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GULF OF MAINE Coastal New England has a rich history. European settlers arrived shortly after 1600 to trade, fish, harvest the forests, farm the land, settle and live. Native North Americans had done the same - though perhaps more passively - for close to 10,000 years. And though the natural beauty of the region is undeniable, both Europeans and native peoples before them settled and remained because harvest from both land and sea was plentiful. The accompanying brief description of the Gulf of Maine introduces some of the natural features that make it such a highly productive system. Cod, herring, whales, and other species - including man - have thrived on this productivity for thousands of years. As we come to better understand how this unique system works, we hope newfound information may be used to better manage and protect its priceless living resources. Physical Description The Gulf of Maine is a roughly rectangular, partly enclosed sea occurring along northeastern North America, from Cape Cod, Massachusetts, north to the northern Bay of Fundy, and east to Cape Sable, Nova Scotia. The Gulf is bounded along its Atlantic margin by Georges Bank and Browns Bank, which are large, shallow shoals that greatly reduce water exchange with the open ocean. The entire Gulf occupies an area of approximately 91,000 square km (36,000 square miles). A series of 21 basins lie within the Gulf of Maine. The deepest (300-400 m) of these include Georges, Wilkinson and Jordan basins. 
A limited volume of Atlantic Ocean water flows as a cold coastal current over the shallows of
Browns Bank to enter the Gulf of Maine near Cape Sable. Additionally, deeper nutrient-rich oceanic
water surges tidally into the central basins of the Gulf of Maine through the Northeast Channel -
a narrow, deep (230 m) passage between Georges and Browns banks.
Once water has entered the Gulf, it is directed to the northeast toward Nova Scotia and the Bay of Fundy by the rotation of the Earth. The water is then deflected to the southwest by the northern coast of the Gulf. A large, counterclockwise circulation pattern that results is called the Gulf of Maine Gyre. Circulation is further driven by the phenomenal tidal cycles that flood into the Bay of Fundy along its eastern shoreline and then ebb back into the Gulf. The gyre moves surface waters at a rate of approximately 7 nautical miles a day. A single revolution around the entire Gulf takes about three months. Bottom waters in the deep basins also circulate, but they do so more slowly. It takes about a year for deep Gulf water to cycle through the basin system. Water exits the Gulf primarily through the 75-m deep Great South Channel, which lies between western Georges Bank and Nantucket Shoals. Water also flows out of the Gulf over the eastern portion of Georges Bank. The System That Glaciers Built As restricted as the communication between the Gulf of Maine and the open ocean is now, it was even more isolated in the very recent geological past. The last of the Great Glaciers that scoured the Gulf's basins began to retreat some 13,000 years ago. Once freed from the tremendous weight of an ice sheet more than a mile thick, the terrestrial and submarine land rebounded and the sea withdrew to a level that was about 60 m less than the current sea level. At this point (around 11,000 years ago), Georges and Browns banks were emergent land masses. Water entry into Gulf was limited to the tidal flow through the narrow Northeast Channel. The hydrology of this ancient landlocked regime - which geologists refer to as the DeGeer Sea - was strikingly different than that of the present-day Gulf of Maine. For example, the tidal range of the Bay of Fundy was probably 30-60 cm, whereas today it can exceed 15 m. Eventually, Georges and Browns banks again became submerged, as glacial ice continued to melt and sea level rose to essentially its present height (approximately 4,000 years ago). Basins, Shelf, Slopes, and Canyons Before the deep basins of the central Gulf were scoured by glacial action, they occurred as shallower cuts carved by ancient rivers at a time when this part of the seafloor existed as dry 
land. Georges and Browns banks were apparently resistant to the glacial action that in large part
shaped the central Gulf. Instead, the mammoth ice sheet was diverted by Georges Bank to the east,
where it carved out the deep submarine valley that became the Northeast Channel.
The entire southern flank of Georges Bank is cut by a series of deep (1,200 m) submarine canyons whose geological origins are not fully understood. The larger canyons extend deep enough into the shelf and slope to expose rock strata deposited as much as 120 million years ago. Over geologic time, these canyons have been eroded, filled, and emptied again by a combination of seismic, hydrologic, glacial, and other processes. The current mission will be conducted in Wilkinson Basin, the shelf/slope waters along the southern edge of Georges Bank, and Oceanographer Canyon. These distinct environments occurring within the Gulf of Maine region differ from one another in their potential for supporting primary production. As such, each gives rise to it's own distinct living systems. By investigating a range of environments, scientists are likely to gain a broader understanding of the trophic importance of the siphonophore Nanomia cara. Marine Productivity The Gulf of Maine is among the most productive marine habitats in the world, accounting for a rich fishery heritage and history whose origins predate European settlement of North America. This high rate of productivity is made possible by a combination of physical system morphology (topography) and the seasonally changing climate of the Gulf region. Here, as elsewhere, the basis for a large and productive fishery is the photosynthetic phytoplankton that constitute the base of the coastal marine food web. If we look at the central Gulf of Maine, we see a phytoplankton primary production that is typical of high latitude coastal environments. Blooms in the central Gulf occur in the spring (April-May), and again in the fall (September-October). During these times, stratified upper (warm) and lower (cold) layers of water destabilize allowing vertical mixing to occur. This turbulent mixing is important because it brings photosynthetic phytoplankton from surface waters in contact with nutrients from deeper water. The result is a phytoplankton bloom. The spring bloom event persists until upper and lower water masses again stabilize (become stratified), as solar heating warms surfaces and makes the water less dense. Thereafter, the phytoplankton primary producers deplete the nutrients in surface waters and the bloom dissipates. In spite of the presence of ample solar energy in the summer months, a lack of nutrients in the surface waters acts to limit primary production in the central Gulf. In contrast, a fall bloom event is limited not by nutrients but by the availability of sunlight. As surface waters cool, vertical mixing again brings phytoplankton and nutrients together. But the depth to which sunlight penetrates and allows photosynthesis (called the photic zone) is shallower and decreases further as fall turns into winter. In contrast to the central Gulf of Maine, the photic zone over Georges Bank and other shoal habitats extends through much of the water column. Furthermore, the shallow depth of these habitats ensures strong vertical mixing that is not seasonally mediated. Rather, tidal currents cross the shoals and mix the entire volume of overlying water. These conditions promote a higher degree of primary production that is spread more evenly throughout the year. Although they occupy only thirty percent of the total area of the Gulf, shoal habitats contribute about sixty percent of the total Gulf primary production. Note: Much of the information contained on this page has been adapted from the following sources: "From Cape Cod to the Bay of Fundy: An Environmental Atlas of the Gulf of Maine" (P.W. Conkling, ed.) 1995 MIT Press. "Georges Bank" (R.H. Backus and D.W. Bourne, eds.) 1987 MIT Press.  |  |
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