If you look up a world atlas you would notice that most of the earth's surface is covered by the waters of the oceans (about 71%). Beneath the water surface is a fascinating world of a wide variety of habitats and living communities comparable in diversity to the terrestrial biomes but different enough from them to seem to belong to a different world altogether. The main factors affecting the type and numbers of organisms found in aquatic ecosystems are water salinity and depth to which sunlight penetrates; amount of dissolved oxygen and temperature of water.