Nitrogen removal by streams and rivers of the Upper Mississippi River basin

Abstract

We used stream chemistry and hydrogeomorphology data from 549 stream and 447 river sites to estimate NO3–N removal in the Upper Mississippi, Missouri, and Ohio Rivers. We used two N removal models to predict NO3–N input and removal. NO3–N input ranged from 0.01 to 338 kg km−1 d−1 in the Upper Mississippi River to <0.01–54 kg km−1 d−1 in the Missouri River. Cumulative river network NO3–N input was 98700–101676 Mg year−1 in the Ohio River, 85961–89288 Mg year−1 in the Upper Mississippi River, and 59463–61541 Mg year−1 in the Missouri River. NO3–N output was highest in the Upper Mississippi River (0.01–329 kg km−1 d−1), followed by the Ohio and Missouri Rivers (<0.01–236 kg km−1 d−1) sub-basins. Cumulative river network NO3–N output was 97499 Mg year−1 for the Ohio River, 84361 Mg year−1 for the Upper Mississippi River, and 59200 Mg year−1 for the Missouri River. Proportional NO3–N removal (PNR) based on the two models ranged from <0.01 to 0.28. NO3–N removal was inversely correlated with stream order, and ranged from <0.01 to 8.57 kg km−1 d−1 in the Upper Mississippi River to <0.001–1.43 kg km−1 d−1 in the Missouri River. Cumulative river network NO3–N removal predicted by the two models was: Upper Mississippi River 4152 and 4152 Mg year−1, Ohio River 3743 and 378 Mg year−1, and Missouri River 2277 and 197 Mg year−1. PNR removal was negatively correlated with both stream order (r = −0.80–0.87) and the percent of the catchment in agriculture (r = −0.38–0.76).