Trends in the nutrient enrichment of U.S. rivers during the late 20th century and their relation to changes in probable stream trophic conditions

We estimated trends in concentrations of total phosphorus (TP) and total nitrogen (TN) and the related change in the probabilities of trophic conditions from 1975 to 1994 at 250 nationally representative riverine monitoring locations in the U.S. with drainage areas larger than about 1,000 km2. Statistically significant (p, 0.05) declines were detected in TP and TN concentrations at 44 % and 37 % of the monitoring sites, and significant increases were detected at 3 % and 9 % of the sites, respectively. We used a statistical model to assess changes in the probable trophic-state classification of the sites after adjusting for climate-related variability in nutrient concentrations. The probabilistic assessment accounts for current knowledge of the trophic response of streams to nutrient enrichment, based on a recently proposed definition of ‘‘eutrophic,’ ’ ‘‘mesotrophic,’ ’ and ‘‘oligotrophic’ ’ conditions in relation to total nutrient concentrations. Based on these trophic definitions, we found that the trophic state improved at 25% of the monitoring sites and worsened at fewer than 5 % of the sites; about 70 % of the sites were unchanged. Improvements in trophic-state related to declines in TP were more common in predominantly forested and shrub– grassland watersheds, whereas the trophic state of predominantly agricultural sites was unchanged. Despite the declines in TP concentrations at many sites, about 50 % of all monitoring sites, and more than 60 % of the sites in predominantly agricultural and urban watersheds, were classified as eutrophic in 1994 based on TP concentrations