TEMPORAL DYNAMICS IN STABLE ISOTOPE RATIOS OF TRANSPORTED ORGANIC MATTER IN THE 
UPPER MISSISSIPPI RIVER

Rebecca J. Bown and Michael D. Delong
Large River Studies Center and Biology Department, Winona State University, 
Winona, Minnesota  55987

Stable isotope studies have identified autochthonous sources of organic matter 
as a major driver in large river food webs, particularly transported organic 
matter (TOM). The temporal scope of these studies, however, is limited.  A 
better understanding of the temporal aspects of TOM composition and quality is 
needed.  The objective of this study was to examine temporal shifts in 
composition and quality of transported organic matter in the Upper Mississippi 
River (UMR) using carbon and nitrogen stable isotope ratios and 
carbon:chlorophyll a ratio.  Water samples were taken from sites in the main 
channel, side channels, and backwaters April - October 2004.  Water samples (15 
- 60 L) were step-filtered in the field through 105-?m and 53-?m sieves. Each 
filtered sample was mixed with a colloidal silica solution and centrifuged to 
separate the algal and detrital constituents of each size fraction of TOM.  
Samples were prepared and shipped for determination of stable isotope ratios.  
Chlorophyll concentrations were determined using spectrophotometry.  Preliminary 
results indicate that carbon and nitrogen stable isotope ratios do shift 
temporarily in response to hydrological patterns of the UMR, but little change 
in isotopic ratios occurs during periods of hydrological stability.  The 
carbon:chlorophyll a ratio for detrital TOM indicates high quality, probably due 
to a large component of senescent phytoplankton and benthic algae.  A better 
understanding of the temporal and spatial dynamics of potential organic matter 
sources will allow for a broader recognition of trophic processes, thereby 
providing functional measures to support management plans in the rehabilitation 
of river ecosystems.
	

Keywords:  transported organic matter, temporal, stable isotope ratios, 
hydrological patterns, Upper Mississippi River