Eutrophication from nitrogen and phosphorus pollution is a major stressor of freshwater ecosystems globally. Despite recognition of this problem by scientists and managers, qualitative and quantitative synthesis of scientific evidence is still needed to inform nutrient-related management decisions and policies, especially for streams and rivers. We conducted systematic reviews of the literature on nutrient stressor-response relationships and potential confounding environmental factors to support identifying, managing, and restoring aquatic resources impaired by eutrophication. We asked two questions: (1) What are the responses of chlorophyll-a (chl-a), diatoms, and macroinvertebrates to total nitrogen (TN) and total phosphorus (TP) concentrations in lotic ecosystems? (2) How are these relationships affected by other environmental factors? Consistent with best practice in systematic review, we published our review protocol before the review began and established consistent data searching, screening, extraction, and evaluation methodologies to increase transparency and reproducibility and reduce bias. When completed, our dataset will be publicly available via an interactive database to facilitate user-driven data exploration and analysis.
Results/Conclusions
Our preliminary results are based on ~300 publications (screened from >20,000 initial search results) that document quantitative cause-effect relationships between relevant nutrients and endpoints around the world. Overall, we found that chl-a has stronger relationships than diatoms or macroinvertebrates to nutrients. This confirms a common understanding in stream ecology that autotrophic indicators are better predictors of nutrient enrichment. However, certain macroinvertebrates show broadly consistent responses to both TN and TP: burrowers, chironomids, and dipterans show consistent positive responses, while coleopterans, plecopterans, and EPT indices show consistent negative responses. For studies that report relationships between chl-a and both TN and TP, those that show a strong response to TN also tend to show a strong response to TP. This reaffirms the importance of controlling both nitrogen and phosphorus to limit eutrophication. Finally, we are beginning to describe the effects of over 20 other environmental factors on nutrient stressor-response relationships. For instance, the strength of the benthic chl-a response declines with increasing stream size. Our reviews represent a state-of-the-science body of evidence for assessing nutrient impacts on widely used indicators of biological responses to nutrients.