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. Water supports the bodies of living things. Water life forms do not need to be as concerned about supporting their bodies as land life forms. Living things that live in water are not in danger of dehydration. Water is more dense than air.