TEMPORAL CHANGES IN ORIGIN OF ESSENTIAL INORGANIC NUTRIENTS ASSOCIATED WITH 
PRIMARY PRODUCERS IN A LARGE RIVER ECOSYSTEM 

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

      Phytoplankton within a large river ecosystem are an integral part of 
the food web. Previous studies have shown that the carbon and nitrogen 
isotopic ratios of phytoplankton fluctuate in response to variation in the 
hydrology of the Upper Mississippi River, which may have an effect on food 
web interactions. We propose that these hydrologically driven changes are a 
result of shifts in the nature of inorganic nitrogen and carbon available to 
phytoplankton for growth and reproduction. Stable isotope ratios and 
concentrations of inorganic carbon and nitrogen were used to determine 
changes over time in backwater and main channel habitats in Reach 6 of the 
Upper Mississippi River.  Samples were collected Aug and September 2006 and 
March – August 2007.  Both nitrate concentrations and stable isotope ratios 
decreased in 2006, but were their highest in April 2007.  They continued to 
decline for the remainder of the study period.  Nitrate concentrations and 
isotopic ratios were lowest in closed backwaters, whereas no differences were 
evident between main channel and open backwater sites.  Between-habitat 
differences were also observed for inorganic carbon, although not as profound 
as changes observed for nitrate.  This study demonstrates that temporal 
changes in the characteristics of inorganic nutrients do occur in the 
riverscape of the Upper Mississippi River as both as function of habitat and 
time.  We suggest that closed backwaters show a marked difference from open 
backwaters and the main channel because they are isolated from the delivery 
of nutrients afforded to open backwaters as a result of strength of 
connectivity to the main channel. 
      


Keywords: nitrate, inorganic carbon, temporal, hydrology, stable isotope

Aaron Hefty
Large River Studies Center, Winona State University, Winona, MN 55987
507-457-2458

Poster only
Student presenter