Freshwater Ecosystems

There are two general terms that categorize freshwater ecosystems. Lentic refers to standing bodies of water (lakes, ponds, and inland wetlands) while lotic refers to flowing systems (streams and rivers).

Lakes

Lakes are “large, natural bodies of standing water formed when precipitation, runoff, or groundwater seepage fills depressions in the Earth’s surface” (Miller).

The four zones of a lake from top to bottom are: the littoral zone, the limnetic zone, the profundal zone, and the bathyal zone. The littoral zone encompasses the area near the shore at the top of the lake that receives sunlight, extending down to the depth where rooted plants stop growing. This zone has high biodiversity. The limnetic zone is the sunlight part at the top of the lake, similar to the littoral zone. However, the limnetic zone is the open area away from the shore. Most photosynthesis occurs in this part of the lake. The profundal zone is the deep open water, where it is too dark for photosynthesis. The benthic zone is the very bottom of the lake. Organisms here tend to tolerate cooler temperatures well. For the profundal and benthic zones, low levels of photosynthesis result in low levels of dissolved oxygen.

The parts of a lake can be classified by temperature. The epilimnion is the upper layer of warm water. The hypolimnion is the lower layer of colder, denser water. The thermocline is the area in between in which water temperature decreases rapidly with depth. As depth increases, dissolved oxygen levels decrease, so the epilimnion has the highest amount of oxygen and the hypolimnion the lowest amount.

In general, lakes can be one of three types: oligotrophic, mestrophic, or eutrophic. An oligotrophic lake is a newly-formed lake with a small supply of plant nutrients, deep and steep banks, and low primary productivity. A eutrophic lake has a large, excessive supply of nutrients (mostly nitrates and phosphates) and a high primary productivity. Eutrophic lakes can be created through human influences, such as pollution. A mesotrophic lake is somewhere between the two extremes.

Streams

Like lakes, streams are also divided into different zones. The source zone is the area where the stream begins. It usually consists of mountain streams with cold, clear water the flows quickly downhill. This area usually has a high level of dissolved oxygen mixed in from the air. The transition zone is the area where mountain streams merge to form wider, deeper streams that flow down more gradually and with fewer obstacles. At this point, the stream is still moving fairly quickly. The floodplain zone is the area where streams merge into wider, deeper rivers that flow slowly across broad, flat valleys. Increased erosion occurs here.

Other terms that refer to streams are: surface water, precipitation that does not sink into the ground or evaporate; runoff, surface water that ends up flowing into a stream; and watershed, the land area that contributes runoff sediment and dissolved substances to a stream.

Thinkquest Team "Fish," March 2005, Disclaimer and copyright information