The oceans which cover about 71 per cent of the earth's surface are crucial to the maintenance of the environment for they contribute to the basic oxygen-carbon-dioxide balance of the biosphere, upon which the human and animal life depend. About 70 per cent of earth's oxygen is produced by ocean phytoplankton.
Water from the oceans evaporates and forms clouds which are carried away to hills and plains where they rain. Ocean is the major source of available water on earth. Any human intervention, addition of pollutants, exploitation of ocean resources such as pollutants from oil spillage and nuclear testing are likely to bring about drastic imbalance in the global water cycle and this may change the pattern of the climate.
Estuaries are the tail end of rivers located on the sea shore. They are one of the most productive ecosystems of earth. Estuaries play an important role in protecting coast line from erosion and damage. Being full of lush green vegetation they provide a buffer zone between coastal and river waters. They are also recognized as delicately balanced ecosystems.
Coral reefs, similarly, are among the habitats of richest biological diversity. They are formed by calcareous remains of colonial coelenterate animals and their skeletons and offer a variety of micro-habitats to many species of invertebrates and fishes. Pollution in the marine ecosystem largely arises out of economic activities of man. While a few problems are chronic and therefore quite complex in some countries, however, they are relatively simpler than in others. India has a coastline of about 7,000 km. Communities located near the seashore, industries and agricultural farmers dispose 1,645 km3 of used waste water into the sea. You know that all rivers, canals and streams ultimately find their way into sea. India ^fourteen major rivers drain 85 per cent of their run off into Indian Ocean, the pollutants carried by them-are also carried into the sea, ultimately causing pollution of marine waters. This, however, is a highly irrational way of using marine resources. You will find after going through this unit that oceans deserve much greater care from human beings than is being accorded at present.
Apart from the above three sources, the pollution of marine waters takes place in ways unique to seas and oceans. In the section we will discuss marine water pollution caused by oil spills, industrial effluents, discharge of hot waters and mining of polymetallic nodules.