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220 GREAT BASIN NATURALIST [Volume 55 Bray-Curtis Distance 1.00 I 0.75 I 0,50 r 0,25 r -0,00 r Seasonal Werlond - June Ri.er Chonnel - July Ri.er Channel - August Seasonol Werland - July River Backwater - August Seosonol Werland - August Ri.er Backwoler - July Side Chonnel - June Fig. 3. UPG MA cluster analysis of Green River habitat types located in the Ouray National Wildlife Refuge. Side Chonnel - July the original Bray-Curtis distances. Ephemeral side channel samples show the greatest simi- larity (least distance), and wetland and back- water sites are more similar to one another. DISCUSSION Nematoda The importance of free-living nematodes in aquatic systems has not been extensively stud- ied. Aquatic nematodes are known to be micro- botrophic, predaceous, and/or parasitic during one or more of their life stages (Poinar 1991). Due to the scarcity of adequate keys and their small size, nematodes are seldom listed beyond the phylum designation in most studies and may not even be quantified. In studies of aqua- tic systems where nematodes are quantified, highest densities have been found in lakes. Strayer (1985) and Nalepa and Quigley (1983) reported that nematodes comprised 60% and 80%, respectively, of all benthic metazoans in Mirror Lake, NH, and in Lake Michigan with means of680,OOO/m2 (Mirror Lake) and 260,000/ m2 (Lake Michigan). In contrast, Palmer (1990) in Goose Creek and Gladden and Smock (1990) on the floodplain of Colliers Creek reported that nematodes comprised a much smaller percentage (6% of total invertebrates) and occurred at diminished densities (1000-15,000/ m2 and 1746/m2, respectively) in lotic systems. In our study nematode density estimates from the seasonally inundated wetland June sample (7133/m2) and the July and August river channel samples (24,881/m2 and 2421/m2, respectively) are comparable to densities pre- viously reported from lotic systems (Gladden and Smock 1990, Palmer 1990). Density esti- mates for all other sites and dates (54,872- 302,603/m2) are more similar to densities in lentic habitats (see above). Greater densities are achieved in the more stable benthic envi- ronments provided by calmer waters and finer sediment particle size. In their study of White Clay Creek, Bott and Kaplan (1989) found that nematode densities were greater in silt than in sand. In our study the highest densities are also associated with a low sand content in the substratum. Low densities reported for the June sample of the seasonally inundated wet- land site reflect the relatively short time that water had been on the sample site. Of the four major invertebrate groups collected in this study, nematodes accounted for 8% of the individuals in the river channel August sample and 98% in the June ephemeral side channel. Nematodes accounted for 67.7% of all organisms observed, Palmer (1990), using a 3.3-cm-dia. core and 44-p.m mesh, reported that nematodes constituted only 4-15% of the Goose Creek community, with a mean of 9%, Her data are similar to our river channel values, High nema- tode densities and their high percentage of the total invertebrates that we report from the ephemeral side channel, river backwater, and seasonally inundated wetland are unusual and should be compared to samples taken at similar locations in this and other large rivers using comparable methods. Oligochaeta Freshwater oligochaetes are a well-studied and diverse group found in every type of estu- arine and freshwater habitat. They feed mostly on bacteria living in soft sediments (Brinkhurst and Gelder 1991). The amount and quality of ~