FOOD-MATTERS IN THE SEA 321 



possible organic life in the sea is limited by the quantity 

 of whatever necessary substance is present in minimal amount 

 — it being supposed, for example, that the necessary 

 nitrogen has to be obtained from the small quantities present 

 in the form of ammonia salts, nitrates and nitrites. But 

 these recent experiments show that, to quote the words of 

 Moore's Royal Society paper : — 



" The source of the nitrogen is the atmospheric elemental 

 nitrogen dissolved in the sea-water, and not ammonia, 

 nitrates or nitrites. The source of the carbon is the carbon 

 dioxide of the bicarbonates of calcium and magnesium 

 dissolved in sea- water." 



This reaction is so large in amount in the sea, in spring 

 at the time of the plankton maximum, that if it takes place 

 to the same extent down to a depth of 100 metres, then the 

 carbon made available would suffice for a crop of phyto- 

 plankton amounting to at least ten tons of moist vegetation 

 per acre. 



In the application of oceanographic investigations to sea- 

 fisheries problems, one ultimate aim, whether frankly 

 admitted or not, must be to obtain some kind of a rough 

 approximation to a census or valuation of the sea — of the 

 fishes that form the food of man, of the lower animals of 

 the sea-bottom on which many of the fishes feed, and of 

 the planktonic contents of the upper waters which form 

 the ultimate organized food of the sea — and many attempts 

 have been made in different ways to attain the desired 

 end. 



Our knowledge of the number of animals living in different 

 regions of the sea is for the most part relative only. We 

 know that one haul of the dredge is larger than another, or 

 that one locahty seems richer than another, but we have 

 very little information as to the actual numbers of any kind 

 of animal per square foot or per acre in the sea. Hensen, 

 as we have seen, attempted to estimate the number of food- 

 fishes in the North Sea from the number of their eggs caught 



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