Nutrient Trends in Streams and Rivers of the United States, 1993-2003

Trends in streamflow and concentrations and loads of total phosphorus, total nitrogen, and nitrate were determined for the period from 1993 to 2003 in selected streams and rivers of the United States. Flow-adjusted trends in concentration (the trends that would have occurred in the absence of natural changes in streamflow), non-flow-adjusted trends in concentration (the trends resulting from both natural and human factors), and trends in load (trends in the nutrient mass transported downstream) were determined, and the results were examined spatially to determine whether a consistent pattern of trends occurred across groups of sites at multiple locations. Relations between the trends and changes in nutrient sources and streamflow were examined. At the majority of sites nationwide, concentrations and loads of total phosphorus, total nitrogen, and nitrate did not change significantly from 1993 to 2003. Where significant changes did occur, there were more upward and fewer downward flow-adjusted trends in total phosphorus and total nitrogen concentrations as compared to nitrate concentrations. Increases in nutrient sources and(or) transport contributed to net upward flow-adjusted trends at 33 percent of sites for total phosphorus, 21 percent for total nitrogen, and 12 percent for nitrate; whereas decreases in nutrient sources, implementation of pollution-control strategies, or other anthropogenic activities contributed to net downward flow-adjusted trends at 16 percent of sites for total phosphorus and total nitrogen, and 25 percent of sites for nitrate. There were fewer upward non-flow-adjusted trends than upward flow-adjusted trends in total phosphorus (24 and 33 percent of sites, respectively), total nitrogen (11 and 21 percent, respectively), and nitrate (6 and 12 percent, respectively) concentrations. This was particularly evident at sites in the Central and Southwestern United States, where most of the downward trends in streamflow occurred as a result of drought conditions in the latter part of the study period. At these sites, the increase in source indicated by the upward flow-adjusted trend likely was offset by the effects of decreasing precipitation and surface runoff indicated by the downward streamflow trend. Without the decrease in surface runoff, in-stream concentrations at these sites probably would have been higher. Changes in population density were related to total phosphorus trends at sites in undeveloped areas and total phosphorus and nitrate trends at sites in the Western United States. Changes in nonpoint-source loads were related to total phosphorus trends (fertilizer and manure) at sites in some agricultural areas, as well as nitrate trends (manure) at sites in the Central United States. The more frequent increases in total phosphorus concentrations as compared to total nitrogen and nitrate concentrations cannot be explained easily by changes in nonpointsource loads; there were slightly more sites with increases in nitrogen loading from fertilizer and(or) manure than with increases in phosphorus loading from fertilizer and(or) manure. Instead, the differences may have been due in part to phosphorus saturation of soils in areas that have been subject to long-term manure or fertilizer application and(or) lag time between anthropogenic changes on the land surface and changes in nitrogen concentration in streams caused by the transport of nitrogen in ground water. At nearly 20 percent of sites with nutrient enrichment over natural background conditions, anthropogenic activities contributed to net decreases in nutrient concentrations from 1993 to 2003, and overall nutrient quality in the streams improved. At sites minimally affected by human activities, downward trends occurred at fewer than 10 percent of sites, and there were over three times more upward trends than downward trends in total phosphorus concentrations and over eight times more upward trends than downward trends in total nitrogen concentrations.