Headwaters to the Sea - EMAP's Estimate of Nutrient Transport in the Mississippi River Basin
Brian H. Hill
US Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Research Laboratory, Mid-Continent Ecology Division, Duluth, MN
The US Environmental Protection Agency has conducted two major efforts to assess the ecological conditions in wadeable streams and the Great Rivers in the Mississippi-Atchafalaya River basin (MARB). The Office of Water’s Wadeable Stream Assessment sampled stream chemistry, physical habitat and biology at 510 MARB sites. Similarly, the Office of Research and Development’s EMAP Great Rivers sampled 381 sites on the Upper Mississippi, Missouri and Ohio Rivers. Using carbon (C), nitrogen (N) and phosphorus (P) data collected from these sites in combination with the National Hydrography Data-Plus estimates of cumulative watershed area, mean annual discharge and watershed land cover information associated with each stream, I modeled nutrient loading from the sub-basins of MARB. Total N and P were highest in Upper Mississippi River basin sites and lowest in the Ohio River basin sites. Mean annual discharge (m 3 s -1) was strongly dependent upon watershed area (r 2=0.85-0.99), and this resulted in similarly strong regressions of N and P loads (MT y -1) vs. watershed area (r 2=0.83-0.95). A significantly greater proportion of the Upper Mississippi River basin was in agriculture (41%) compared to the Missouri (18%) and Ohio (12%) Rivers basins, and stream N concentrations in Upper Mississippi and Missouri Rivers basins were significantly related (r 2=0.44-0.49) to the percentage of the watershed in agricultural land cover. A similar relationship was not demonstrated for P. The ratios of C, N and P suggest significant nutrient enrichment within the MARB. Regression models estimate an annual N load from the MARB to the Gulf of Mexico of 1.15 million MT, of which 46% flows from the Upper Mississippi River basin, 36% from the Ohio River basin, and 10% from the Missouri River basin.