Benthic invertebrates are the small animals, such as clams, worms, and crustaceans that live on or in the bottom substrate of a water body. These organisms are an important food source for many fish and crustaceans, including many recreationally and commercially important species. In addition, benthic organisms can be important organisms for monitoring the adverse effects from anthropogenic impacts. Numerous studies have described benthic invertebrate communities in coastal South Carolina, a few of which have occurred in the ACE Basin study area.
Benthos can be classified into two categories based on their size: meiofauna (63 to 500 m m) and macrofauna (>500 m m). Studies of meiofaunal communities and their distribution patterns in both the estuarine and freshwater portions of the ACE Basin drainage system are lacking. Research on meiofaunal organisms from other coastal areas of South Carolina indicate that they have an important role in the estuarine food web complex since meiofauna consume bacteria, other microfauna and flora, and detritus; and they are, in turn, consumed by many larger macrofaunal invertebrates and juvenile finfish (Bell and Coull, 1978; Smith and Coull, 1987; Coull, 1990).
Studies of the benthic macrofauna in the ACE Basin have been more extensive than for the meiofauna, although sampling has largely been limited to subtidal estuarine and nontidal freshwater habitats. A diverse array of epifaunal species was found at most of the estuarine stations sampled in the ACE Basin. Species present at more than 70% of the stations sampled by Calder and Boothe (1977a) and Van Dolah et al. (1979) included several species of arthropods, cnidarians, and bryozoans.
Infaunal species which frequently occurred in collections in the ACE Basin included the polychaetes and amphipods (Calder and Boothe 1977b; Calder et al. 1977; Van Dolah et al. 1984 1991; and Hyland et al. 1996, 1998). A few species were found at only one or a few sites, but were very abundant (>1000 individuals/m2). Other taxa that were commonly found at many of the sites in the ACE Basin, often at high abundances, and were not identified to the genus or species level included oligochaetes, nemerteans, and actinarians. Although benthic invertebrates from tidal creeks, sand beaches and marsh flats have not been sampled in the ACE Basin, species collected from these habitats in other parts of South Carolina should be representative of those found in the ACE Basin system.
The dominant taxa observed at riverine sites in the ACE Basin include insects, an isopod, crayfish, and oligochaetes. In general, insect taxa form a much greater component of the invertebrate taxa found in freshwater habitats compared to estuarine.
In general, there are many factors that play an important role in regulating the distribution and abundance of the meiofaunal and macrofaunal communities in the ACE Basin. Since these biota represent an important food source for many other larger taxa, predation effects are often a major regulating factor. Competition, both among individuals within a species as well as among species, can also play a major role in limiting faunal abundances and distribution. These factors, when combined with the effects of various physico-chemical factors such as salinity, temperature, dissolved oxygen, sediment grain size, depth of the redox (reducing) layer within the sediments, and distribution along the intertidal-subtidal depth gradient in estuarine environments, result in very complex spatial and temporal patterns of these assemblages. Both sediment characteristics and salinity conditions appear to be the primary environmental factors influencing the distribution of the infaunal species, as well as some of the epifaunal taxa that were collected from the ACE Basin.