TEMPORAL PATTERNS OF BENTHIC-PELAGIC COUPLING IN MICROALGAE: A STABLE ISOTOPIC 
EVALUATION

Laura E. Luger and Michael D. Delong 
Large River Studies Center and Biology Department, Winona State University, 
Winona, Minnesota 55987

Stable isotope ratios were used to examine temporal relationships in sestonic 
and benthic microalgae.  Samples were collected April-September 2004, from the 
main channel of the Upper Mississippi River near Winona, Minnesota.  Samples 
were collected to create a time series of 13C and 15N stable isotope ratios of 
transport organic matter (TOM) and benthic microalgae that would allow us to 
ascertain if these two algal forms are linked.  Water column samples (TOM) were 
pumped from 1 m below the surface.  Microalgae were scraped from substrate at 
three different sites.  Hydropsychid caddisflies were collected from rocky 
substrate and held in aerated buckets for 48 hr to allow the gut to clear, then 
were frozen.  Algal and detrital TOM fractions were separated using colloidal 
silica centrifugation separation.  The same process was used to separate 
sediment and detritus from benthic microalgae. Isotopic ratios of algal TOM were 
used to evaluate potential benthic-pelagic interactions between sestonic and 
benthic habitats. A two-source mixing model will be applied to ascertain the 
contribution of sestonic vs. benthic microalgae to the diet of Hydropsychid 
larvae and as another measure for assessing the presence of benthic-pelagic 
coupling in the trophic dynamics of the Upper Mississippi River.  We hypothesize 
that isotopic ratios of TOM and benthic microalgae will remain similar over time 
if there is a rapid turnover in benthic algae.  Pronounced differences will 
suggest that residence time of benthic algae is more protracted, meaning that 
turnover from TOM is slow or that the extent of benthic-pelagic coupling is 
limited.

Keywords: stable isotope: transported organic matter; Hydropsychid; benthic 
microalgae; benthic-pelagic coupling