METABOLISM IN THE SEA 193 



80,000 of these little animals. In some parts of 

 the Antarctic Ocean the diatoms are so abundant 

 as perceptibly to colour the surface water, and in 

 the same region the deposit at the sea-bottom is a 

 soft white ooze which consists almost entirely of 

 the dead flinty shells of these organisms. The 

 diatoms and the copepods form the fundamental 

 organic food-material of the sea. 



It is possible to estimate the productivity of 

 the sea, just as one can estimate the productivity 

 of the land. Hensen and Brandt found that each 

 square metre of the Baltic produced on the 

 average about 150 grammes of dry organic food- 

 substance in the shape of plankton. A similar 

 area of cultivated land produces (according to 

 Biebahn and Rodewald) about 180 grammes of the 

 same ultimate food-substance. The productivity 

 of the sea, judged by the amount of plankton 

 generated, is therefore about 20 per cent, less 

 than that of cultivated land. The productivity 

 of the sea may also be compared with that of 

 the land by comparing the quantity of produce 

 obtained in each case from similar areas ; that is, 

 the weight of fish obtained from an acre of sea 

 may be compared with the weight of agricultural 

 produce obtained from an acre of cultivated land. 

 Some interesting observations of this kind are 

 contained in the Report of the Fisheries Commis- 

 sioners of 1866, according to which "an acre of 

 good land carefully tilled produces [per annum] 



13 



