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Aquatic ecosystems can be freshwater (such as ponds and lakes, streams and rivers, and wetlands) or marine (such as oceans, coral reefs, mangrove swamps, salt marshes, and estuaries).

Freshwater Regions.

Water is classified as fresh if it contains less than 1 percent salt (sodium and potassium chloride and other salts) in solution. Most freshwater plants and animals are unable to tolerate higher concentrations of salt in the water.

Ponds may be only a few square meters, whereas the largest lakes cover thousands of square kilometers. Ponds and lakes are scattered all over Earth. Many important ponds are seasonal and last for only one or two months at a time. These seasonal ponds may still be important resting places for migrating birds and other animals. Some large lakes have lasted for tens of thousands of years.

Lakes and larger ponds can be divided into three different "zones," determined by depth and distance from the shore. The littoral zone is the top layer of water near the shore. It may be warmer than the average temperature of the lake, but it also may have wider seasonal temperature variation. This is generally the most biologically diverse community in the lake, including algae (like diatoms), rooted and floating aquatic plants, grazing snails, clams, insects, crustaceans, fishes, and amphibians.

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