Life in the oceans, especially the upper layers

The ocean is the cradle of life. Evolution of life in this aqueous environment has given rise to a wide variety of lifestyles. While some evolved into freeliving plankton that drift passively in the water, others developed into nekton that swim about actively. Species that dwell on the sea bottom are called the benthos. These may be sessile and attached, or possess only weak mobility. Organisms in the sea are governed by its physical and chemical properties, such as temperature, salinity, nutrients, light, and hydrostatic pressure. All of them are adapted to cope with salinity. Intertidal and estuarine organisms thrive in salinities that vary from freshwater conditions to that in the adjoining sea. Temperature tolerance varies from nearfreezing to tropical conditions, but, of course, any species is restricted only to a relatively narrow range of salinity and temperature.

Organisms require energy for growth and multiplication, which they acquire through photosynthesis (plants), chemosynthesis (autotrophic bacteria anywhere, and animals living in great profusion at the “hot springs” of the mid-oceanic ridges, Fig. 6 and Fig.9) or uptake of organic matter as particles (most animals) or dissolved materials (heterotrophic bacteria). These different mechanisms can be broadly classified as ‘trophic levels’.

• The photosynthetic organisms that constitute the lowermost trophic level, both on land and in the sea, are called primary producers. In the water column, unicellular phytoplankton (Fig. 31) are responsible for photosynthetic fixation of carbon dioxide in the presence of sunlight and nutrient salts. This process of primary production takes place within the “euphotic zone“ or the upper sunlit zone. In clear tropical offshore waters the euphotic zone extends beyond 100 m, but is much shallower in near-shore waters because of increase in turbidity. Among the phytoplankton, the pico-plankton are less than two microns in size, the nanoplankton between 2-20 microns, and the micro-plankton 20-200 microns (human hair is about 80 microns thick).

• The trophic level above that of the primary producers consists of the primary consumers, namely zooplankton, which graze upon the phytoplankton. Like their food, zooplankton are also classified as nanoand micro-zooplankton. The meso-zooplankton, between 200 microns to 20 mm in size, are important inhabitants of the sea. Among these, the copepods are the most abundant.
• Animals (zooplankton, fish) that feed on the “herbivorous” zooplankton are the secondary consumers and, in theory, constitute the next higher trophic level.
• Carnivorous fish, squid and turtles constitute the top of the trophic level, and are sometimes called tertiary consumers.

Since all trophic levels are connected, any attempt to understand fisheries should also take into consideration the other trophic levels (Fig. 32). Some 80-90% of organic matter or energy is lost during each feeding step, principally by incomplete digestion and the metabolism of the predator. Therefore, relatively little food can reach the top carnivores. The metabolism regenerates the nutrient salts and carbon dioxide, while some of the “lost” organic matter is in the form of organic secretions, faeces, and dead tissues. These are utilized by heterotrophic organisms such as bacteria and fungi. Every ml of seawater contains on an average a million cells of bacteria, which perform the important task of recycling nutrient salts . The bacteria are fed upon by many microscopic animals, such as the flagellates and ciliates, which in turn are eaten by the mesozooplankton.

Actually, the food relations in the water column do not constitute a chain as on land (grass - cattle - tiger), but a web in which much feeding is by size rather than kind. The mass of a phytoplankton cell of 200 micron is 1 million times that of a 2-micron cell, similar to the ratio between elephants and small mice: the ratio is very much larger between the small planktonic and the nektonic animals. Just as no net catches elephants and mice, marine animals also can hunt only for limited size ranges. However, also in contrast to land, a large “goat” easily and commonly eats a young “wolf” and thus obliterates the trophic level concept, aside from the fact that many phytoplankton species can also eat particles (mixotrophs). We cannot predict fish yield from primary production rates alone because the uncertainty about the level(s) of the food web on which a particular species feeds, and because of our ignorance of how much food is available to the target species from competition with the other members of the web.

All organisms in the sea, to whichever trophic level they belong, are linked to each other in a complex food web, the structure of which is determined by the physics and chemistry of the ocean. The Arabian Sea and the Bay of Bengal, being tropical waters, provide congenial temperatures and light conditions throughout the year for sustained primary production. This, however, does not always happen because nutrients can often be seasonally limiting. Mineral nutrients (nitrate, ammonium, phosphate, silicate, and iron) are essential for phytoplankton growth and multiplication. Physical processes such as upwelling (a process in which cooler waters from below are brought up), hydrographic fronts (regions over which a property like temperature or salinity changes sharply in the horizontal), eddies, and cyclones are responsible for bringing up nutrients from deeper waters to the surface, thus stimulating primary production. The southwest monsoon along the west coast of India brings up nutrients from deeper waters through upwelling and is important for the high biological productivity. Cyclones are among the major cause of nutrient injection to the surface waters and elevated primary production in the Bay of Bengal. The fertility of the oceanic water column can be assessed by its chlorophyll content as a measure of phytoplankton. Ocean Colour Remote Sensing technology offers a real-time estimate of this fertility (Fig. 33).

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