AN ANALYSIS OF CHANGES IN TROPHIC DYNAMCS IN THE ST. CROIX RIVER: A STABLE 
ISOTOPIC EVALUATION

Deepshika Ramanan1, Michael D. 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

      The Upper Mississippi River flows from the headwaters at Lake Itasca, 
Minnesota to Cairo, Illinois creating a capacious range of physical, chemical 
and biological diversity. One of the tributaries, the St. Croix River flows 
through Wisconsin and Minnesota qith much of the river designated as a National 
Scenic Riverway under the protection of the National Park Service. The St. Croix 
River is one of the rivers of the Mississippi River basin least affected by 
human actions. The objective of this research was to study the alterations in 
food webs over the past century in the St. Croix River using stable isotope 
ratios of carbon and nitrogen as indicators.  Fish and mussel samples of 0.5g 
were collected from past to present years using preserved specimens from museum 
collections.  The samples were dried, ground and 1.5 – 2.0 mg of powder, and 
packed in tin capsules. Samples were sent to Kansas State University for 
determination of isotopic ratios. Carbon and nitrogen stable isotope ratios were 
determined separately for piscivores, insectivores, omnivores/detrivores and 
planktivores. Previous studies show that terrestrial vegetation present on the 
St. Croix today is similar to what was present prior to settlement. Similarly, 
the results of this study show that there has been very little change from 1900 
to 2000 in the stable isotope ratios of fish. This indicates that there has not 
been a significant change in the assimilated diet of fishes of all feeding 
guilds. Hence, it is evident that the trophic dynamics in the St. Croix River 
have been relatively stable over the past century.

Keywords: trophic dynamics, food webs, stable isotopes, St. Croix River, 
temporal