MONITORING OF NITROGEN CYCLING AT AN UPPER MISSISSIPPI RIVER BACKWATER SITE

R.M. Kreiling1,2, W.B. Richardson2, E.A. Strauss1,2, L.A. Bartsch2, J.C. 
Cavanaugh1,2.
1River Studies Center, University of Wisconsin-La Crosse, La Crosse, WI 54601.  
2 US Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, 
WI 54603.
       
Backwaters of the Upper Mississippi River (UMR) contain highly organic sediments 
that are anoxic throughout most of the year.  This is an optimal environment for 
nitrogen removal from this high nitrogen system; however, measured rates of 
denitrification are typically low.  We monitored a backwater site on the UMR 
near navigation Pool 8 at La Crosse, WI to assess temporal trends in nitrogen 
cycling.  We measured water quality, sediment chemistry, and denitrification 
rates weekly for a three-year period.  Denitrification rates ranged from 0-1.6 
ugN/cm2/hr, but were greater in winter months and lower during summer, and were 
correlated to surface and sediment porewater nitrate levels (r=0.63 and 0.34).  
During winter we observed huge algal blooms creating oxic conditions (3.5-25 
mg/L O2); when oxygen was present, nitrate was likely generated by nitrification 
and promoted increased denitrification, despite low temperatures.  This study 
suggests that denitrification in backwaters of Pool 8 may be controlled 
primarily by the availability of nitrate in the sediment, regardless of 
temperature.

Keywords:  nitrogen cycling, denitrification, Mississippi River, backwater areas