SPATIAL AND TEMPORAL CHANGES IN PHYTOPLANKTON QUALITY IN 
A LARGE FLOODPLAIN RIVER.

Fareeda Taher Nazer Hussain and Michael D. Delong
Large River Studies Center, Biology Department, Winona State University, Winona, MN 
55987
 
Food web studies in the past have shown that instream primary production, particularly 
phytoplankton, is a major organic matter source for large rivers. These studies, however, 
only examined trophic linkages at only a single point in time. Questions remain as to if 
phytoplankton quality remains high enough for a sufficient period of time to sustain the 
food web throughout the year. To help resolve these questions, we compared patterns and 
changes in the concentration of chlorophyll a and carbon mass:chlorophyll a ratio of 
phytoplankton in main channel and backwater habitats in Reaches 5 and 6 of the Upper 
Mississippi River over four sample dates (June – August 2004). Living and detrital 
components of ultrafine (UTOM) and fine transported organic matter (FTOM) were 
separated using colloidal silica centrifugation. The algal fraction of TOM, which 
represents phytoplankton, was used in our analyses. Chlorophyll concentration was 
measured using spectrophotometry and carbon mass data were generated as part of stable 
isotope analysis.  Chlorophyll concentrations in the main channel increased as discharge 
decreased over the course of the study. Chlorophyll concentrations in backwaters also 
increased over the same period, but not at a lower rate than observed in the main channel 
Carbon: chlorophyll a ratios remained relatively constant throughout the study.  This 
study, in addition to similar work done in 2004, suggests that phytoplankton abundance 
and quality in the main channel is directly influenced by hydrological retention time, 
whereas other factors are important in backwaters.

Keywords:  transported organic matter, temporal, phytoplankton, chlorophyll, backwater, 
main channel