HISTORICAL TIMELINE OF LARGE RIVER FOOD WEBS THROUGH STABLE ISOTOPE ANALYSIS 

Emily E. Zelenka1, Michael M. Delong1, and James. H. Thorp2

1Large River Studies Center and Biology Department, Winona State University, 
Winona, Minnesota 55987

2Kansas Biological Survey and Department of Ecology and Evolutionary Biology, 
University of Kansas, Lawrence, KS 66047


      For as long as humans have lived they impacted the large river systems 
that they live near.  Yet, for all this time, it has only been in the last few 
decades that we have begun to study these impacts.  This study was conducted to 
further our understanding of how trophic dynamics of river systems are affected 
by human disturbances using stable isotope ratios of carbon and nitrogen. Tissue 
samples of fish collected between Winona, MN and Clinton City, IA were obtained 
from museum and university collections.  Fish were available in collections 
dating from 1880 to present time.   A 0.2 g tissue sample was removed from each 
preserved fish.  Samples were dried at 60?C for 48 hr and then ground into a 
fine homogenous powder using a Wig-L-Bug or, when necessary, with a mortar and 
pestle.  A 1.5-2.0 mg sample of the powder was placed in a tin capsule and sent 
to Kansas State University for isotopic analysis.  Isotopic ratios were used to 
establish a timeline of events that may have impacted the river to see if they 
coincide with shifts in trophic positions, and thus ecosystem dynamics.  There 
is a trend of increase in Delta15N after 1920 and again after 1950.  These times 
coincide with the expansion of intensive agricultural land use, followed by the 
introduction of man-made fertilizers following World War II.  There seems to be 
a similar increase in Delta13C in 1900 then again in 1920 and decrease in 1980.  
This may coincide with changes in pH at those times, but other physical-chemical 
factors require further consideration.  From this, it can be concluded that 
anthropogenic events have a clear impact on food web dynamics on a temporal 
scale.
 
Keywords: food web; Mississippi River; stable isotope; trophic dynamics; 
timeline; human disturbance