Any unit that includes all the organisms i.e. the communities in a given area, interact with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycle (exchange of materials between living and non living components) within the system, known as an ecological system or ecosystem.
The term ecosystem is derived from two different words, eco = environment; system = an interacting, interdependent complex.
Major oceans of the world, Atlantic, Pacific, Indian, Artic and Antartic cover approximately 70% of the earth’s surface. Each ocean indeed represents very large and stable ecosystem. Marine environments appear to be more stable in their chemical composition due to being saline, and moreover other such physio-chemical as dissolved oxygen content, light and temperature are also different.
The biotic components of an ocean ecosystem are of the following orders:
i. Primary consumers – The herbivores that feed directly on producers are chiefly crustaceans, mollusks, fish etc.
ii. Secondary consumers – These are carnivorous fish, as Herrin, Shad, Mackerel etc, feeding on the herbivores.
iii. Tertiary consumers – Still in the food chain, there are other carnivorous fishes like Cod, Haddock, halibut etc. that feed on other carnivores of the secondary consumers level. Thus, these are the top carnivores in the food chain.