286 FISHES AND MAN xi 



after it has died and passed through a bottom-living invertebrate. 

 Thus plaice feed on lamellibranchs and worms, soles on worms alone, 

 haddock on star-fish, molluscs, and worms. The phytoplankton is, 

 of course, built up by photosynthesis from carbon dioxide, water, 

 nitrates, phosphates, and small quantities of other substances. The 

 limiting factors in the plankton growth, granted adequate temperature 

 and sunlight, may be nitrate, ammonia, and phosphate, the latter 

 present only to the extent of less than one part in 20 million. During 

 the spring and early summer nearly all the phosphate in the water may 

 be taken up into the phytoplankton, which increases rapidly. The 

 zooplankton (Cala?ius, Sec.) increases somewhat later and by mid- 

 summer grazes down the phytoplankton to a considerable extent. It 

 is at this midsummer period that the pelagic fishes feed most easily, 

 and a little later it is the turn of the bottom-living invertebrates and 

 fish, as some of the zooplankton dies and falls to the bottom. At the 

 end of the season there may be a second increase in the phytoplankton, 

 the recovery being due to breakdown of the discontinuity layer (see 

 below), and then with the onset of winter the plants die off and most 

 of the phosphate is returned to the inorganic condition. 



Thus to take the case of the cod, whose food chain is one of the 

 longest: the activity of the plants turns carbon dioxide, nitrate, and 

 phosphate into organic matter: the activity of the copepods makes 

 this available to the herrings and the cod eat the herrings. There are, 

 of course, many intermediate effects: Sagitta eat some of the cope- 

 pods, fish-fry and large cod eat younger cod, and so on, but in general 

 we shall not go wrong if we assume that the cycle depends on the 

 activity of diatoms, calanoids, herrings, and codfish— all seeking, 

 eating, growing, reproducing, and dying. 



The pressure of these various activities produces a complicated set 

 of interrelations that may without undue extravagance of fancy be 

 called a macro-organism. The activities of all the members contribute 

 to the balance that is set up. Sometimes shortage of materials, such as 

 phosphorus, at key-points may determine the whole cycle. 



The phosphate in the sea is increased to a small extent by drainage 

 from the land and, further, in a restricted area such as the North Sea, 

 by the influx of water from more open areas, in that case the Atlantic. 

 Not all the phosphate in a given area of sea is available for organic 

 growth, because there is usually a separation between upper and lower 

 layers owing to the upper layer becoming warmer and less dense. The 

 phosphate in the water below this limiting discontinuity layer, usually 

 lying 20-100 metres down, is thus not available, though it may be 



