SPATIAL AND TEMPORAL VARIABILITY IN FOOD QUALITY OF TRANSPORT ORGANIC MATTER

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

Patterns of chlorophyll a and particulate organic matter (POM) in the water 
column determine food quality, which can be a driving force in system 
energetics.  An increase in the chlorophyll a/POM ratio indicates an increase in 
food quality.  Additionally, food quality is likely to change as a function of 
habitat type and time of the year; the results of which could influence consumer 
productivity.  Chlorophyll a and POM concentrations were examined monthly for 
main and side channel habitats to ascertain spatial and temporal patterns of 
food quality.  One-liter samples were collected from main channel and side 
channel habitats in Reaches 5, 6, and 8 of the upper Mississippi River from July 
through September 2001.  Samples were collected from near shore areas in 
association with snags.  From each sample, 500 mL were vacuum filtered through 
two separate 
1.0-µm glass fiber filters.  One filter was used to determine chlorophyll a 
concentration using spectrophotometric techniques and the other filter was used 
to determine biomass of POM.  Biomass was measured by placing filters in oven at 
60º C for 48 hr, sampled was weighed and placed in a muffle furnace at 550º C 
for 4 hr to determine ash free dry mass. There was no significant difference in 
chlorophyll a concentration, POM biomass or in chlorophyll a/POM ratio.  The 
ratio remained unchanged seasonally in the main channel but reached a peak of 
22.5 in late July before returning in September to a level similar to that of 
early July.  This response was not observed in side channels, but that may be 
due to high concentrations of POM resulting from the abundance of dying 
macrophytes we observed during this period.

Keywords: chlorophyll a, particulate organic matter, food quality, main channel, 
side channel