Characterization of the Ashepoo-Combahee-Edisto (ACE) Basin, South Carolina

Zooplankton

Zooplankton,small animals (mainly invertebrates) resident in the water column but with limited mobility, are important for their trophic position in aquatic environments. Microplankters, such as heterotrophic protists, are the primary link between phytoplankton and bacterial biomass and higher trophic levels. Mesoplankters, particularly copepods and cladocerans, are essential as food for early fish larvae and macroplankters, which in turn are fed upon by late larval and postlarval fish and other organisms.

Knott (1980) described the abundance of mesozooplankton in the North Edisto River at Bluff Point, near the boundary of the ACE Basin Characterization Area, where copepods were the dominant group. Although the abundance of meroplanktonic zooplankton (spending only a portion of their lives as plankters) was uniformly low at this site, organisms such as decapod crustaceans appeared to be dominant components of the macroplankton community. Recent studies by researchers working in the North Inlet estuary of South Carolina provided some insight into the composition and dynamics of a macroplanktonic community that is likely to closely resemble that in the ACE Basin. The most abundant organisms in a 6-month series of daily samples were fish larvae, larval and postlarval decapod crustaceans (including noncommercial species), juvenile bivalves, and holoplanktonic organisms (permanent zooplanktonic residents of the water column) (Houser and Allen 1996). The existing data on freshwater zooplankton communities suggest that free-living non-photosynthetic protists, rotifers and microcrustaceans are the dominant components (Sandifer et al. 1980).

For many larval fish and crustacean species that depend on wetlands and shallow water bodies as spawning or nursery grounds, an abundant zooplanktonic population is necessary. In estuaries, macroplankters such as mysid shrimp and gammarid amphipods are an important food source for large animals (e.g., Atlantic croaker, Atlantic menhaden, seatrout, drum, blue crab, and white shrimp) (Ragotzkie 1959; Van Engel and Joseph 1968; UGMI 1971). In freshwater floodplain wetlands, larger animals preferentially select cladocerans.

Very little data exist on the dynamics of zooplankton populations in the southeast, including the ACE Basin study area. However, like other organisms, the assemblages of zooplankton in marine, estuarine, and freshwater habitats are subjected to the vagaries of their habitat. For example, in coastal marine and estuarine habitats, zooplankters are influenced by strong tidal forces, wind stress, bottom friction, and buoyancy fluxes. An abundance of both food items and predators, as well as other environmental cues such as light, salinity, and temperature, influences the activity and production of estuarine and freshwater zooplankters.