ECOSYSTEM RESPONSES TO AN EXPERIMENTAL DRAWDOWN ON THE UPPER MISSISSIPPI RIVER: 
A SUMMARY OF FINDINGS

Michael D. Delong
Large River Studies Center, Biology Department, Winona State University, Winona, 
MN 55987

      Experimental drawdowns have been tested on several occasions as a 
potential water-level management tool in the Upper Mississippi and other large 
regulated rivers.  The most recent drawdown was executed on Reach 5 of the Upper 
Mississippi River, where water levels were reduced approximately 0.5-m from the 
end of June into September 2005.  The Large River Studies Center, as part of the 
broader effort funded by the U.S. Army Corps of Engineers through the Navigation 
and Environmental Sustainability Program (NESP), monitored changes in factors 
indicative of ecosystem-level processes in backwater and main channel habitats 
throughout the reach.  Factors measured May – August 2005 included: community 
metabolism; phytoplankton chlorophyll concentrations; benthic and sestonic 
organic matter carbon and nitrogen isotope ratios; and trophic linkages.  
Standard methods used for collecting organic matter and invertebrates in past 
studies through the LRSC were employed.  Metabolism was measured using 
AquaDevice continuous oxygen/temperature recorders deployed for 7-d periods as 
part of the monthly sampling regime.  The same sample procedures, except trophic 
measures were repeated in Reach 6.  Gross primary production increased in the 
main channel of Reach 5 increased during the study, whereas GPP of backwaters 
decreased in both Reaches, but at a slower rate in Reach 5.  
Production:Respiration remained >= 1 throughout the study in Reach 5 but 
declined over time in Reach 6.  Nitrogen isotope ratios of phytoplankton 
increased over time in Reach 5 while decreasing in Reach 6 just as they did in a 
2004 study.  The differences in ?Delta?15N suggest that inorganic nitrogen 
sources differed between the two Reaches during the drawdown period.  
Chlorophyll concentrations were, for the most part, similar between the two 
Reaches.  Findings indicate that riverine primary production was enhanced by the 
drawdown, which would be expected given the increase in hydrological retention 
resulting from maintaining lower water levels.

Keywords: ecosystem health, biomonitoring, assessment, ecosystem processes, 
function