Freshwater ecosystems include ponds, lakes, streams, rivers and wetlands. Freshwater means the body of water has a salt concentration of less than 1 percent. Plant and animal life of freshwater ecosystems do not survive in salt water.
Ponds and lakes vary from a few square yards to thousands of square miles. They occur across the globe but have a high concentration in Northern Hemisphere areas that were affected by Ice Age glaciers. The sun heats the layer near the shore of a lake, the littoral zone. Species there include algae and other plants, snails, insect larvae, crustaceans, and fish. The waterfowl, snakes and turtles in the littoral zone feed on them. The top layer away from the shores, the limnetic zone, is home to plankton and fish. The profundal zone, the deepest part, is colder and home to heterotrophs, creatures that eat dead organisms.
Rivers and streams have headwaters at springs and lakes or in snowmelt. At the mouth, the river empties into another body of water, sometimes another river or a sea or ocean. Water is cleaner at the headwaters, has more oxygen and has more freshwater organisms such as trout and heterotrophs. At the mouth where water is often cloudy from sediments, there may be less oxygen, and there are less diverse life forms and different types of organisms, such as carp and catfish. Organisms also differ from nearer the banks to mid river.
Wetlands are standing water bodies that include swamps, bogs and marshes. Plant species here are known as hydrophytes, which are adapted to the moisture and humidity. Birds, reptiles, amphibians and furry animals make their home in wetlands. Some wetlands are salt marshes and are not considered freshwater ecosystems.