Multimetric macroinvertebrate indices for mid-continent US great rivers

We developed a set of great-river macroinvertebrate indices of condition (GRMINs) for the littoral benthos of mid-continent great rivers in the US (Missouri, Upper Mississippi, and Ohio). We used a multiscale (site, reach, landscape), multimetric abiotic stressor gradient to select macroinvertebrate assemblage metrics sensitive to human disturbance. We used the same stressor gradient to derive reference expectations by empirical modeling. We developed GRMINs for each of 5 reaches: the upper Missouri River, the lower Missouri River, the unimpounded Upper Mississippi River, the impounded Upper Mississippi River, and the Ohio River. We created 2 versions of each index. One version was based on the absolute value (AV) of the correlation between invertebrate metrics and the stressor gradient and potentially included all types of metrics (e.g., diversity, composition, tolerance to pollution, feeding guild). The second version included no tolerance-value-based metrics (NTV). Each index included up to 10 metrics. The indices were reliable for general assessment of great-river aquatic conditions except in the unimpounded Upper Mississippi River. The AV and NTV versions of the GRMIN were highly correlated (r = 0.94). The indices revealed longitudinal variation in condition on great-river reaches and among-river variation in relative condition. No sites near or <100 km downriver of Kansas City, Missouri, on the Missouri River or Minneapolis–St. Paul, Minnesota, on the Upper Mississippi River were in least-disturbed condition. GRMIN indices are an indicator of biotic response to general human disturbance. They are appropriate for local and regional-scale ecological assessments and long-term monitoring.