USING STABLE ISOTOPES TO MEASURE CHANGES OVER TIME IN THE MINIMALLY DISTURBED 
ST. CROIX RIVER: A POTENTIAL GUIDE TO REHABILITATION

Daniel J.Crawford1, Michael D. Delong1, James H. Thorp2, and Jeffrey 
Anderson3

1Large River Studies Center, Biology Department, Winona State University, 
Winona, MN 55987
2Kansas Biological Survey, University of Kansas, Lawrence, KS 66047
3Large River Studies Center, College of Science and Engineering, Winona State 
University, Winona, MN 55987

Throughout the course of human history, populations have dwelled alongside 
the banks of many fertile rivers.  Technological advances and increasing 
human population size have taken their toll on large rivers globally. Human 
impacts have drastically altered the ecosystems of these rivers by, but not 
limited to, pollution, alterations, impediments, and introduction of exotic 
species.  Altering ecosystems and introducing foreign species to non-native 
habitats can cause many unforeseen problems.  Troubles arise in the question 
of restoring these habitats: In what capacity can an ecosystem be returned to 
its natural state and also, what was the natural state before a particular 
disturbance occurred?  Museum specimens of fish, freshwater mussels, and 
gastropods were analyzed for carbon and nitrogen stable isotope ratios to 
calculate food chain length by trophic position.  Nitrogen and carbon 
isotopic ratios in the St. Croix River from the 1880’s through the end of the 
20th century have been consistent compared to other rivers in the Mississippi 
River basin.  Minimal environmental disturbances have occurred in the St. 
Croix River over the past 60 yr and, as a result, little change has occurred 
in specimen isotope levels during that time.  Stable isotopic ratios from the 
St. Croix River can be used as an accepted reference measure. Correlations 
with isotopic changes in other rivers with man induced alterations to the 
other rivers can serve as a means for assessing the extent of change and 
potential for rehabilitation in these complex and dynamic ecosystems. 

Keywords:  St. Croix River, disturbance, food chain, museum samples, 
rehabilitation