Distribution and abundance of thaliaceans and copepods off the southeastern U.S.A. during winter

The abundance and distribution of planktonic copepods and tunicates were 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 vertically 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, whereas at the 75 m isobath the currents showed a mixed response to wind and Gulf Stream events. Chlorophyll concentrations in the study area usually surpassed 0.5 and reached levels of 3 μg l−1. Zooplankton was 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 doliolids was inversely related to those of the calanoids. We hypothesized that this inverse relationship could be the result of (a) doliolids removing food particles, thus reducing food particle concentrations, and through that calanoid reproduction rates; (b) doliolids ingesting calanoid eggs and nauplii, thereby reducing copepod recruitment; (c) active avoidance of doliolids by downward migration of copepods; and (d) vertical shear between the upper and lower part of the stratified water column. At stations on the middle shelf the abundance of photo- and heterotrophic nanoplankton was inversely related to the abundance of doliolids. 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 that thaliacea due to their high rates of feeding, individual and population growth could impact zooplankton communities two-fold: indirectly, by reducing phytoplankton concentrations rapidly thus affecting reproduction of certain calanoid taxa, and directly, by consuming copepod eggs.