Permeating the water column, crawling across the sea floor, flowing unobstructed with the currents, living creatures rove the Gulf of Maine in incredible numbers. To us a cold and even dangerous place, the Gulf's oceanographic features make it a superbly nurturing environment to other animals.
The waters of the Gulf of Maine surpass even the most fertile terrestrial landscape in sheer biomass. Great schools of fish, birds, and marine mammals ply its waters to graze on rich blooms of phytoplankton. For centuries cod reigned as the chief commercial fish harvested from the Gulf, so enriching to early settlers in Massachusetts that a gold likeness of the fish was placed on the roof of the old State House.
While the Gulf of Maine is well known for its vast schools of groundfish and other cold-water species, relatively few species are found here. Unlike the tropical regions of the earth, where water temperature may vary little during the course of a year, the tremendous variation in temperature and salinity in the Gulf of Maine from season to season means that only a few well-adapted species will thrive here. The few that do flourish in great abundance.
The reasons for this richness lie in the water. The Gulf of Maine is a semi-enclosed sea. Its major connection to the North Atlantic is the Northeast Channel, a deep submerged valley separating elevated areas known as Georges and Browns Banks. Through the Northeast Channel, cold nutrient-rich water from the Atlantic sweeps into the Gulf of Maine. Because it is cold and saline, this water is more dense than the water above. Hence it quickly settles into the mid and lower regions of the Gulf, where it would remain if not for an unusual confluence of tides and seasonal weather patterns.
Each winter the water that lies at the surface of the Gulf of Maine loses its heat to the cooler atmosphere. Because cold water is heavier than warmer water, this cold water sinks down from the surface. As it settles, it mixes with the nutrient-rich water brought in from the North Atlantic. This is called "winter turnover" or convective mixing. How deep this mixing occurs depends on a number of factors including the density of the underlying water, amount of heat lost to the atmosphere, and surface winds. When spring comes and temperatures rise, this mixing slows. The layers of water in the open Gulf then become more stable.
As a result of this winter mixing, by March and April cold, nutrient-rich water lies throughout the upper water column of the Gulf. In addition, the sun grows ever stronger as the weeks pass. This combination of nutrients and sunlight creates a great bloom of phytoplankton. These tiny drifting plants form the basis of the Gulf's intricate food web.
In the summer, waters in the mid-Gulf continue to stabilize. As the surface water warms, convective mixing slows. As a result, cool, salty water, rich in nitrogen and other nutrients, stays below. Without these necessary nutrients, phytoplankton density in the open Gulf drops. However, closer to shore, water layers remain mixed due to the great energy of the tides.
The range of the tide in the Gulf of Maine is greater than any other place in the world.
The strength of the tide provides the energy necessary to churn together the different layers of water near the coast, constantly combining oxygen and vital nutrients necessary for phytoplankton growth.
While the tides are strongest in the northeast Gulf of Maine, the Gulf's currents ensure that the well-mixed water is circulated throughout the region. The Gulf of Maine current travels in a counter-clockwise direction around the Gulf. Its relatively rapid speed means that water can move from southwest Nova Scotia around the Gulf's perimeter and out onto Georges Bank in approximately three months.
Envision a great water highway. Bearing nutrients and phytoplankton as it passes along the coast, the Gulf of Maine current also picks up valuable passengers. The larvae of lobsters, herring, and other species are carried from northeastern spawning grounds along the coast to sheltering habitats within bays and around islands. The current sweeps all along the coasts of New Hampshire and Massachusetts, finally bearing north and east to spin itself out on Georges Bank. It truly links the three states and two Canadian provinces in an unmistakable if unseen bond.