors and environmental changes have influenced the lake. Recent shifts in diatom assemblage composition can be attributed to increasing regional air temperatures and changes in lake thermal structure.