Physical/Biological Interactions

## Physical/Biological Interactions on Georges Bank

Physical processes exert important influences on the biological processes which control the distribution and abundance of populations on the Bank (see Table 1 and Table 2). Understanding the connections between these physical and biological processes is the primary focus of the Northwest Atlantic/Georges Bank Study. For each of the six life stages of cod and haddock, physical and biological factors may have different effects on a cohort's survival, depending upon its location, degree of aggregation, behavior, and coincidence with planktonic prey and predators (Tables 1, 2). The principal spawning time and location of cod and haddock have evolved so that developing larvae coincide with developing plankton populations in the spring. These populations are advected along the southern flank of the Bank and arrive at the western end just as recirculation intensifies, further serving to retain these cohorts on the Bank. Examples of the important physical processes are:

- Advection - The mean inflow of water onto the Bank from the Gulf of Maine brings nutrients, which support the high primary production on the Bank (Schlitz and Cohen, 1984), and younger stages of zooplankton, which seed the spring increase in some of the zooplankton species on the Bank (Davis, 1984a). For Calanus finmarchicus, a key aspect of their seasonal biology is lateral transport on and off the Bank. The main stocks must come onto the Bank from the Gulf of Maine, Scotian Shelf, and/or from Slope Water to the south, in late-winter/spring. The stock which builds up on the Bank during spring and early summer must either die off, or lateral flow carries them over deep water where they once again enter diapause. The ultimate population size achieved by Calanus in late-spring may depend initially upon the size of the colonizing propagule of females.

  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.

- Turbulent Mixing - Tidally-induced turbulent mixing over the shallow areas of the Bank provides for rapid availability of regenerated nutrients within the photic zone and promotes the high rate of primary production observed in the Bank's central area (O'Reilly and Busch, 1984). Surface wind stress and waves also can produce significant turbulent mixing over the Bank, and the combined effects of surface and horizontal buoyancy fluxes and tide- and wind-induced mixing determine the location of the tidal mixing front found around the top of the Bank (Loder and Greenberg, 1986). Secondary circulation associated with the tidal-mixing front may cause surface convergence, which may act as a concentrating mechanism of organisms (Loder et al., 1992). Turbulent mixing, due to either surface or internal tides or winds, also can affect the contact rate between planktonic organisms (Rothschild and Osborn, 1988). Variability in turbulent intensity may have an important influence on the feeding success of zooplankton and larval fish populations (e.g., Sunby and Fossum, 1989; Davis et al., 1991).

- Stratification - Springtime development of density stratification on the southern flank of the Bank is associated with increased concentrations of chlorophyll and zooplankton in the pycnocline. This concentrating of food organisms is believed to be an important factor in the growth and survival of larval fish on Georges Bank (Buckley and Lough, 1987). Disruption of the density structure and the associated concentration of larval prey organisms by storm events could have significant negative impacts on year-class recruitment.

- Frontal Exchange - Exchange across the fronts along the edges of the Bank can transfer properties and organisms across the Bank. Along the northern flank, a tidally induced internal hydraulic jump and internal waves are believed to enhance mixing and cross-frontal exchange during stratified conditions (Loder et al., 1992). The frontal zone has a zooplankton composition that is transitional between the well-mixed area and the stratified waters in the Gulf of Maine. The trajectory of the current jet across the northeast region of the Bank acts to retain both zooplankton and phytoplankton on the Bank and contribute to their accumulation in the well-mixed area (Perry et al., in press). Along the southern flank, shear instabilites in the shelf/Slope Water front can result in the removal of water and organisms from the Bank (Ramp et al., 1983).

- Bottom-boundary Layer Processes - The bottom-boundary layer plays an essential role in the generation of tidally induced turbulence and mixing, and thus in determining the structure of the temperature and density fields. The large near-bottom shear in the oscillating current field allows for the differential movement of a vertically migrating organism relative to the average horizontal motion of the water column. The interaction of current shear and vertical migration is believed an important mechanism for organisms to maintain their location or enhance their residence time on the Bank (e.g., Lough and Trites, 1989). An on-bank (shoalward) component to the flow near bottom in late spring and summer may contribute to the retention of older cod and haddock larvae on the Bank (Lough and Bolz, 1989).

- Control of Phytoplankton Abundance and Species Composition by Mixing Processes - In shallow water columns, both wind and tidal mixing are significant processes in resupplying nutrients to the photic zone after the spring bloom and during summer months, thus enhancing primary production during months when high productivity would not normally be expected (Pingree et al., 1976). Spatial variations in tidal mixing can lead to ecological zonation along a gradient determined by levels of turbulence and water depth; a well-mixed shallow water column is often separated from deeper stratified water by a marginally stratified transition (or frontal) zone. Phytoplankton (and copepods) tend to be uniformly distributed with depth within well-mixed water columns, but concentrated at or above the pycnocline in stratified columns (Holligan and Harbour, 1977; Holligan et al., 1984a). Primary production tends to be highest in the frontal zones (Holligan et al., 1984b).

  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).