## The Georges Bank Ecosystem

### General Description

Georges Bank is a shallow submarine bank lying along the outer continental shelf about 180 km east of Cape Cod, Massachusetts (Figure 2). Along with Browns Bank, Georges Bank forms the southern sill of the Gulf of Maine with its many basins. Georges Bank is bounded on the east and west by the Northeast and Great South Channels, respectively. The Bank is roughly elliptical in shape with a very steep flank along its northern edge and a more gently sloping flank out to the shelf break at 100 m along its southern edge. The Northeast and Great South Channels have sill depths of about 230 and 60 m, respectively, and the Northeast Channel is the primary conduit for slope water flowing into the Gulf of Maine. Formed by glacial action during the last Ice Age, the Bank has a rough crest covered with large sand waves and shoals which cause the bottom depth there to vary from 10-40 m. Currents over the Bank are dominated by strong semidiurnal tidal components and a mean clockwise gyre that intensifies with increasing stratification in the summer. During the summer period, the gyre becomes partially closed with water parcels recirculating around the Bank with an approximate period of 50 days (Flagg et al., 1982). Bank waters exhibit a large seasonal temperature variation, typically increasing from 5oC in late winter to 15oC in late summer (Flagg, 1987). The biology of the Bank is characterized by high plankton and fish biomass, and high primary and secondary production. Extensive background information on the physical oceanography and biology of Georges Bank is given in Backus (1987), yet fundamental questions remain about the dynamics of populations living on the Bank and their coupling to the physical environment.

### Criteria Used in Selecting Georges Bank.

The criteria for selection of this region as the first U.S. GLOBEC study site are outlined in the U.S. GLOBEC Initial Science Plan (GLOBEC, 1991c; pp. 8-10). These reasons include the following:

- An analysis of historical data collected since 1951 shows that the Georges Bank area has experienced some of the North Atantic's largest changes in sea surface temperature (Gordon et al., 1992). Georges Bank is an area that coupled ocean-atmosphere climate models suggest will be subject to larger change than other parts of the North Atlantic Ocean (Manabe et al., 1991). The most significant regional changes are expected to be in temperature and precipitation associated with changes in surface pressure and wind stress patterns.

  The flow of water along the shelf from Newfoundland to the Mid-Atlantic Bight is largely buoyancy-driven from local and distant freshwater sources including Hudson Bay runoff and glacial or sea-ice melt from the Labrador Sea. Climatic changes in the net precipitation minus evaporation patterns between 40-65o N could alter both local runoff and the strength and relative freshness of the along-shelf flow entering the Gulf of Maine and Georges Bank. Changes in the magnitude of the subtropical Gulf Stream and subpolar Labrador Current transport due to differences in wind forcing, air temperature, and salinity could significantly impact the position and dynamics of ocean fronts, rings, and other features in the Georges Bank area. Such changes may also affect the timing, intensity and location of stratification on the region's submarine banks. Other important processes, including water formation in the Labrador Sea (Lazier, 1981) and sea ice development (Hill and Jones, 1990), may be linked to climate change and other long-term fluctuations.

- The ecosystem on Georges Bank is also sensitive to climate change (Frank, et al., 1990). Georges Bank is located in a faunal transition zone, with colder boreal water fish and plankton species to the north and warmer water species to the south (Figure 1). With the large annual cycle of water temperature on the Bank, it is both the northern limit for many of the warm water organisms and the southern limit for many of the cold water species. A climate-induced change in mean water temperature of 2oC or more could cause a significant latitudinal shift in the faunal transition zone and thus in the seasonal occurrence of different organisms on the Bank. This is in addition to the ecological consequences of other potential climate change effects on the environment, as discussed above.

- Changes in the timing and intensity of seasonal stratification due to a changing climate could effect on cod and haddock populations differentially. Cod larvae may be less sensitive to water column stratification (because they are winter spawners), whereas haddock larvae are highly sensitive to stratification (because they are spring spawners), and survive best in stratified waters.

- Important target species are present on Georges Bank and are entrained in the clockwise circulation which increases the residence time of the populations on the Bank (Walford, 1938; Clarke, et al., 1943; Davis, 1987b; Lough and Bolz, 1989). These species exhibit substantial in-situ population growth and development on the Bank because of enhanced retention and elevated food concentrations.

- A variety of physical processes including tidal mixing, stratification, frontal dynamics and storm events, together with definable populations of dominant target species allow generic physical/biological mechanisms and interactions to be studied. Another comparison made possible on Georges Bank that has broad application is a comparison of the phytoplankton food environments in well-mixed vs. stratified water columns, and concomitant study of the effects these different environments might have on the rates of growth and production of herbivorous zooplankton.

- A significant historical research and data base exists for the biology and physics of the Bank and the surrounding region. While much of this work is summarized in Backus (1987), new studies have been (and should be) undertaken on smaller-scale physical and biological processes and circulation modeling.

- Although this area has been extensively sampled in the past and a substantial base of information is available about the resident populations, the level of understanding is still not sufficient to enable predictions about the response of the populations to climate change to be made with any accuracy or confidence. This is because integrated and coupled physical/biolgical measurements either have not been made or not made with sufficient resolution. New instrumentation and techniques that are being developed, integrating both biological and physical measurement, should be readily adapted to the spatial and temporal requirements of a comprehensive study of physical/biological interactions of the Bank.

- A significant opportunity exists for comparative research through international collaboration with several programs, including the Canadian Ocean Production Enhancement Network (OPEN) program, Northern Cod Science Program (NCSP) and the International Commission for the Exploration of the Sea (ICES) Cod and Climate Change Program (CCC). In particular, the OPEN program is focussed on understanding the physical and biological interactions on bank systems on the Scotian shelf. Comparison of results between these ecosystems and Georges Bank should benefit both programs. Also, since cod has been an important fishery throughout the North Atlantic fishing grounds, coordination of the Georges Bank Study with the ICES program should improve our ability to predict broader-scale climatic influences on fisheries.

### Space-Time Boundaries of Georges Bank Study.

A working definition of the geographical boundary of Georges Bank is useful to place a space and time focus in the Northwest Atlantic/Georges Bank Study. The target fish species, cod and haddock, spend much of their early life history - from egg to demersal stages - within the shelf waters covering the Bank from winter through summer. The target zooplankton populations, which are prey for the cod and haddock larvae, also reside for most or all of their life cycle in the Bank waters. We therefore define the Bank as the area within the 100 m isobath with the western boundary located near 69o W along the center of the Great South Channel (Figure 2). The Northwest Atlantic/Georges Bank Study will focus on the key processes controlling population dynamics of the target species within the Bank during the period from December to the following August. Additional sampling in the regions around the Bank (e.g., the Gulf of Maine and Slope Water) and during the rest of the year will be necessary to better understand the physical and biological properties of the water flowing onto and off the Bank. Most or all of this will be accomplished using moored instrumentation.