Gulf of Maine Area Program

Hypsometric Characterization of the Gulf of Maine, Georges Bank, Scotian Shelf and Neighboring Continental Slope

Nicholas Wolff and Lewis Incze, University of Southern Maine

Data Use Statement and Policy: These data are being compiled as part of a description of the Gulf of Maine study area by the authors. Please cite this source if you use these data informally. Consult with L. Incze before using the data in any published form.

This hypsometric characterization of the Gulf of Maine Area (GoMA) program provides data on the areas, depths and volumes of the various physiographic regions (physioregions) of the study area that we have defined for this program (Fig. 1). Please see the Physioregion section of the website for more information about the oceanography, circulation and substrate of each region. The Digital Bathymetry for the Gulf of Maine, used for this analysis, was constructed by the USGS (see Open-file Report 98-801). The eastern portions of the Scotian Shelf and Continental Slope regions extend beyond the available data in this dataset. The following hypsometric analysis covers those areas shaded in blue in Figure 1.

The delineation of these regions is somewhat subjective, but is convenient for packaging the information and calling attention to the varied characteristics of the area with respect to its size and depth. Some of the delineations follow smoothed isobaths constructed from the bathymetric data using GIS. For example, we define the coastal shelves and Browns Bank as those areas inside the smoothed 100 m isobath (100 m or shallower). Jordan and Wilkinson Basins we define as those areas deeper than the 200 m isobath, and Georges Basin as deeper than the 250 m isobath. The three major basins (Georges, Jordan and Wilkinson) are, of course, part of the central Gulf of Maine, and are singled out for analysis only because they are distinct and large features. Some of the other boundaries were determined on the basis of changes in depth gradients (change in depth per change in geographical position). Using these criteria, the northern boundary of Georges Bank is defined by the 100 m isobath, while the southern boundary of Georges Bank and the Scotian Shelf are defined by the 200 m isobath. This method is also used to define the southern, seaward boundary of the Continental Slope at the 2,000 meter isobath.

Other boundaries: We defined the Bay of Fundy by a straight line between West Quoddy Head, Maine and Cape St. Mary, Nova Scotia. We could have defined this differently, but any choice would have been arbitrary. The mouth of the Bay of Fundy is an extension of the Gulf of Maine and an open boundary, and we define it here simply to provide some measurements. A line from Cape Sable to the intersection of the Continental Slope and the eastern edge of the Northeast Channel defines our boundary between the Gulf of Maine and Scotian Shelf regions. This is one of the transect lines (Browns Bank) used in the Canadian Atlantic Zone Monitoring Program (AZMP) ref. The southwestern edge of the study area extends from southeastern Cape Cod around Nantucket Shoals, along the western edge of the Great South Channel, and finally south along 69° 20′ longitude to the 2,000 m isobath. Another straight line defines a useful boundary between Georges Bank/Great South Channel and the Southern Coastal Shelf. The Bear Seamount physioregion encompasses the seamount and some of the deeper surrounding waters. Please see the seamount section of our website for more information about Bear and several other New England seamounts.

Fig.1. Gulf of Maine Census study area and physiographic regions defined for this study.

Surface areas, total volumes and mean depth of the physioregions are given in Table 1. For comparison, data are also shown for the North Atlantic. The three deep basins singled out in this analysis (Georges, Jordan and Wilkinson) should be added to the Central Gulf of Maine region to get the true area and volume of the interior of the Gulf. As far as we can discern, the Eastern Coastal Shelf has no universally accepted common name (such as SW Nova Scotian Shelf), and our appellation is intended only as a convenient name consistent with nomenclature used for other coastal shelf areas in the Gulf of Maine. [SW Nova Scotian Shelf would be an excellent name except that the Scotian Coastal Shelf and Scotian Shelf are together commonly referred to as the Western Scotian Shelf, and it is a little awkward to have a SW shelf that is west of the western shelf.]

Table 2 gives the surface area and volume of the physiographic regions for each 10 meter depth plane. The area is the area at the depth plane indicated. The volume refers to all the water below the depth plane. The area decreases as the shore, or edges of the basin, or the bottom intersects with the depth plane being considered. The volume decreases with depth for these same reasons and because part of the overlying water column is excluded.

Fig. 2. Hypsometry (area at depth) of major regions in the study area: Gulf of Maine (regions 2-7,9-12), Scotian Shelf and Georges Bank. Note differences in the shallow (<100 m) areas.

Fig. 3. Hypsometry of the three major basins in the Gulf of Maine: Georges, Wilkinson and Jordan Basin.