Every year June 8 is observed as the World Ocean Day to underscore a need to protect the great body of salt water, which covers 70.78 per cent of the earth’s surface. Over two-thirds of the surface of our planet is covered with water. Although this great water expanse is divided into five ocean bodies, it is in fact one vast water body. The ocean acts as a gigantic weather machine for its surrounding land mass and controls wind pressure and temperature of a region. Like landmass, a huge ocean system has several distinct regions. Differences in depth of water, temperature, degree of salinity and presence of oceanic nutrients create habitat for growth of a huge variety of oceanic flora and fauna. It is said that if one cubic of sea water evaporates, it produces 40 million tons of salt. There are three zones in the water of the ocean. The photic zone, euphotic zone or sunlight zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur. It extends from the surface down to a depth where light intensity falls to one per cent of that at the surface, called the euphotic depth. Accordingly, its thickness depends on the extent of light attenuation in the water column. Typical euphotic depths vary from only a few centimetres in highly turbid eutrophic lakes, to around 200 metres in the open ocean. It also varies with seasonal changes in turbidity. Since the photic zone is where almost all of the primary productivity occurs, the depth of the photic zone is generally proportional to the level of primary productivity that occurs in that area of the ocean. About 90 per cent of all marine life lives in the photic zone. A small amount of primary production is generated deep in the abyssal zone around the hydrothermal vents which exist along some mid-oceanic ridges.

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