CHAPTER 8 

 Gco^rapiiii and seasonal distrihiitioii 



As PLANKTON is SO dependent on the conditions of its environment — the 

 weather above the sea, the temperature and the chemical content oi the 

 water itself, its biolc^gical history — it is only to be expected that there will 

 be great geographic and seasonal variations. The most important physical 

 factor is temperature and, because temperature has such profound effects, 

 it would be worth-while giving a page or so to it. Temperature differences 

 in the water are of course fundamentally dependent on the climate above 

 and it is almost too obvious to mention that tropical seas will be warmer than 

 polar seas, but this fact is of such major importance in so many ways that 

 it just has to be stated. With the winds, particularly the fairly constant trade 

 winds, temperature is one of the causes of the main current systems, the 

 plankton distributions and the location of the world's dominant fisheries. 



Warm water is lighter than cold, and where the tropical sun warms the 

 seas the warm water stays on top and so gets warmer still, hi cold areas the 

 cold water sinks. A general circulation is thus set up, cold water sinking in 

 ihe polar regions; this heavier cold water spreads along the ocean floor 

 towards the tropics and is counterbalanced by the movement of warm water, 

 northwards in the Northern Hemisphere and southwards in the Southern, 

 to replace it. One might thus expect that the colder the air near the poles the 

 more water would sink, and as more warm water would then replace it, 

 the temperature near the poles would rise and conditions would be kept 

 fairly stable. This is not so, however, due to freezing. Fresh water, when 

 cooled, expands just before it freezes and continues to expand as ice forms 

 (and bursts the unprotected water pipe or car radiator), and so it becomes 

 lighter and stays on top to freeze solid and form a stable layer there. Sea 

 water behaves differently and continues to contract as it cools to temperatures 

 below the freezing point of fresh water. It thus sinks away from the atmo- 

 spheric cold before it freezes and is absorbed into and warmed by the 

 slightly warmer water below, ff the water is deep there is a very great 

 reserve of warmth, and deep sea water is thus very difficult to freeze. But if 

 there is a less saline — and therefore lighter — layer overlying the deep water, 

 the increased density due to cooling may not be enough to let it sink into 

 the warmer saltier water below, and this stable surface layer can thus freeze. 

 Ice crvstals are made from more or less fresh water and are therefore light 

 and float leaving even saltier water below. Less saline waters are, of course, 



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