Are nutrient concentrations in streams changing over time?
USGS findings on trends from about 1993 to 2003 generally show minimal changes in concentrations of nitrogen and phosphorus in the majority of studied streams across the Nation. Specifically, findings show no significant trend in flow-adjusted concentrations at the majority of sites for nitrogen (63 percent) or phosphorus (51 per cent). At sites with significant trends in nutrients, more sites had increasing than decreasing trends. Flow-adjusted concentrations increased at 33 and 21 percent of sites for phosphorus and nitrogen, respectively, and decreased at 16 percent of sites for both nutrients. Increasing nutrient concentrations related to human and land-use activities from 1993 to 2003 are more common in less impacted streams (streams with nutrient concentrations in 1993 that were below U.S. Environmental Protection Agency’s recommended ecoregional nutrient criteria). Nearly 40 and 30 percent of the less impacted sites showed upward trends in phosphorus and nitrogen, respectively.
