Monitoring the health of large rivers with macroinvertebrates: Do dominant taxa help or hinder the assessment?

Abstract Macroinvertebrate communities in the Mississippi River are often dominated by a few taxa (e.g. oligochaetes in the fine sediments and hydropsychid caddisflies on rocks) that exploit the natural abundance of fine organic particles. These taxa are moderately to highly tolerant of pollution, and the combination of high abundance and high pollution tolerance could result in the river being assessed as highly polluted. We examined how the presence or absence of dominant oligochaetes or hydropsychid caddisflies affected biometric values and responses used to monitor environmental stress in large rivers. Responsiveness was assessed across five simulated impairments created by removing different subsets of sensitive taxa from the original data for macroinvertebrates in the Upper Mississippi River near Cape Girardeau, MO. For many metrics, removal of the dominant taxa changed values, and increased the difference between existing conditions and simulated impairments. For macroinvertebrate data from fine‐sediment habitat, 8–9 of 12 metrics responded to the two most severe impairments simulated with or without dominant oligochaetes, but none changed for less severe impairments. For rock habitat, more metrics responded to moderate or severe impairments simulated when dominant hydropsychids were excluded. Few metrics performed better with dominant taxa. Overall, removal of dominant macroinvertebrate taxa improved both the accuracy and interpretability of several metrics commonly used to monitor effects of water pollution on large‐river faunas dominated by a few taxa. Removing them also makes observed responses more comparable to those frequently quantified for wadeable streams and rivers. Copyright © 2009 John Wiley & Sons, Ltd.