It is believed that life began in the ocean, possibly around deep volcanic vents in the ocean floor, or perhaps in warm, shallow seas. The chemicals of life move easily in water, and water environments support many forms of animals and plants. Aquatic life is dependent on access to sunlight and nourishing chemicals that are dissolved in the water. Some of these chemicals are carried into the ocean from the land, some are reclaimed from the decaying bodies of dead animals and plants. The availability of these elements limits the numbers of life forms that live in different areas.
Light is an important factor in determining the makeup of aquatic populations.
Plants can only grow where they have light. Because of limitations in the supply of nourishing chemicals and light, some parts of the ocean are almost lifeless, while other parts are thickly populated with life forms. There are many varieties of life forms near the land, especially where parts of the ocean are shallow, and where nutrients are being washed off the land into the water. Aquatic environments share certain characteristics . Temperatures in water change slowly and are not so extreme as temperature changes on land.