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Communication at the NABS Annual meeting, Keystone, 2000
(141) DENITRIFICATION IN THE UPPER MISSISSIPPI RIVER: POTENTIAL LIMITATION BY HYDRAULIC EXCHANGE AND NITRATE DELIVERY.
W.B. Richardson, E.A. Strauss, E. Monroe, L. Rabuck, L. Bartsch, and D.M. Soballe. Upper Midwest Environmental Sciences Center, U.S. Geological Survey, 2630 Fanta Reed Rd., La Crosse, Wisconsin, 54603, U.S.
Models of nitrate loading to the Gulf of Mexico from the Upper Mississippi River suggests nitrate passes conservatively from the upper river to the Gulf. Yet, the Upper Mississippi River still has large expanses of riparian wetlands and backwater lakes containing submerged vegetation - hypothetically capable of supporting substantial denitrification. We initiated studies to test this hypothesis in a 27km reach of the Mississippi River, near La Crosse, Wisconsin. During October 1999, we sampled 60 sites, equally distributed across an organic carbon gradient. At each site we measured denitrification, total sediment organic carbon, porewater and exchangeable NH4+ and NO3-; in the overlying water we measured NH4+ and NO3-. Isolated backwaters tended to have the lowest mean denitrification rates (11.1 mg N m-2 d-1 +2.9 SE), lowest surface water NO3- and highest sediment carbon and NH4+ concentrations; conversely, sediments near large channels tended to have the highest rates (22.5 mg N m-2 d-1 +3.6) and lower sediment carbon. Monitoring data supports our contention that much of the area with highest denitrification potential is hydrologically isolated from the NO3 - source. Denitrification across the entire reach would likely increase by increasing connectivity between the main channel and backwaters during summer and fall.
Presented at 1:30 PM on Tuesday, May 30, 2000 in Nutrient Processes in Sediment
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