TO the untrained eye, the Gulf of Maine is just a chunk of open ocean embraced by Cape Cod to the south and Nova Scotia to the north. In fact, it's often called ``a sea within a sea'' because of its unusual bathymetric characteristics. To the south, its sea floor is bounded by Nantucket Shoals and Georges Bank - where the water is sometimes only 13 feet deep. Browns Bank wraps around from the northeast. To the east lies a narrow stretch of deep water known as Northeast Channel - the only escape for the water pouring in from the region's many rivers.
Contained by these boundaries, water within the Gulf circulates counterclockwise in what scientists call a gyre. The influx of fresh water brings up nutrients from the bottom, causing an abundance of plant and animal life. As a result, the Gulf differs significantly from the open ocean in temperature, salinity, and biota.
``The Gulf of Maine is not the Atlantic Ocean,'' says David Keeley, director of the Maine Coastal Program. ``It's not like New Jersey, where you look out to Spain. The Gulf of Maine is really an ecosystem.''
And so far, it's a healthy one - unlike the Gulf of Mexico, which in some ways it resembles.
Keeping the Gulf of Maine pristine, he says, is especially important. That means controlling not only visible trash but less visible chemicals and storm-sewer runoff. Because of the gyre effect, pollutants get trapped in the region. ``What somebody puts in in Massachusetts circulates - it doesn't wash away,'' he says, noting that trash dumped off Cape Cod can find its way to Nova Scotia - and back again.