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222 GREAT BASIN NATURALIST [Volume 55 11m2 on over-flooded meadows in Czechoslo- vakia, In Lake Norman, NC, Bowen (1983) reported a mean larval ceratopogonid density of 767/m2. Ceratopogonid densities reached a peak in the August river channel sample (13,0261 m2)_ much higher than any reported in the litera- ture above. In their study of the Green River, Grabowski and Hiebert (1989) did not report densities, but did conclude that ceratopogo- nids were more abundant in river channel samples than in backwaters. Our study supports this conclusion, Average densities for the river channel July and August samples were 3608/m2 and 13,026/m2, respectively, compared to 961m2 and 461/m2 for the backwater July and August samples. Ceratopogonid larvae were complete- ly absent from the ephemeral side channel as well as the June and July seasonally inundated wetland samples. Chironomidae Chironomidae are typically the most abun- dant macroinvertebrates in lentic (Strayer 1985) and lotic (Grzybkowska and Witczak 1990) sys- tems. Studies of relatively small geographical areas have reported impressive numbers of species. For instance, Douglas and Murray (1980) found 142 species in Killarney Valley, Ireland. High diversity of chironomids makes them important as indicators of environmental condition (Wingard and Olive 1989). They are also abundant and provide an important food source for fish (Brown et al. 1980, Winkel and Davids 1987, Grabowski and Hiebert 1989), waterfowl (Titmus and Baddock 1980), and other migratory birds (Bowman 1980). We identified 19 chironomid genera from our sites within the Green River ecosystem. Other investigations oflotic systems have yield- ed similar numbers-12 genera in the upper Tuscarawas River, OH (Wingard and Olive 1989), 24 genera in the River Frome, England (Pinder 1980), 25 genera in the Mississippi River (Beckett et al. 1983), and 36 genera in Juday Creek, IN (Berg and HellenthaI1991). Grabowski and Hiebert (1989) studied the Green River in the same general area consid- ered in our study and also identified 19 genera. However, only seven of the genera reported by the latter authors were found in our study: Chironomus, Cricotopus, Cryptochironomus, Polypedilum, Procladius, Tanypus, and Tanytarsus. Densities of chironomids in aquatic sys- tems can vary substantially. In a study of Lake Vissavesi, Finland, Paasivirta and Koskenniemi (1980) reported densities of 641m2 in a coarse debris habitat and 2997/m2 in a moss-grown site. Jonasson and Lindegaard (1979) reported 59,000/m2 from Lake Myvatn, Iceland. Vari- ability in lotic systems has also been docu- mented. Pinder (1980) reported densities from a low of 481m2 to 6273/m2 in a chalk stream in England, and Grzybkowska (1989) found 1O,664/m2 in the River Grabia, Poland. While no distinct trends exist when comparing chiro- nomid densities in lentic and lotic systems, den- sities are influenced by sediment size (Paasivirta and Koskenniemi 1980, Beckett et al. 1983). Chironomid densities from the July and August river channel samples were 4148/m2 and 3516/m2, respectively. River backwater samples were 31,125/m2 and 22,864/m2 for the same times. Grabowski and Hiebert (1989) reported maximum chironomid densities in the same area of the Green River of less than 100/m2 for the river channel and 2800/m2 for river backwaters-substantially less than our estimates. It is possible that annual differences in seasonal discharge, area of the sampling device, and later sampling period all contrib- uted to this discrepancy. However, because of significant differences in mesh size (63-tLm ours, 600-tLm Grabowski and Hiebert's), data of Grabowski and Hiebert and ours cannot be considered equivalent. It is worth noting that mesh sizes larger than 100 tLm have been shown to negatively bias density estimates (Strayer 1985). Community Similarity Cluster analysis of the data showed that, in general, habitat types clustered together inde- pendent of sample date, suggesting that the different habitat types studied in the Green River are distinct. Beckett et al. (1983), for ex- ample, studied five habitats in the Mississippi River and also found them to remain composi- tionally distinct regardless of flow and sample date. Distribution and abundance of benthic macroinvertebrates characteristic of these habitat types have been attributed to flow con- ditions and sediment size in our study. Since flow conditions are the major determinant of particle size, flow conditions are likely the determining factor. This conclusion has also t