Trends are computed for nutrient inflows from 37 streams discharging into the Gulf of Mexico. The drainage areas of these streams represent about 86 percent of the drainage area to the Gulf from the conterminous United States. The period analyzed varies for each stream, but generally includes water years 1972-93. Stations included in this analysis primarily are part of the National Stream Quality Accounting Network operated by the U.S. Geological Survey. Short-term trends for each station are indicated by LOWESS smooth lines superimposed on graphs of the relations between flow-adjusted residuals of concentration and time or load and time. Long-term trends were evaluated using Kendall's tau and the slope of the Kendall-Theil robust line. Long- term trends for each station are indicated by Kendall-Theil robust lines superimposed on the aforementioned graphs. Annual loads are estimated with regression analysis and corrected for log-transformation bias with the Minimum Variance Unbiased Estimator. Trends in annual streamflow are presented to aid in the interpretation of trends in nutrient inflows. Statistically significant, long-term increases in flow-adjusted residual concentrations of total nitrogen were detected at 19 stations, decreases were detected at 7 stations, and no significant trends were detected at 11 stations. Long-term increases in total nitrogen load were detected at 3 stations, decreases were detected at 4 stations, and no significant trends were detected at 30 stations. Long-term increases in flow-adjusted residual concentrations of total phosphorus were detected at 7 stations, decreases were detected at 11 stations, and no significant trends were detected at 19 stations. Long-term increases in total phosphorus load were detected at 3 stations, decreases were detected at 12 stations, and no significant trends were detected at 22 stations. The median yields (mean annual 10ad divided by drainage area) of total nitrogen and total phosphorus were significantly lower (p < 0.05) for the 13 streams in Texas than for the 24 streams east of the Sabine River (Texas-Louisiana boundary). Statistically significant trends in annual streamflow were detected at only four stations. However, annual streamflow influences trends in load, even when the streamflow trends are not statistically significant.
Additional Publication Details
USGS Numbered Series
Trends in nutrient inflows to the Gulf of Mexico from streams draining the conterminous United States, 1972-93