Pond and lake are fresh water ecosystems in which, like other ecosystems, there are two main components:
(A) Abiotic component
(B) Biotic component
(A) Abiotic component:
Abiotic component of pond consists of water, dissolved minerals, oxygen and carbon dioxide. Solar radiations are the main source of energy.
(B) Biotic component:
It includes the following:
(iii) Decomposers and transformers.
On the basis of water depth and types of vegetation and animals there may be three zones in a lake or pond littoral, limnetic and pro-fundal. The littoral zone is the shallow water region which is usually occupied by rooted plants. The limnetic-zone ranges from the shallow to the depth of effective light penetration and associated organisms are small crustaceans, rotifers, insects, and their larvae and algae. The pro-fundal zone is the deep water parts where there is no effective light penetration. The associated organisms are snails, mussels, crabs and worms (Fig. 3.3).
The main producers in pond or lake ecosystem are algae and other aquatic plants, such as Azolla, Hydrilla, Potamogeton, Pistia, Wolffia, Lemna, Eichhornia, Nymphaea, Jussiaea, etc. These are either floating or suspended or rooted at the bottom. The green plants convert the radiant energy into chemical energy through photosynthesis. The chemical energy stored in the form of food is utilized by all the organisms. Oxygen evolved by producers in photosynthesis is utilized by all the living organisms in respiration.
In a pond ecosystem, the primary consumers are tadpole larvae of frogs, fishes and other aquatic animals which consume green plants and algae as their food. These herbivorous aquatic animals are the food of secondary consumers. Frogs, big fishes, water snakes, crabs are secondary consumers. In the pond, besides the secondary consumers, there are consumers of highest order, such as water-birds, turtles, etc..
(iii) Decomposers and Transformers:
When aquatic plants and animals die, a large number of bacteria and fungi attack their dead bodies and convert the complex organic substances into simpler inorganic compounds and elements. These micro-organisms are called decomposers chemical elements liberated by decomposers are again utilized by green plants in their nutrition (Fig. 3.4).