Lakes, ponds, rivers, streams and wetlands that have a low salt concentration (usually below 1%) and serve as habitats are called freshwater ecosystems. The two major divisions of freshwater ecosystems are the lentic ecosystems and the lotic ecosystems.
Lentic ecosystems are those whose water is still, and are made up of ponds, marshes, ditches, lakes and swamps. These ecosystems range in size from very small ponds or pools that may be temporary, to large lakes. Lakes and ponds are broken down into three different zones, each with its own different society of organisms. The littoral zone is the one closest to the shore. Because these areas are shallow, light is able to reach to the bottom, making this warmest area. The abundance and diversity of plants and animals living in this zone are the food for the other, larger animals that reside there. Moving inward away from the shore, the limnetic zone still receives plenty of sunlight. This zone is very rich in microorganisms called plankton. The plankton that live here include plants (phytoplankton) and animals (zooplankton). As you get deeper into the limnetic zone, less light is present. Under the limnetic zone is and are that is much colder and denser region, known as the profundal zone. Here, there is little light that makes it all the way down. The result is that photosynthesis, or the process by which plants take energy from the sunlight and use it to create their own food, is less prominent. Lack of photosynthesis means the plants here die. The bottom area of the lentic ecosystems is the benthic zone. In this area, dead and decaying organisms fall from above, and are consumed by the bottom-dwelling animals.
Lotic ecosystems are characterized by flowing waters. Examples include rivers, streams, brooks and springs. Here, the oxygen level is higher due to the continually moving waters of the current and the water is clearer. The flow of the water is different depending on the area of a river or stream. This impacts what type of plants and animals can live in those areas.