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comprised 65-72% of the total floodplain microcrustaceans quantified in our study. <br />Cladoceran community shift <br />The most dramatic community shift in our study occurred in the floodplain. The <br />floodplain cladocerans experienced the greatest change with planktonic and benthic <br />densities decreasing from 33,097 and 72,809/mZ respectively in June to 2,058 and <br />526/mZ in August. Others have also reported peak densities of cladocerans in early <br />summer with abrupt declines (Osmera 1973, Kahn 1987, Twombly and Lewis 1987, <br />Robertson 1990). This decrease has been attributed to increases in mortality rather <br />than decreases in natality (Twombly and Lewis 1989, Robertson 1990). Causes of <br />mortality include increased predation, food limitation, temperature changes, and <br />changing water quality (Twombly and Lewis 1987, 1989, Robertson 1990). The redox <br />measurements (Table. 2) of the substrate shows that it became anoxic by August <br />which would decrease the benthic cladocerans. The cladocerans typical in the <br />floodplain benthos in July were absent from the plankton in August. <br />Comparison of habitat types <br />The microcrustacean densities of the floodplain are considerably greater than <br />those of the backwater or the river channel. In a comparison of the first floodplain <br />sample period in June and the first sample in July for both the Ouray backwater and <br />the river site, the following generalizations can be made: 1) The density of the benthic <br />microcrustaceans is 41x greater in the floodplain than in the other habitats, and 2) <br />17 <br />