The tendency for the around-bank circulation to become closed with increasing stratification provides a mechanism for retaining planktonic organisms on the Bank and is believed to be important for survival of larval fish populations on the Bank (Iles and Sinclair, 1982; Bolz and Lough, 1984). In contrast, storm events and warm-core ring entrainments which remove water from the Bank represent potentially important mechanisms for episodic exchanges of organisms from the Bank.
The phytoplankton community is often characterized by a dominance of diatoms in the well-mixed and frontal zones, and flagellates in the stratifed waters (Holligan and Harbour 1977). As noted earlier in this document, this is known to be true for Georges Bank (O'Reilly et al., 1987; O'Reilly and Busch 1984). Highest phytoplankton biomass is often associated with a mixed water column (see Figure 10). Such variations in food web structure could influence the egg production, growth and recruitment rates of herbivorous zooplankton. This was one of the key hypotheses of "Foodweb I", the final CEPEX experiment (Grice et al., 1980). A recent attempt to study the effects of a well-mixed vs. stratified water column on copepod dynamics was carried out at the Marine Ecosystems Research Laboratory at the University of Rhode Island. Sullivan (in press) compared zooplankton abundance and community structure in well-mixed and stratified tanks. She found that copepods were much more abundant in the stratified tanks, and that dominance in the two tanks shifted from Eurytemora and Acartia tonsa in the well-mixed tanks to Oithona colcarva in the stratified tanks. It is intriguing that these changes in abundance and species composition were remarkably similar to those described by Holligan et al., (1984a) for well-mixed and stratified waters of the English Channel. How (or if) a larger species like Calanus finmarchicus would respond to a spatial gradient in water column mixing is not known. For any species, it is not known whether a response is due to solely to physical disturbance resulting from to increased turbulence or to biological differences resulting from the increased mixing (i.e., differences in phytoplankton biomass and species composition).