Gulf of Maine Habitat

The Gulf of Maine is highly productive. Currents and seafloor topography work together to give rise to a diverse assemblage of species. The northerly Labrador Current and the southerly Gulf Stream meet in the Gulf of Maine, creating a counter-clockwise gyre that delivers nutrients around the entire Gulf (1). The Gulf's combination of deep basins and channels and shallow banks supports a rich variety of water temperatures and correspondingly high biodiversity (1). Together, the structure of the waters and the structure of the bottom determine the mix and arrangement of species in the Gulf of Maine. 

The Gulf of Maine is a cold-water ecosystem -- much colder than other New England waters, with a surface temperature of 10°C (50°F) in the winter and 17°C (63°F) in summer. Cape Cod is considered a strong physical barrier to migration. Many of the inshore species in the Gulf of Maine do not appear in Southern New England, and vice-versa. 

The Gulf of Maine is also deep – 1,236 ft. at its deepest, and 490 ft. on average – with a thermocline (distinct temperature change) occurring 300-1,000 ft. below the surface. Cold waters beneath the thermocline are high in dissolved oxygen, and then they are stirred up, they replenish nutrients throughout the Gulf. The ample Gulf of Maine nutrient base supports a thriving phyotplankton community, which in turn supports zooplankton, and explains the high abundance of planktivorous fishes in the Gulf. Horizontal layers of water caused by differences in temperature and salinity, tidal surges, and seasonal upwelling events provide concentrations of plankton that attract various species. 

The Gulf of Maine is a partly encircled body of water, bounded on the west, north, and east by the coastlines of Massachusetts, New Hampshire, Maine, the Bay of Fundy, and Nova Scotia, and on the south by the raised underwater plateau of Georges Bank. Water flows into the Gulf of Maine on either side of Georges Bank, via the Great South Channel and the Northeast Channel. Water entering through the Northeast Channel combines with the force of the tides from the Bay of Fundy to cause a counterclockwise gyre that rotates around the whole Gulf. Thanks to the underwater boundary created by Georges and Brown’s Banks, this 36,000 square-mile area possesses a nearly enclosed circulation system. It takes about three months for the surface waters of the Gulf of Maine to make a full rotation of its perimeter.

Although the Gulf of Maine can be thought of as one big ecosystem, it can also be viewed as a collection of smaller ecosystems. In the west, Cape Cod Bay, Stellwagen Bay, Jeffreys Ledge, and the Wilkinson Basin provide a varied seafloor topography that provides habitat to different assemblages of species. In the east, Georges Bank, Browns Bank, Georges Basin, and the Northeast Channel play that role. In the North, tidal mixing driven by the Bay of Fundy moves away fine sediments, leaving gravelly habitat favored by herring for spawning. The Gulf’s rocky coasts are prime habitat for rockweed, kelp, blue mussels, barnacles, sea stars, sea urchins, and juvenile lobsters, and provide haul-out sites for marine mammals and nesting areas for seabirds.

The Gulf of Maine watershed pours 250 billion gallons of fresh water, laden with nutrients, into the Gulf each year.  The 7,000-mile coastline of the Gulf of Maine is interspersed with various small estuaries where rivers meet the sea: Massachusetts Bay, Great Bay, Saco Bay, Casco Bay, Merrymeeting Bay, Sheepscot Bay, Muscongus Bay, Penobscot Bay, Blue Hill Bay, Frenchman Bay, Narragaugus Bay, Englishman Bay, Machias Bay, Cobscook Bay, and Passamaquady Bay.

The Gulf of Maine shoreline is dotted with salt marshes, seagrass beds, and rockweed beds. Salt marshes are extensive around the head of the Bay of Fundy and the perimeter of Cape Cod, and at the mouths of major rivers. Seaweed beds occur in high energy environments with exposed boulders. At high tide, rockweed beds (Fucus and Ascophylum) become three-dimensional, and their floating fronds creating a refuge from predators for tomcod, pollock, sculpin, alewives, white hake, cod, and winter flounder. At low tide, the moist fronds protect invertebrates from the light of the sun. In deeper water, kelps like Laminaria and Alaria grow, down to a depth of 60 feet. Kelp habitats provide refuge for lobsters when they are shedding their old shells. Seaweeds, including kelps, oxygenate water as a byproduct of photosynthesis, and when the decompose, they enter the detrital food chain.

The Gulf of Maine has a denser concentration of islands than anywhere else on the Atlantic coast. Islands interrupt tidal and current flows, helping to oxygenate water and enhance local upwelling processes. By multiplying the area covered by intertidal habitats like salt marshes, islands enhance coastal productivity and greatly increase the availability of habitat for inshore creatures.

The highly productive waters of the North Atlantic, combined with strong tidal flows, complex circulation, and varied seafloor topography, result in a diversity of fish species (3). Fish gravitate towards habitats defined by structural complexity, temperature changes, areas of high plankton concentration, and certain sediment types. The deep basins, shallow banks, and major channels of the Gulf of Maine are consistently important for fish productivity (3).

Fish are not distributed randomly within the Gulf of Maine; their location is related to the ecological structure provided by the seafloor and the waters (2). Observers have noted that demersal fishes seem more abundant along the western Gulf, where waters are more stable and phytoplankton blooms once each year in the spring before sinking to the bottom to support a rich bottom community. In the east, where waters are more mixed and phytoplankton productivity is to be high throughout the year, small pelagic fish like herring abound (2).

Nor is the distribution of fish species consistent in time. Cod, haddock, ocean perch, and six species of flounder are year-round residents, while sturgeon, sharks, menhaden, alewives, shad, dogfish, and squids move into the Gulf to feed in the summer, and leave in the fall to journey south.

Species’ life histories account for their usage of specific parts of the Gulf of Maine environment.

• Herring require tidally mixed areas bounded by oceanographic fronts in order to spawn; thus herring distribution during the fall spawning season is a function of depth and tides.
• Shrimp, which require cold water on the bottom, are found in highest abundance where water is most stratified: in the southwestern part of the Gulf of Maine. Part of the reason for this preferences may be that shrimp swim upwards during the night, crossing into the layer of warm water above the bottom; scientists have found that shrimp egg production is highest where the temperature difference between the cold bottom layer and the warm layer above it are greatest.
• Redfish are found in the deepest parts of the Gulf: Truxton, Jordan, and Crowell Basins. These places provide stable environments where the slow-growing, late-maturing redfish can live out their long lives.
• Members of the cod family (gadids) – cod, haddock, pollock, silver hake, red hake, and white hake – are found throughout the Gulf, but the distribution of each species differs. Apollino and Mann (2) say that “with each species able to function best in a certain part of the Gulf, it is as if nature has hedged its bet, so to speak, to ensure that whatever environmental disturbances there may be, there is always a species of this family to fill the gadid niche within the system (88).” The temporal patterns of these species also complement each other, with spawning patterns following one another, leading Apollino and Mann (2) to surmise that “nature, perhaps, doesn’t care whether a cod or a haddock or a cusk performs the function (91-92).”
• Green urchins at times become so abundant that they consume whole kelp beds, creating what are called urchin barrens, where nothing grows. The urchin harvest industry has helped keep this phenomenon in check.
• Mussels are found along rocky shores, particularly under rockweeds.
• Soft-shell clams are found in high densities on mudflats, though abundance varies interannually as a result of changes in larval transport. An overall decline took place in the 1990s, particularly in Eastern Maine, possible linked with the appearance of think mats of green algae.

Sources:

(1) Cook and Auster 2007

(2) Apollino and Mann 1995. In Conkling, P.W., ed. From Cape Cod to the Bay of Fundy: An Environmental Atlas of the Gulf of Maine. Cambridge, Massachusetts: MIT Press.

(3) Greene, J.K., M.G. Anderson, J. Odell, and N. Steinberg, eds. 2010. The Northwest Atlantic Marine Ecoregional Assessment: Species, Habitats and Ecosystems. Phase One. The Nature Conservancy, Eastern U.S. Division, Boston, MA.

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