Establishing thresholds in nutrient concentrations related to biological community response: Implications for nutrient criteria development

Human influence on the landscape has caused contributions of nutrients in surface waters to increase to the point where their presence has substantially altered biological communities. This cultural eutrophication has become a major source of water-quality impairment throughout the United States (US). A nationally recognized problem, the United States Environmental Protection Agency tasked each State, Tribe, and Territory to adopt numeric nutrient criteria. Through the efforts of three studies we investigated the effects of increased nutrients on benthic macroinvertebrate and diatom communities. First we studied the response of more than 100 individual invertebrate taxa, developing species nutrient optima and response thresholds. Then, using this information we constructed a new biological metric to assess eutrophication in wadeable streams. Second, we treated large rivers separately from wadeable streams and investigated the response of biological communities unique to these systems across New York State. Using various newly developed threshold statistics such as nonparametric deviance reduction we identified thresholds related to water-column nutrient data from 40 large river sites. Thresholds were based on shifts in biological community structure (benthic macroinvertebrate and diatom). Third, we related nutrient concentrations to the probability of causing biological community impairment. In 100 wadeable streams across New York State's ecoregions we studied the response of invertebrate and diatom communities along a gradient of nutrients. We showed how certain community metrics exhibit threshold responses to nutrients, sometimes varying based on region. Using these thresholds we suggest nutrient values protective of aquatic life based on biological impairment criteria already established for New York State.