We sampled macroinvertebrates in the benthos and on the surface of woody snags in three mid-continent USA great rivers, the Upper Mississippi River, the Lower Missouri River, and the Ohio River, all of which flow through several large urban areas. We defined urban and non-urban zones of each river based on mean percent impervious surface extracted from recent land cover data. We estimated that 28-36% of the rivers, by length, was urbanized. Based on multivariate ordination, the overall structure of the great river assemblages was not different between urban and non-urban zones for either the benthos or snags in any river. Most taxa (82 %) did not exhibit a significant positive or negative effect of urbanization in either river. Many of the taxa that were responsive to zone type were relatively rare (river- and habitat-specific relative abundance < 5 %). For responsive taxa, the effects of urbanization were generally weaker on the Upper Mississippi and Ohio Rivers than on the Lower Missouri River and weaker for snag-surface assemblages than for the littoral benthos. Among the taxa that were more abundant in urban sections were several insect taxa generally considered intolerant of pollution. We speculate that urbanization, possibly due to nutrient enrichment and resulting increased food resources (algae, seston), may improve conditions for these taxa. There appeared to be a threshold in mean percent impervious surface for a great river subcatchment near 5 % above which urban-intolerant taxa were always rare and urban tolerant taxa were often abundant.
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