Long-term assessments of ecological effects of anthropogenic stressors on aquatic ecosystems from paleoecological analyses: challenges to perspectives of lake management

Paleolimnological techniques are important for determining background ecological conditions and ecosystem responses to stressors when long- term data sets are absent. Research at the Dorset Environmental Sciences Centre (Ontario, Canada) has included paleolimnological studies to determine the effects of anthropogenic stressors on shield lakes, including eutrophication, acidic deposition, and climate change. Diatom- based total phosphorus (TP) inferences suggest that [TP] has declined in some lakes, despite increased inputs from anthropogenic sources. When coupled with siliceous algae- based pH inferences that suggest landscape- scale long- term acidification, these results indicate that the ecological effects of anthropogenic P inputs are being masked by the cumulative effects of multiple stressors. Detailed stratigraphic analyses of lakes that have experienced severe anthropogenic disturbances indicate that despite measured and inferred recovery in epilimnetic [TP] to predisturbance concentrations, pelagic communities have not recovered to predisturbance community composition; profundal communities and the variables associated with water quality (e. g., hypolimnetic oxygen) have also not recorded recovery. This suggests that (i) typical modeling approaches to quantify the effects of anthropogenic inputs on lake water [TP] should be used with caution and (ii) lake management approaches that follow the paradigm of "water quality recovery = biological recovery" may not be applicable to ecosystems being affected by multiple anthropogenic stressors.