* Aquatic ecosystems refers to plant and animal communities occurring in water bodies
* On the basis of salinity ,Aquatic ecosystems are classified as follows-
a- Fresh water ecosystems -Water on land which is continuously cycling and has low salt content (always less than 5 ppt) is known as fresh water
b-Marine ecosystems — the water bodies containing salt concentration equal to or above that of sea water (i.e., 35 ppt or above).
c-Brackish water ecosystems — these water bodies have salt content in between 5 to 35 ppt.
*Again there are two types of fresh water ecosystems:
They are – a) Static or still water (Lentic) ecosystems e.g. pond, lake, bogs and swamps.
b- Running water (Lotic) ecosystems e.g. springs, mountain brooks, streams and rivers.
* Shallow seas and open ocean come under Marine ecosystems
* Estuaries, salt marshes, mangrove swamps and forests come under Brackish water ecosystems
* On the basis of zone of occurrence, the aquatic organisms are classified as follows-
a-Plankton: Microscopic floating organisms such as algae, diatoms, protozoans and larval forms are called plankton.
Both microscopic plants like algae (phytoplankton) and animals like crustaceans and protozoans (zooplankton) come under this group.
b-Nekton: This group contains powerful swimmers that can overcome the water currents.
c-Neuston: These organisms live at the air-water interface e.g. floating plants.
d-Periphyton: These are organisms which remain attached to stems and leaves of rooted plants or substances emerging above the bottom mud such as sessile algae
e-Benthos: The benthic organisms are those found living in the bottom of the water mass.
* The nutrient-enrichment of the lakes promotes the growth of algae, aquatic plants and various fauna. This process is known as natural ‘eutrophication’.
* Similar nutrient enrichment of lakes at an accelerated rate is caused by human activities (discharge of wastewaters or agricultural runoff) and the consequent ageing phenomenon is known as ‘cultural eutrophication’.