Meta-analysis on the effects of chemical stressors on freshwater ecosystem functions

Background

Despite considerable progress in our capacity for predicting the response of different trophic levels to chemical stressors, generalizable relations between chemical stressors and ecosystem functions are lacking. We addressed this knowledge gap by conducting a meta-analysis (159 studies; 350 observations) on the responses of freshwater ecosystem functions (community respiration, organic matter decomposition, nutrient cycling, photosynthesis, and primary production) under controlled conditions (laboratory or outdoor mesocosms) to pesticides, pharmaceuticals, and metals.

Results

We identified monotonic dose–response relationships between standardized chemical concentrations, in terms of toxic units, for selected chemical use groups and organic matter decomposition by decomposer-detritivore-systems as well as photosynthesis by algae and macrophytes. By contrast, consistent relationships were not found for other ecosystem functions, such as organic matter decomposition by microbial decomposers alone and primary production under the conditions studied. Importantly, the shape and direction of the relationships identified here match those reported in field-based studies, indicating a decrease in functioning as chemical stress increases, strengthening the ecological relevance of our findings. Finally, we found a disconnect between regulatory ecological quality targets and ecological outcomes, highlighting a need to re-evaluate risk assessment approaches if they are supposed to be ecologically meaningful and protective of ecosystem functions.

Conclusions

Our analyses demonstrate the potential to derive chemical stress–ecosystem function relationships from currently available data, which already reveal adverse effects on specific ecosystem functions. However, several key research gaps must be addressed to improve the specificity and predictive power of such relationships. These include expanding the range of chemical stressors investigated, accounting for the context-dependency of chemical effects on ecosystem functions, and complementing laboratory studies with semi-field and field experiments to verify and validate laboratory-derived findings under more realistic environmental conditions.