Mesohabitat-specific macroinvertebrate assemblage responses to water quality variation in mid-continent (North America) great rivers

We compared the responsiveness of macroinvertebrate assemblages to variation in water quality (ions, nutrients, dissolved metals, and suspended sediment) in two mesohabitats within the main channel of three North American great rivers, the Upper Mississippi, Missouri, and Ohio. Based on about 400 paired samples, we examined the responsiveness of benthic assemblages sampled in the littoral zone and assemblages sampled from the surface of woody snags in the main channel. The assemblages in the two mesohabitats were different in all rivers. Taxa richness was much higher in the benthos than on snags. Macroinvertebrate assemblage response to water quality variation was weak on the Mississippi River, but the reasons for this are unknown. Based on analysis of the similarity between the composition of assemblages from groups of sites with high and low concentrations of water quality variables, benthic assemblages were only slightly more sensitive to water chemistry variation than were snag assemblages. Results of two-sample comparisons between groups of sites with high and low concentrations of water quality variables were consistent with rank correlations of assemblage metrics with water quality. In general, there was little difference between habitats in response to variation in water quality on any river. Our simple method of snag sampling in great rivers is usually much easier than littoral benthic sampling because it does not require wading. Snag sampling in large rivers has some limitations (e.g., natural snags are sometimes absent, samples are semi-quantitative), but lack of sensitivity to water quality gradients compared to the benthos is not among them.