EFFECT OF WATER LEVEL DRAWDOWN ON NITROGEN CYCLING PROCESSES IN UPPER 
MISSISSIPPI RIVER POOL 8.  

Eric A. Strauss, William B. Richardson, Lynn A. Bartsch, Jennifer C. Cavanaugh, 
Denise A. Bruesewitz, Heidi J. Imker, and Dave M. Soballe.  US Geological 
Society, Upper Midwest Environmental Sciences Center, 2630 Fanta Reed Rd., La 
Crosse, Wisconsin 54603.

A water level drawdown was conducted in Navigation Pool 8 of the Upper 
Mississippi River for the primary purpose of stimulating vegetation growth along 
the river margins.  We used this water level manipulation to investigate the 
effect of a drawdown on nitrogen cycling processes.  Since spring 2000 we have 
seasonally monitored nitrogen cycling processes and sediment chemistry at 
approximately 25 backwater sites in Pool 8 and sediments at 5 of these sites 
were desiccated during the drawdown.  Neither nitrification or denitrification 
rates varied significantly among the sites prior or during the drawdown (P > 
0.05, t-tests).  However, nitrification rates did differ in the fall of 2001 
after the dried sediments were re-flooded.  Previoiusly dry sediments had lower 
nitrification rates (P = 0.035) than those that remained saturated throughout 
the drawdown event.  The delayed nitrification response was likely due to the 
decrease in exchangeable ammonium at the dried sites during (P = 0.037) and 
after (P = 0.19) the drawdown.  The drawdown did not appear to have a 
significant effect on denitrification rates.  Our data demonstrates that 
nitrogen cycling processes may respond to river drawdown management, but 
responses are likely mild and probably temporary. 

Keywords: nitrogen, drawdown, nitrification, denitrification, Mississippi River