Differences in fine-scale structure and composition of zooplankton between mixed and stratified regions of Georges Bank

Transects were made with the Video Plankton Recorder (VPR) in different water masses on the southern flank of Georges Bank in May 1992. CTD-data, chlorophyll fluorescence, and attenuation were measured simultaneously. Images were recorded at a rate of 60 fields per second, as the instrument was towed at 2 m s−1 (4 knots). Tapes from high and low magnification cameras (imaging volumes of 0.62 ml and 33 ml, respectively) were analyzed with respect to the distribution of copepods and other grazers, as well as invertebrate predators. This paper describes the differences in patterns of occurrence of important zooplankters in well-mixed and stratified waters on the Bank and in Slope Water south of these stations. Planktonic taxa were sampled over the same range of scales as the fluorescence and hydrography, allowing direct visual comparisons of the spatial distributions of these variables. Late copepodites of Calanus finmarchicus were strongly concentrated near the surface in the stratified area, while a dense belt of Limacina sp. and Oikopleura sp. occurred below the pycnocline. Other fragile forms were also found to be dominant. Colonies of hydroid polyps were very abundant at the mixed station, especially deeper in the water column, indicating that they may have been transported up from the bottom. Colonies of the diatom Chaetoceros socialis were abundant in the cold bottom water in the stratified region and also in the mixed area. These colonies may have been in the process of sedimenting out of the water column, as similarly sized and shaped marine snow was abundant in the same area. In the Slope Water, acantharia and Trichodesmium were found in the chlorophyll maximum above the pycnocline. Other important genera encountered were: Pseudocalanus, Oithona, Centropages, Obelia, Pleurobrachia and Sagitta. Taxa that occurred in all three areas were often differently distributed with respect to depth and physical parameters, indicating that vernal stratification is an important structuring factor of plankton populations.