DENITRIFICATION IN THE UPPER MISSISSIPPI RIVER: THE ROLE OF NITRATE 
DELIVERY AND SEDIMENT CARBON

W. Richardson, E. Strauss, E. Monroe, L. Rabuck, L. Bartsch, and D. Soballe.
Upper Midwest Environmental Sciences Center, U.S. Geological Survey, 2630 Fanta 
Reed Road, La Crosse, WI 54603.

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 basin contains large expanses of riparian wetlands and 
vegetated backwater lakes hypothetically capable of supporting substantial 
eodenitrification.  We initiated studies to test this hypothesis in a 27 km 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 (14.9 mg N/m2/d + 
4.28 SE), lowest surface water N03- and highest sediment carbon and NH4+ 
concentrations; conversely, sediments near large channels tended to have the highest 
rates (43.0 mg N/m2/d + 9.3 SE) and lower sediment carbon.  In high sediment 
carbon areas, denitrification = 4.8 (surface water NOx) + 0.11 (sediment carbon) 
-0.37; rư=0.72, p=0.0001.  Monitoring data supports our contention that much of 
the area with the 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. 

Keywords:  nitrogen, denitrification, connectivity, ammonia, sediment carbon
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