TRANSPORTED ORGANIC MATTER IN A FLOODPLAIN RIVER: EVIDENCE 
OF HYDROLOGICAL CONTROLS ON TROPHIC DYNAMICS

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

Food web studies have generated increasing evidence that phytoplankton and benthic 
microalgae are principle drivers of energy flow in large rivers. These snapshots of trophic 
linkages made it possible to begin testing new hypotheses on the functioning of large 
river ecosystems. We examined changes in composition and quality of the algal and 
detrital components of transported organic matter (TOM) in the Mississippi River by 
assessing chlorophyll concentration plus carbon and nitrogen stable isotope ratios April–
September 2004 in main channel, secondary channel, and backwater habitats. Rather than 
follow expected climatic-based temporal shifts, all measures of phytoplankton and 
detrital TOM responded to more strongly to hydrological conditions. Phytoplankton 
chlorophyll concentrations were highest in spring and summer when hydrological 
retention time was high (low discharge). Stable isotopic ratios also responded to 
hydrological conditions, possibly reflecting changes in inorganic nutrient sources. 
Moreover, quality of detrital TOM was also highest during low discharge, suggesting that 
senescent phytoplankton were a major part of detritus during these periods.  Isotopic 
ratios of the filter-feeding caddisflies Hydropsyche orris and Cheumatopsyche sp. from 
the main channel were examined concurrent to this study.  The ?15N of both caddisflies 
corresponded closely to the temporal pattern observed for the ?15N of phytoplankton.    
This study illustrates the importance of hydrologic dynamics to resource availability and 
trophic dynamics in floodplain rivers.


Keywords: phytoplankton, transported organic matter, stable isotopes, food web, habitat, 
hydrology