Invertebrate Drift Dynamics in a Subtropical Blackwater River

Abstract

Invertebrate drift was studied for one year in a Coastal Plain blackwater river in southeastern Georgia, USA. Post-dusk samples were collected monthly at two sites, 160 km apart. Diel drift patterns were assessed seasonally at both sites, and a detailed analysis of diel drift was performed for two summer months at one site. Post-dusk drift densities (2-5 individuals/m³) and biomass (0.2-0.5 mg dry mass/m³) were relatively high throughout the year at both sites. Drift was composed primarily of larval black flies, caddisflies, beetles, midges, mayflies and copepods. Seventy-two to 82% of drift numbers and biomass consisted of animals normally found on submerged woody debris (the snag habitat). The remainder was from sandy and muddy benthic habitats. Drift had a relatively uniform distribution throughout the water column, suggesting that snag animals are swept far from their habitat. As a result, animals appear to drift for rather long distances (0.18-1.8 km), much further than found in most small streams. Drift periodicity was strongly night-active throughout the year, but the pattern was especially strong in summer. The summer diel analysis indicated that drift densities do not vary greatly in spite of wide fluctuations in discharge. However, a changing discharge pattern is reflected by a change in community composition. Black flies, in particular, increase rapidly in response to rising waters. Our results are consistent with the finding of concurrent analyses of production dynamics indicating that snags are an extremely important habitat for the invertebrate community in Coastal Plain rivers. We suggest that season and discharge together will determine temporal patterns of production and drift dynamics.