TEMPORAL TRENDS OF BENTHIC MICROALGAE AND PHYTOPLANKTON 
IN THE UPPER MISSISSIPPI RIVER.

Briana L. Flattum and Michael D. Delong
Large River Studies Center, Biology Department, Winona State University, Winona, MN 
55987
      
Benthic microalgae and phytoplankton have been identified as key food sources in large 
rivers.  These studies, however, have not considered temporal variability of the 
autotrophic sources resulting from changes in nutrient availability. This study was done 
to investigate temporal changes in carbon and nitrogen stable isotope ratios of benthic 
microalgae and phytoplankton.  Stable isotope samples of microalgae were collected 
from main channel sites in Reach 6 of the Upper Mississippi River June – September 
2004.  Epilithic algae were collected by scraping rocks with razorblades.  Epipelic algae 
were collected in 50-mL vials.  Epiphytic algae were collected by hand from macrophyte 
leaves and stems. Phytoplankton samples were collected from a depth of 1-m along 
transects in the main channel to provide a composite representation. All samples were 
placed in 50-mL vials and placed on ice until they could be returned to the laboratory.  
Benthic microalgae and phytoplankton were separated from detritus and sediments  using 
colloidal silica centrifugation. Carbon (?13C) and nitrogen (?15N) stable isotope ratios 
were determined at Kansas State University.  The ?13C of benthic algae and 
phytoplankton followed the same pattern, increasing from June – August, before 
declining in September.  ?13C was always greater for benthic microalgae, ranging from -
24 to -26 o/oo as compared to -28.5 to -32 o/oo for phytoplankton.  ?15N was similar for 
phytoplankton and benthic microalgae, although ?15N of ultrafine phytoplankton 
(UTOMA) was generally significantly different from ?15N of benthic algae.  ?15N 
decreased for all autotrophic sources June – August, before increasing in September.  
Differences in ?13C have been reported in other comparisons of benthic algae and 
phytoplankton.  Our study, however, indicates that the nature of inorganic carbon sources 
for both change over time, as does the nature of inorganic nitrogen.  We suggest that 
these changes in inorganic carbon and nitrogen assimilated by riverine autotrophs is 
directly associated with changes in hydrological retention time as a function of prevailing 
discharge conditions. 

Keywords: benthic algae, phytoplankton, stable isotope, hydrological retention time, 
main channel