NATURE ADUIFT 



amount will depend on the varying factors, for instance, a short turbulence 

 at just the right time will enrich the surface waters without too much 

 interference, but a continuous autunm turbulence will distribute too many 

 of the plants into waters too deep and dark to make use of the nutrients. 

 This of course happens in the winter when there is a complete mixing of 

 the waters. 



In polar seas the stabilizing effect of a thermocline does not exist to the 

 same extent. There we have a six month period of light and growth and a 

 six month period of darkness and no plant growth. In spite of this six months 

 of darkness, plankton is richer in polar waters than in the tropics due partly 

 to the continuous supply of nutrients for the plants in the continuous light 

 of summer and partly because the organisms in the plankton tend to have a 

 longer individual life span in the cold water. This is due to a slower meta- 

 bolic rate, and they take longer to use the same amount of food. Under the 

 ice of the poles, of course, plankton is reduced but is not absent any more 

 than it is in deep water. 



Why arc the deeper waters rich in nutrients ? Only in the light zone does 

 the utihzation of dissolved nutrients take place. Feeding on the plants in the 

 light zone are the herbivorous animals; these migrate up and down in a 

 night and day movement (p. 155) and they in turn form the food of the 

 carnivores, fed on by other carnivores deeper and deeper in the water. 

 During life all these animals release their waste products into the sea and on 

 death these organisms, plant and animal, gradually sink so that regeneration 

 of nutrients always tends to be in the deeper water. Any regeneration that 

 occurs in the light zone is usually used up on the spot. 



The major plankton distribution is thus largely based on temperature, but 

 it is locally upset by other factors, especially those that cause upwelling of 

 deep water into the productive light zone. For example, the almost constant 

 trade winds off the coast of Peru are offshore winds which blow the surface 

 waters away from the land with the result that the deep water must upwell 

 to replace it. Here, then, we have light, warmth and nutrients and one of the 

 richest areas of production of plankton in the world. Millions of small plants 

 via the zooplankton feed small pelagic fish, millions of birds harvest some 2| 

 million tons of these fish each year, reckoned to be about 10 per cent of the 

 total catch of the world's fisheries, and their droppings on the land form the 

 rich guano deposits. There is now a fishery for these small fish, mostly 

 anchovies, which are used for fish meal, and this has rapidly increased in recent 

 vears. In 1956, 27,791 tons of meal were exported, but in 1959 this had risen 

 to 200,000 tons, and to produce this figure a million tons of anchovies were 

 landed by some 500 purse seine vessels manned by 5,000 men. And yet if a 

 trick of fate changes the wind for a short time the cycle stops and there is 



112 



