Using a weight of evidence approach to define nutrient criteria protective of aquatic life in large rivers

In keeping with the United States Environmental Protection Agency’s (USEPA) national strategy for developing regional nutrient criteria, New York State is revising its narrative nutrient standard for nitrogen and phosphorus. Using the USEPA’s recommended weight of evidence approach, this investigation identified nutrient thresholds using percentile, changepoint and cluster analysis. The thresholds are associated with shifts in biological community structure (benthic macroinvertebrate and diatom) as related to water column nutrient data from 40 large river sites throughout New York State. Using the weight of evidence approach combining all three methods, proposed values for large river nutrient criteria are TP 0.03 mg/L, TN 0.7 mg/L, NO₃^- 0.3 mg/L, and Chl-a 2.2 mg/m³. These values are similar to other work and provide meaningful aquatic life protective endpoints for nutrient levels in large rivers.