Why are oceans salty? Over four billion years, the water cycle has been dissolving trace amounts of salts, including chlorine, sodium, sulfur, magnesium, calcium, and posassium from the crust of the Earth, delivering it to marine waters. The cumulative buildup of those small concentrations of salts now adds to a concentration of roughly 3.5 percent, which we label as salt because that's how it tastes to us. When the water cycle continues and moves into the air by evaporation from the surface of the ocean, the salts are left behind to increase in concentration.

There are other elements, too, in salt water. Particularly important are the mineral nutrients taken in by phytoplankton (from Greek planktos, wandering), the microscopic marine photosynthesizers that derive energy from sunlight and support the web of marine life. Phytoplankton convert dissolved carbon dioxide into the structural parts of their bodies, and into the organic molecules of life. We can't see them with the naked eye, but they are the miracle that brings the ocean to life.

Zooplankton, microscopic animals that graze on phytoplankton, are fed on by larger predators such as small fish and squid, which are fed on by larger fish with which we are more familiar because we sometimes find them on our dinner plates.

The average marine catch of the world's fisheries is currently about 87,000,000 metric tons (1 metric ton = 2,205 pounds). Roughly 25 percent of that catch--22,000,000 metric tons--is taken from the Atlantic Ocean.

In addition to its commercial fish, the North Atlantic contains a rich diversity of marine species, including 5 species of sea grasses, 13 species of corals, 432 species of mollusks ("shellfish"), 77 species of shrimps and lobsters, 87 species of sharks, 56 species of seabirds, and 48 species of marine mammals (whales, seals, dolphins, porpoises, and so on).

In the North Atlantic Ocean, prevailing winds blow in a clockwise motion out from the subtropical high, a semi-permanent high pressure belt located at 30 degrees latitude. Surface waters drift along with this whirl in the same direction, nudging waters from North America toward Europe. One thing leads to another, and pretty soon all that moving water begins to pile up. Pressure differences arise in the water column and soon, deep below, a slow reactive motion begins. This is the birth of the Atlantic's surface currents.

Perhaps the best known of these currents are the Gulf Stream and the Labrador Current. Both are part of an immense whirl of water in the Atlantic called a gyre. The Gulf Stream flows northward along the East Coast of the United States, carrying quantities of warm water from the tropics to higher latitudes. Close to Cape Cod, the Gulf Stream veers away from shore and moves eastward, where it eventually slows and mixes with a much broader current called the North Atlantic Drift. In the far north, a smaller subpolar gyre shunts icy arctic water into the southward-moving Labrador Current. These two great currents collide close to the Grand Banks off the shore of Newfoundland, where intense fogs are often created by the contact of warm and cold waters.

Maine's coastal climate is influenced dramatically by the course of these two great Atlantic currents. Neither the Gulf Stream nor the Labrador Current flows directly into the Gulf of Maine, which is separated from the greater Atlantic by a series of underwater banks and mountains. Occasionally, the Gulf Stream spills onto Georges Bank. When this happens, the sudden temperature rise and change in water chemistry results in massive die-offs of Gulf of Maine fish.