NITROGEN CYCLING DURING SEDIMENT DESICCATION AND REWETTING IN THE UPPER 
MISSISSIPPI RIVER, NAVIGATION POOL 8.  

Jennifer C. Cavanaugh, Eric A. Strauss, Lynn A. Bartsch, William B. Richardson, 
and Dave M. Soballe.  U.S. Geological Survey, Upper Midwest Environmental 
Sciences Center, 2630 Fanta Reed Rd., La Crosse, Wisconsin 54603.

In summer 2002, a 14-week water level drawdown was conducted in Navigation Pool 
8 of the Upper Mississippi River for the primary purpose of consolidating 
sediments and stimulating vegetation growth along the river margins.  We used 
this opportunity to investigate the effect of water level manipulations on 
nitrogen cycling processes.  Nitrogen concentrations and nitrogen cycle 
processes were measured bi-weekly before, during, and after the drawdown from a 
10-site experimental transect that dried during the drawdown and from another 
10-site reference transect that remained inundated.  During the dry period, 
porewater nitrate (NO3-) concentrations and rates of nitrification and 
denitrification increased in the experimental transect whereas denitrification 
enzyme activity (DEA) rates and exchangeable ammonium (NH4+) concentrations 
decreased (p < 0.05, RM ANOVA), compared to those in the reference transect.  
The increase in nitrification in the experimental transect was likely in 
response to increased aeration in the sediments.  This resulted in higher levels 
of available NO3-, which subsequently enhanced denitrification.  The inverse 
relationship between denitrification and DEA rates could be attributed to 
concurrent desiccation-induced degradation of denitrification enzymes and 
enhanced enzyme efficiency from an increase in denitrification.  The decrease in 
NH4+ likely resulted from increases in nitrification and plant uptake.  After 
rewetting, nitrification, denitrification and pore water NO3- returned to levels 
similar to the reference sites, although DEA rates and NH4+ remained lower in 
the sediments that previously dried.  Overall, there was an apparent, but not 
significant reduction in total nitrogen in the sediments of the experimental 
transect.  Although nitrification and denitrification rates increased, 
indicating a net nitrogen loss, it was not sufficient to show a significant 
affect in the high total nitrogen levels in the sediment (near 2000 mg/L).  Our 
results demonstrate that water level manipulations do promote some nitrogen loss 
from the sediment by stimulating nitrogen cycling processes, however a 
pronounced reduction in total sediment nitrogen may require extended dry periods 
or multiple manipulations.    

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