Aquatic ecosystems refer to plant and animal communities’ occurring in water bodies. Aquatic ecosystems are classified as:

• Fresh water ecosystems: lakes, ponds, swamps, pools, springs, streams, and rivers;
• Marine ecosystems: shallow seas and open ocean;
• Brackish water ecosystems: estuaries, salt marshes, mangrove swamps and forests.

Aquatic Organisms

The organisms in the aquatic ecosystem are unevenly distributed but can be classified on the basis of their life form or location into five groups:

• Neuston: These are unattached organisms which live at the air-water interface such as floating plants and several types of animals. Some spend most of their lives on top of the air-water interface, such as water striders, while other spend most of their time just beneath the air-water interface and obtain most of their food within the water, e.g., beetles and back-swimmers.
• Periphyton: These are organisms which remain attached or clinging to stems and leaves of rooted plants or substances emerging above the bottom mud. Usually sessile algae and their associated group of animals fall in this group.

• Plankton: This group includes both microscopic plants, chiefly algae (phytoplanktons) and animals, primarily crustaceans and protozoans (zooplanktons) found in all aquatic ecosystems, except certain swift moving water. The locomotory power of the planktons is limited so that their distribution is controlled largely by currents in the aquatic ecosystems. Most phytoplanktons and zooplanktons are capable, however, of at least some movement.
• Nekton: This group contains animals which are swimmers. The nektons are relatively large and powerful as they have to overcome the water currents. The animals range in size from the swimming insects, which may be only about 2 mm long, to the largest animals that have lived on earth, namely the blue whale.
• Benthos: The benthos or the benthic organisms are those found living in or on the bottom or benthic region of the water mass.They exhibit a variety of adaptations to the environment since the bottom is a more heterogeneous habitat than either the open water or the surface. Benthos includes crabs, lobsters and sponges.

Freshwater Ecosystem

Fresh water ecosystem depends on the terrestrial ecosystems for large quantities of organic and inorganic matter which are constantly added into them by the communities growing on nearby land. The fresh water ecosystems can be conveniently divided into two main divisions:

• Lentic (from ‘lenis’, calm) or standing or basin series ecosystems. Examples of this division are lakes, pools, ponds, swamps and marshes.
• Lotic (from ‘lotus’, washed) or running or channel series ecosystems. Examples of this division are rivers, streams and springs.

Lotic Environment (Running water series)

• It includes “water bodies which move continuously in a definite direction”. It includes all stages and forms of river systems such as spring, stream, rivulet and river along with flood plains and swamps.
• A river has its highest elevation at its headwaters. At first, the headwaters flow as tiny, narrow channels called rills, the channels are filled only during rain storms. As the rills travel down they come together, forming wider, deeper channels called brooks. In turn, brooks combine to form streams called tributaries that carry water to a river. A river and its tributaries form a rivers system, and all stages of riverine system are defined differently.
  • Spring: A spring is a source of water body, normally the origin point of a river.
  • Creek: A creek is a natural stream of water normally smaller than a tributary.
  • Brook: An upland bubbling brook is a small stream, smaller than a creek. It has a rocky bottom and a foamy white colour.
  • Rivulet: A rivulet is also synonymous to brook or creek, This is also designated as a small stream.

Lentic environment (standing water series)

• Lentic environment consists of “static” water and includes all forms of inland water in which water is essentially stagnant or with water motion not forming a continuous flow in a definite direction. Small water movements, however, occur in the form of wave action, internal currents or flow in the vicinity of inlets and outlets. Lentic habitat includes lakes, reservoirs, ponds and swamps or bogs.

Lakes

• A lake has been defined as a body of standing water, occupying a basin and lacking continuity with the sea. A large lake is known for its stability of environmental factors. There is no sharp distinction between lakes and ponds except for the size. But they differ from one another ecologically. In lakes three distinct zones are recognized by limnologists. They are as follows: 
• Littoral zone: It includes the shallow waters of the lake which are occupied by plants.
• Limnetic zone: It includes the open water zone to the depth of effective light penetration and
• Profundal zone: It includes the bottom and deep waters which is beyond the depth of light penetration. Profundal zone is generally absent in ponds. 

Based upon the productivity; lakes are classified under three distinct heads:

• Oligotrophic lakes are characterized by great depths in which the water is transparent. The amount of nitrogen present is negligible. It is poor in organic materials. The pH is very low and hence the planktonic content is negligible. The depth of these lakes, on the other hand, are rich in their faunal content and seem to be the refuge for many peculiar deep water fishes.
• Eutrophic lakes are known for the abundance of organisms. They are shallow in nature. Eutrophic waters are rich in phosphorus, organic materials and plankton.
• Dystrophic lakes may be shallow or deep and they are rich in humus and poor in oxygen content. Such lakes often contain insect larvae and few deep water animals. Natural dystrophic waters are absent in India. Dystrophic waters in general are known for their poor faunal content. The drystrophic condition is met with in shallow waters during summer.

Ponds

Ponds are generally defined as shallow standing water bodies and they are characterized by relatively quiet waters and abundant vegetation. Usually three distinct types of ponds are recognized:

• ponds derived from large lakes
• ponds which never had any connection with lakes and which have been quite small right from the beginning of their origin and
• artificial ponds created due to human activities.

Like lakes, ponds also have two distinct zones, i.e. littoral and limnetic, and profundal zone is seldom present. Based upon the seasonal duration, two distinct categories of ponds are recognized (i) temporary or seasonal and (ii) permanent or perennial.