Relative Importance of Water-Quality Stressors in Predicting Fish Community Responses in Midwestern Streams

Evaluation of relations among water-quality stressors, fish communities, and land use allows for the identification of important stressors linked to human activities that adversely affect fish communities.  Fish were collected from streams in the Midwestern U.S., an area dominated by intense cultivation of row crops but with growing urban development.  We focused on ten stressors, including total nitrogen, total phosphorus, riparian disturbance, riparian vegetative cover, stream bed sedimentation, instream fish cover, streamflow variability, dissolved oxygen, pesticides, and bed sediment contaminants.  Fish community response was assessed using a fish stressor index (FSI).  The FSI was developed as an abundance-weighted expression of species-specific tolerances to stressors and adjusted for natural variation.  Regression tree analysis indicated that total nitrogen was the most important stressor predicting the FSI and percent cultivated crop land use was the most important predictor of total nitrogen.  Decision trees provided a quantitative basis for broad geographic scale recommendations that could result in minimizing negative impacts to fish communities and help restore aquatic ecosystems in the Midwest.