DISTRIBUTION AND ABUNDANCE OF THALIACEANS AND COPEPODS OFF THE SOUTHEASTERN USA DURING WINTER

The abundance and distribution of planktonic copepods and tunicates we re studied in relation to physical and biological variables from late January to early February 1990 on the middle and outer shelf off South Carolina, U.S.A. The inner and part of the middle shelf were vertical ly mixed throughout the study period, the middle and outer shelf only after passage of a storm on 4 and 5 February. Subtidal currents at the 40 m isobath were highly correlated with local alongshore winds, wher eas at the 75 m isobath the currents showed a mixed response to wind a nd Gulf Stream events. Chlorophyll concentrations in the study area us ually surpassed 0.5 and reached levels of 3 mu g l(-1). Zooplankton wa s characterized by high abundances of doliolids (Tunicata, Thaliacea) and relatively low concentrations of calanoid and cyclopoid copepods. When doliolids were very abundant the vertical distribution of dolioli ds was inversely related to those of the calanoids. We hypothesized th at this inverse relationship could be the result of (a) doliolids remo ving food particles, thus reducing food particle concentrations, and t hrough that calanoid reproduction rates; (b) doliolids ingesting calan oid eggs and nauplii, thereby reducing copepod recruitment; (c) active avoidance of doliolids by downward migration of copepods; and (d) ver tical shear between the upper and lower part of the stratified water c olumn. At stations on the middle shelf the abundance of photo- and het erotrophic nanoplankton was inversely related to the abundance of doli olids. The frequency of occurrence of thaliacea in neritic regions is evaluated in relation to hydrographic regimes and the availability of seed populations. From previous and present observations we assume tha t thaliacea due to their high rates of feeding, individual and populat ion growth could impact zooplankton communities two-fold: indirectly, by reducing phytoplankton concentrations rapidly thus affecting reprod uction of certain calanoid taxa, and directly, by consuming copepod eg gs.