Over 70% of our planet is covered by water. Most of us experience the edge of the sea as we walk along a beach or gaze out over the waves toward a distant horizon. Yet there is much more to the ocean than the beach. Out of sight lie vast areas where a sailor cannot see land in any direction, where storms raise huge waves and sunlight penetrates only a tiny fraction of the water above the depths.

     Near the shore, nutrients washed down from the land provide supplies for algae growing in shallow water, but further out there are times of scarcity and of plenty.

     In the open ocean, the primary producers are phytoplankton. Phytoplankton include many kinds of microscopic algae and bacteria, which photosynthesize in the bright sun light. The factor limiting growth here is the presence or absence of nutrients.   Where nutrients are lacking there may be areas with very few life forms.  Then the open ocean becomes a virtual desert, with only a few little organisms waiting for the return of essentials such as phosphorous, nitrogen, and iron.

     Tiny animals eat the phytoplankton. These animals include microscopic creatures called copepods, the young of clams, crabs, fishes, and oysters, unicellular animals and krill, which are like tiny shrimp. Although most members of this community are not even visible to humans, they still make a rich food source for fishes, marine mammals, and birds.     

     There is an interesting area in the North Atlantic called the Sargasso Sea.  The water there is caught in a huge oval surrounded by currents.  The sea rotates slowly.  It has calm weather, and so a unique ecology has grown up there:  The picture above this paragraph shows one of them. These large brown seaweeds float in the open ocean where the water currents hold them  The seaweeds grow many small floats or bladders, small grape-shaped ovals filled with gases.  These floats keep the seaweeds up at the surface of the water where they can get lots of light.  A unique community of small animals lives in this seaweed, small shrimp, crabs, and tiny fish.

    Below the surface of the open ocean the water is very deep.  Thousands of meters below the surface lies the abyssal plain where a very different set of life forms lives in the sunless darkness.