EFFECTS OF AN EXPERIMENTAL DRAWDOWN ON FOOD WEBS IN UPPER MISSISSIPPI REACH 5

Brittany Sheehan and Michael Delong

Large River Studies Center, Biology Department, Winona State University, Winona, 
Minnesota 55987

      An experimental drawdown was conducted on Reach 5 of the Upper Mississippi 
River in 2005 to mimic the river’s pre-dam summer flow.  While there has been 
some study on the effects of experimental drawdowns on specific biota (including 
aquatic vegetation and mussels), little is known about the ecosystem response to 
such a drawdown.  The objective of this study was to determine if food sources 
important to benthic invertebrate primary consumers change during the course of 
the drawdown. Linkages between sources and consumers were made using stable 
isotopic ratios of carbon and nitrogen.  Samples were collected June – August 
2005 from backwater and main channel sites located throughout Reach 5.  Source 
and consumer samples were collected using established techniques.  Temporal 
differences in isotopic ratios were evident for both phytoplankton and benthic 
algae, with Delta15N increasing and Delta13C decreasing over the course of the 
study.  Isotopic ratios of consumers followed the same pattern.  Initial 
comparisons of source and consumer isotopic ratios indicate that basal sources 
important to consumers remained unchanged during the drawdown, with 
phytoplankton serving as the main energy source.  A similar food web study in 
Reach 6 also indicated that phytoplankton was the major resource, thereby 
suggesting that trophic dynamics were not adversely impacted by the drawdown. 
Linkages between sources and consumers will be examined more thoroughly using a 
dual-isotope multiple source mixing model.

Keywords: food web, mixing model, stable isotopes, drawdown, invertebrates, 
phytoplankton