VARIATION IN PHYTOPLANKTON COMPOSITION WITHIN LARGER 
RIVERS: STABLE ISOTOPIC APPROACH

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

A number of recent studies have indicated that autochthonous primary production, 
particularly phytoplankton, are a major food source in floodplain ecosystems.  Previous 
studies have also shown phytoplankton concentrations change overtime as a function of 
biological, chemical, and physical attributes of the river ecosystems.  This study 
examined the carbon and nitrogen stable isotope ratios phytoplankton both main channel 
and backwater habitats on four separate dates (June-August). The study was conducted in 
Reaches 5 and 6 of the Upper Mississippi River.  Standard techniques used in past studies 
were employed for collection and processing of TOM samples.  Algal and detrital 
fractions were separated using colloidal silica centrifugation. TOM was divided in the 
field and laboratory into fine TOM (FTOM; 1000-100 ?m) and ultrafine (UTOM; 100-1 
?m).  Samples were shipped to Kansas State University for determination of carbon and 
nitrogen stable isotope ratios.  Carbon isotope ratios of phytoplankton in both the main 
channel and backwaters increased during the course of the study.  Nitrogen stable isotope 
ratios decreased in both habitats over the course of the study, with ?15N of backwater 
phytoplankton decreasing more than observed in the main channel.  This study indicates 
that the inorganic carbon and nitrogen sources used by phytoplankton change as 
hydrological retention time in main channel and backwater habitats increases.  The 
decrease in ?15N reflects a likely shift from biogenic inorganic nitrogen to atmospheric 
nitrogen.  The findings of this study suggest that hydrological retention time is also 
important in shaping system function in the main channel.


Keywords: transport organic matter, Upper Mississippi River, phytoplankton, backwater, 
main channel, hydrology, stable isotope