COMPARISON OF SUMMER ZOOBENTHIC COMMUNITIES IN 4 HABITATS OF A FLOODPLAIN IMPOUNDMENT - 1975 AND 1990

The diversity, evenness, similarity, standing crop and abundance of be nthic macroinvertebrates in Navigation Pool 8 of the upper Mississippi River were compared in 1975 and 1990. Macroinvertebrates were collect ed in midsummer from four habitats: marsh, bay, open water and side ch annel. These habitat types accounted for 64% of the total habitat area in the impoundment. The community structure changed in all four habit ats; the percentage similarities between 1975 and 1990 were lowest in open water (19%), intermediate in bays (44%) and side channels (50%) a nd highest in marshes (62%). Macroinvertebrate diversity and evenness were not significantly different between 1975 and 1990. The standing c rop decreased in all habitat types. The abundance decreased in all hab itats except bays between 1975 and 1990. The greatest changes in stand ing crop and community composition occurred in bays and open water. Be cause the bays and open water habitat comprise 55% of the total area, it is possible that these declines may signal degradation of the aquat ic habitat in the entire reservoir. Changes in macroinvertebrate commu nity structure and standing crop may have been due to unstable sedimen t conditions caused by erosion of islands, resuspension of sediments a nd loss of aquatic macrophytes and depth.