Southern Coastal Shelf

Overview
Adjacent to the most densely populated area in the Gulf watershed, the Southern Coastal Shelf in all likelihood bears the heaviest impact from human activities of any area in the Gulf. Partially cut off from the Gulf of Maine proper by Stellwagen Bank, a shallow undersea plateau, the Southern Coastal Shelf is somewhat self-contained. Currents trap nutrients on the Southern Coastal shelf which support extensive marine productivity; but the same currents also trap contaminants emanating from a highly urbanized environment.

Surface area
3,183 mi2

Depth
Intertidal zone to ~100 m

Bathymetry including key features
The irregular bathymetry of the Southern Coastal Shelf was largely formed during glaciation: glaciers gouged the bottom and deposited sediments. As the ice melted, glacial features were reworked and eroded and larger rocks were deposited. With the return of the sea, erosion and redistribution of sediments occurred in a process that continues today.1 Two prominent features of the Southern Coastal Shelf are Stellwagen Bank, which is as shallow as 20 m in parts, and Stellwagen Basin, just west of the Bank, which reaches depths of 80 m.2,3

Substrate
Higher points of the Southern Coastal Shelf topography are characterized by coarse-grained sediments ranging from boulders and cobbles to sand. Deeper areas contain finer grained sediments such as silt and clay.4 Nor’easters are the predominant factor in the distribution of sediment types on the Southern Coastal Shelf. High winds create currents flowing in a southwesterly direction that winnow finer sediments from shoal areas and deposit them in the basins and on the trailing edges of banks.5

Relationship to large-scale circulatory patterns of the Gulf of Maine
A portion of the coastal current that flows southwestward along the Maine coast continues onto the Southern Coastal Shelf at Cape Ann (the other branch of the coastal current continues south along the edge of Wilkinson Basin). The degree of inflow from the coastal current is partly a function of the prevailing winds. Northeast winds increase the flow of water onto the shelf from the coastal current. Inflow of the coastal current, augmented in the spring by freshwater runoff from the Merrimack and other rivers to the north of Cape Ann, sets up a weak counterclockwise gyre over Massachusetts Bay and Cape Cod Bay. The current that is not entrained in the gyre exits the bays at the tip of Cape Cod where it continues in a southerly direction toward the Great South Channel.6,7,8

Information on average (seasonal/annual) oceanographic conditions
In the spring, currents on the Southern Coastal Shelf are characterized by a weak counter clockwise gyre over Massachusetts and Cape Cod Bays. In late spring and summer, circulation in Cape Cod Bay becomes isolated. Inflow of lower salinity water in the spring begins to stratify the water column; seasonal heating enhances stratification which remains in place until October when surface cooling leads to mixing. Storm events occasionally disrupt summer stratification. Primary productivity follows a pattern typical of coastal waters. Phytoplankton blooms occur in the spring when day length allows algal growth to take advantage of nutrient rich waters. As stratification sets in, nutrients in the upper sunlight layers are depleted. Primary and secondary production slows and decaying material becomes sequestered in bottom sediments. The onset of colder weather in the fall mixes nutrients back into the water column where the cycle starts again once sufficient sunlight is available to support plant growth. Three areas of the Southern Coastal Shelf are particularly productive. Boston Harbor and inner Cape Cod Bay support higher levels of production because their shallow waters allow recirculation of nutrients. Stellwagen Bank is productive because tidal forces in the Gulf keep the water over the Bank well mixed.9,10,11

Presence of GoMOOS buoy
There is one GoMOOS buoy on the Southern Coastal Shelf. Buoy A is located south of Cape Ann.

Whale abundance, distribution and migratory patterns
Cape Cod Bay, within the Southern Coastal Shelf, is designated as a “critical habitat area” for the right whale, designated as an endangered species by the National Oceanic and Atmospheric Administration. Right whales feed on plankton in Cape Cod and Massachusetts Bays during late winter and early spring.

1 USGS, undated
2 Rendigs and Bothner, 2002
3 Oldale, undated
4 Rendigs and Bothner, 2002
5 USGS, undated
6 MWRA, 2003
7 USGS, 1997
8 SBNMS, undated
9 MWRA, 2003
10 USGS, 1997
11 SBNMS, undated