Trends in nutrient concentration and load for streams in the Mississippi River Basin, 1974-94
Trends in nutrient concentration and load are computed for 40 stations on 24 streams in the Mississippi River Basin. The drainage area of the Mississippi Basin covers about 42 percent of the conterminous United States, and the Mississippi River discharges about 420 billion gallons per day of freshwater to the Gulf of Mexico. The period for which trends are analyzed varies for each station but generally comprises water years 1974–94. Stations included in this analysis are part of the National Stream Quality Accounting Network operated by the U.S. Geological Survey. LOWESS (LOcally WEighted Scatterplot Smoothing) smooth lines superimposed on graphs of flow-adjusted residuals of concentration versus time and load versus time indicate short-term trends for each station. Kendall-Theil robust lines superimposed on the same graphs indicate long-term trends. Long-term trends were evaluated using the slope of the Kendall-Theil robust line and Kendall’s tau. Annual loads are estimated with regression analysis and corrected for log-transformation bias with the MVUE (Minimum Variance Unbiased Estimator). Trends in annual streamflow are presented to aid in the interpretation of nutrient trends. Statistically significant long-term increases in flow-adjusted residual concentrations of total nitrogen were detected at 14 stations, decreases were detected at 6 stations, and no significant trends were detected at 20 stations. Statistically significant long-term increases in total nitrogen load were detected at 5 stations, decreases were detected at 4 stations, and no significant trends were detected at 31 stations. Statistically significant long-term increases in flow-adjusted residual concentrations of total phosphorus were detected at 2 stations, decreases were detected at 25 stations, and no significant trends were detected at 13 stations. Statistically significant long-term increases in total phosphorus load were detected at 1 station, decreases were detected at 13 stations, and no significant trends were detected at 26 stations. The mean annual yields were computed from the estimated mean annual loads. Mean annual nitrogen yields are largest for three watersheds in the upper Mississippi Basin; estimated yield of each is greater than 5.0 tons per square mile. Nine stations in the Ohio River Basin have estimated mean annual nitrogen yields greater than 2.0 tons per square mile. Estimated mean annual phosphorus yield is greater than 0.250 ton per square mile at 7 stations, 5 in the Ohio Basin and 2 in the upper Mississippi Basin. Statistically significant trends in annual streamflow were detected at seven stations. Annual streamflow influences trends in load, even when the streamflow trends are not statistically significant.