Aquatic communities are the world's major water habitats. Like land biomes, aquatic communities can also be subdivided based on common characteristics. Two common designations are freshwater and marine communities.
Rivers and Streams are bodies of water that continuously move in a single direction. Both are rapidly changing communities. The source of the river or stream usually differs significantly from the point at which the river or stream empties. A variety of plants and animals can be found in these freshwater communities, including trout, algae, cyanobacteria, fungi, and of course, various species of fish.
Estuaries are the areas where freshwater streams or rivers meet the ocean. These highly productive regions contain widely diverse plant and animal life. The river or stream usually carries many nutrients from inland sources, making estuaries capable of supporting this rich diversity and high productivity. Estuaries are feeding and breeding grounds for a variety of animals, including waterfowl, reptiles, mammals, and amphibians.
Lakes and Ponds are standing bodies of water. Many streams and rivers end in lakes and ponds. Phytoplankton are usually found in the upper layers. Because light is absorbed only to certain depths, photosynthesis is common only in the upper layers. Lakes and ponds also support a variety of plant and animal life, including small fish, brine shrimp, aquatic insects, and numerous plant species.
Oceans cover approximately 70% of the earth's surface. Marine communities are difficult to divide into distinct types but can be classified based on the degree of light penetration. The simplest classification consists of two distinct zones: the photic and aphotic zones. The photic zone is the light zone or area from the surface of the water to the depths at which the light intensity is only around 1 percent of that at the surface. Photosynthesis occurs in this zone. The vast majority of marine life exists in the photic zone. The aphotic zone is an area that receives little or no sunlight. The environment in this zone is extremely dark and cold. Organisms living in the aphotic zone are often bioluminescent or are extremophiles and adept at living in extreme environments. As with the other communities, a variety of organisms live in the ocean. Some include fungi, sponges, starfish, sea anemones, fish, crabs, dinoflagellates, green algae, marine mammals, and giant kelp.