POPULATION AND COMMUNITY PATTERNS OF INVERTEBRATE DRIFT IN AN UNREGULATED COASTAL-PLAIN RIVER

Drifting invertebrates were sampled monthly during postdusk hours for 2 yr in the Ogeechee River, a sixth-order river in the southeastern United States. Drift quantities were among the highest reported in the literature, with mean biomass varying from 2.41 to 2.46 mg dry mass/m3 between years and mean density from 20.4 to 22.8 individuals/m3. Major contributors to drift biomass were Ephemeroptera (30.3%), Coleoptera (25.1%), Plecoptera (16.6%), and Trichoptera (14.5%). Taxon-specific seasonal patterns of drift biomass for several mayflies (Ephemerellidae and Isonychia) and the dominant stonefly (Perlesta placida) resulted in biomass peaks in winter - early spring. Caddisflies (mostly Hydropsychidae and Chimarra) and beetles (Elmidae and Gyrinidae) complemented the mayfly-stonefly pattern with highest values in late spring - summer. These seasonal patterns coincided with standing stock biomass patterns on submerged wood (snags), their preferred habitat. Drift values were about 5.7 times higher in the Ogeechee than in another Coastal Plain river having only one fifth the amount of snags, suggesting a relationship between habitat abundance and drift. Percentages of snag-dwelling insects found in drift were substantially higher (0.1 to > 1%) than is generally found for smaller streams, indicating drift distances and times much greater than previously shown.