On the basis of salinity, aquatic eco-systems can be freshwater ecosystem and marine eco-system.
Examples of freshwater eco-systems are ponds, lakes, rivers, etc., which are either lotic (running water) or lentic (standing water).
Pond as an Eco-system :
Pond is a freshwater aquatic eco-system. It is a self-sufficient and self- regulating eco-system. The two components of the pond eco-system are: (1) Abiotic component (2) Biotic component.
The abiotic components of the pond are the physical components like sunlight, water-current, heat, inorganic components like water as well as gases like carbon dioxide, oxygen, nitrogen, etc., dissolved in it, sediments and soluble nutrients containing phosphorus, calcium, magnesium, etc., organic compounds like proteins, carbohydrates and lipids are also included in its components.
The biotic components are (i) producers (ii) consumers (iii) decomposers and transformers.
The producers are either microscopic phytoplankton, which are non-rooted floating plants like algae, euglena, etc., or macroscopic plants like Hydrilla, Nymphaea, etc., which may be partly or completely submerged or floating types.
The producers use carbon dioxide from air as well as bicarbonates in water. With the help of chlorophyll they convert solar energy into the chemical energy of food material.
The primary consumers in a pond eco-system are the herbivores like small fish, tadpole, etc., which feed on phytoplankton.
These herbivores become the food of secondary consumers (carnivore order-1) like some small fishes, crabs, frogs, etc. The secondary consumers then become the food of the tertiary consumers (carnivore order-2) like large fish, turtles, etc.
(iii) Decomposers and transformers:
There are a large number of bacteria, actinomycetis, fungi and some invertebrate animals, which in water and in the sediments act as decomposers.
They decompose the complex organic materials of the dead organism and release minerals from them. These minerals again become available to the producers and thus, the cycle of materials continues.