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. Contemporaneous reductions in major nutrient sources to streams, related to wastewater treatment upgrades, phosphate detergent bans, and declines in some agricultural sources, may have contributed to the declines in riverine nutrient concentrations and associated improvements in trophic conditions. Cultural sources of nutrients to streams changed appreciably during the last three decades of the 20th century in the U.S. Advances in wastewater treatment, enabled by the 1972 Clean Water Act, and state-level phosphate detergent bans greatly reduced municipal point sources of nutrients to streams (Litke 1999). Diffuse nutrient sources, including fertilizers and fossil-fuel emissions, rose to peak levels in the late 1970s and early 1980s and have varied considerably since that time (Howarth et al. 2002). Soil conservation and government-sponsored retirements of farmlands have received increasing attention since the 1980s (USDA 1997), although knowledge of their effects on stream nutrients are limited. Despite these widespread changes in nutrient sources and management-related activities, there is surprisingly little information about how the trophic state of U.S. streams has changed over the past several decades, especially in response to changes in nutrient enrichment. Biennial reports of stream-use impairment from state water-quality inventories are generally of limited use for tracking historical changes in water quality and trophic conditions (Mayio and Grubbs 1993; Litke 1999). Moreover, previous studies of nutrient trends in major U.S. rivers (Smith et al. 1987, 1993; Lurry