These ecosystems are characterized as having running water (lotic) or still water (lentic)
Lotic ecosystem includes fresh water streams (springs, rivulets etc) and river trend over their course from being narrow, shallow and relatively rapid to increasingly broad deep and slow moving.
As would be expected, only organisms capable to adhesing to an exposed surface (rocks, limbs etc.) are found in a upper reaches of a stream. Such adhesing organisms are termed as periphyton or aufwuchs. The periphyton includes attached, clumped and filamentous green and blue green algae and various sessile invertebrates, including larvae of insects like black flies and other midges flattened may fly and stonefly nymphs and plan Arians. Further down stream floating and emergent regetation may found, along with sessile invertebrates and those that burrow in the softer substrate such as dams and burrowing may fly nymphs. In both environments we might find crayfish, fish to various sizes from darters to trout and salmons that must expend considerable energy, like alice in wander land, to stay in the same place. Further downstream we also find warm water fish, such as catfish and crab, replacing the cold water trout’s.
Chemically, the upper reaches of lentic environments are rich in oxygen; as the water moves down stream and becomes more sluggish the oxygen level tends to drop. Because of the continual addition of nutrients and detritus enrooted, nutrient levels tend to be higher downstream.
Lentic ecosystems (pods, ponds, lakes). Vary considerably in physical, chemical and biological characteristics. In general they can be considered to have three zones-littoral, limoetic and profundal.
The littoral zones extends from the shoreline to the innermost rooted plants, successively passing from rooted emergent forms such as needs and cattail, rooted species with floating leaves such as water lilies and in deeper waters to various submerged but rooted species. This zone is populated by frogs, snakes, snails, clams and a considerable variety of adult larval insects.
The limpetic zone is the open water down to the depth of light penetration in shallow lentic environments the light may penetrate to the bottom. This zone contains phytoplankton that consists of diatoms, green and blue green algae and a variety of zooplankton from protozoan to microarthopods. It is also the zone for a variety of larger swimming organisms including fish and insects.
The profundal zone occurs below the limnetic zone and in deep-lakes, this zone may constitutes the largest water volume of a lake. The major food source in the profundal zone carries from a detritus rain from the limnetic zone. The bottom consists mostly of decomposers.