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:

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.