ABSTRACT
Water is the most vital component of the abiotic environment without which life cannot exist. Water occurs on the earth in abundance. Water enters the atmosphere principally by evaporation from the surface of oceans, lakes, rivers, land and plants, as well as by transpiration from green plants. The hydrological cycle binds together all components of the earth’s various ecosystems, drives all natural processes and sustains various organisms, plant and animal communities, humans and all life processes and activities. Ecosystems are classified in many ways. They are often categorised on the basis of climate, physiography and dominant vegetation. The differences in the water regime cause different plant and animal species to occupy the habitats and result in the diversity of aquatic, mostly freshwater ecosystems. The movement of water below ground often creates a variety of habitats where the groundwater is a dominant factor influencing and sustaining a specialised community of organisms.
