Anthropogenic nutrient pollution remains a widespread threat to freshwater ecosystems and human health and wellbeing around the world. These threats result from how increased nitrogen and phosphorus concentrations alter primary productivity, biogeochemical rates and food webs, affect biodiversity, and contribute to algal blooms and hypoxia. Characterizing ecological responses to nutrient enrichment provides context needed for determining nutrient concentration targets that can inform the management and protection of freshwater ecosystems and their watersheds (i.e., they provide ecological expectations along nutrient gradients). Here, we used some traditional and emerging ecological indicators and statistics to identify potential nutrient targets for streams. We also discuss concepts for their possible applications and limitations. One study of 74 streams in southern New England developed indicators and potential nutrient targets based on periphyton extracellular enzyme activities (a functional measure intersecting nutrient stoichiometry and metabolism) and nitrogen stable isotopes of periphyton and macroinvertebrates, which also provide insights on food webs. We compared these results to nutrient targets based on diatom responses in 87 other streams in the region. Lastly, in a study of 25 Ohio streams, we used DNA metabarcoding of diatoms as a novel approach to assessing nutrient effects and to identifying possible nutrient targets.
Results/Conclusions
In the southern New England studies, diatom communities and periphyton extracellular enzyme activities indicated that most streams were phosphorus-limited and experienced substantial nonlinear changes above 20 μg total phosphorus (TP)/l. Despite being P-limited, nitrogen pollution in these streams remains problematic because it negatively affects a downstream estuary. Stable isotopes of stream periphyton and macroinvertebrates indicated nitrogen enrichment from human sources with nonlinear responses occurring at 220 and 750 μg nitrate/l. In the diatom metabarcoding study, results from threshold indicator taxa analysis, boosted regression trees, and gradient forest analysis showed multiple threshold responses along the TP gradient and mostly gradual community and taxa turnover along the TN gradient. Collectively, we used results to identify possible targets for TP or TN concentrations that could be protective of low nutrient systems or used as management targets for ones affected by nutrient pollution from human sources. The growing body of evidence shows the importance of new indicator approaches and statistical analyses that quantify nutrient effects and identify targets for protection and management. Given the challenges of nonpoint sources, multiple uses, and priorities in watersheds, stakeholder engagement and novel nutrient management strategies will be critical to implementing nutrient targets and making progress toward ecosystem goals.