ZOOPLANKTON DISTRIBUTIONS AT THE GEORGES BANK FRONTAL SYSTEM - RETENTION OR DISPERSION

The relation of zooplankton distributions to the physical environment across the frontal system on the northern flank of Georges Bank is described for early summer. The goal of this study was to determine if the frontal system is transparent or a barrier to the transport of zooplankton on and off the Bank. Physical characteristics of the frontal system included a tidal front on the Bank, and processes associated with the Bank edge such as a thermohaline front and a strong residual current jet. Persistent patterns in the cross-bank distributions of zooplankton were related to the locations of these physical features such that separate zooplankton communities occurred on and off the Bank. Zooplankton on the Bank were most abundant in the mixed area and often maximal in the vicinity of the tidal front. The separation into Georges Bank and Gulf of Maine communities was related to the edge of the Bank, although some zooplankton from the latter community did occur on the Bank as far as the tidal front. A particle advection model demonstrates that the strength of the current jet at the Bank edge and slope is sufficient to act as a barrier to cross-bank exchange. However, in the lower layer at the Bank edge the along-bank current is greatly reduced, resulting in increased cross-bank transport for plankton which spend time in this deep layer. This contributes to local retention of plankton already on the Bank and provides a mechanism for transport onto the Bank of plankton in deeper waters at the Bank edge. It may represent an important mechanism for supplying zooplankton prey to the important fish habitats of north-eastern Georges Bank.