A comparison of water-column macroinvertebrate communities in beds of differing submersed aquatic vegetation in the tidal freshwater Potomac River

Macroinvertebrates are a major food source for fish species and macrophyte beds are hypothesized to harbor a rich community of these organisms. Macroinvertebrates inhabiting the water column in two macrophyte beds and an adjacent open area were sampled in a small embayment of the tidal freshwater Potomac River. One macrophyte bed consisted of an almost complete monoculture ofHydrilla verticillata, while the second community was a more diverse mixture of plant species. In samples with substantial amounts of submersed aquatic vegetation (SAV), macroinvertebrate density was two orders of magnitude higher than and substantially more taxa were found than at the open water site. Total macroinvertebrate abundance was significantly greater at theH. verticillata site than at the mixed site in July, but no significant difference was observed in August. Taxa richness did not vary between the two vegetated sites in July but was higher in the mixed bed in August. While the two vegetated sites shared similar taxa, they differed in their abundance. TheH. verticillata site harbored more hydrobiid snails, and the mixed site was characterized by more chironomids and hydroptilid caddisflies. Differences between July and August collections were even greater than between sites. Numbers of hydroptilid caddisflies, baetid mayflies, and coenagrionid damselflies were substantially higher in August, while oligochaetes, hydrobiids, and chironomids were reduced. Results support the hypothesis that water-column macroinvertebrates are greatly enhanced in the presence of macrophytes. The ecological significance of the less substantial differences in macroinvertebrates between macrophyte beds requires further study.