PELAGIC PLANT LIFE 



Number of Cells per Litre. 



Z11 



Experiments with pure cultures of different plankton- 

 diatoms, made by Allen and Nelson at Plymouth, show that 

 they do not thrive without a regular supply of nitrogenous 

 compounds. The plan of working which they adopted may 

 also be employed with advantage when we wish to ascertain 

 what concentration of dissolved nitrogenous compounds induces 

 the plankton-algae to augment most rapidly. This is the 

 first thing to find out if we desire to know whether a want of 

 dissolved nutritive substances is the limiting factor of production. 

 It is quite possible that augmentation diminishes from lack of 

 nitrogen long before the total amount of this essential has been 

 fully consumed ; yet augmentation must not fall below a certain 

 minimum if the species is to hold its own, because of the larger 

 or smaller number of individuals that are constantly perishing. 

 Questions like these can only be settled by experiment, so that 

 the cultivation method of Allen and Nelson is bound to be of 

 great assistance to us eventually. But in the meantime our 

 comparative investigations over large areas of the sea are also 

 of considerable value. 



I have already stated that plant life in the Christiania fjord 

 was limited to a very thin surface-layer, which, owing to its 

 lesser density, was differentiated from the deeper infertile 

 water-masses, and this was practically the case along all 

 the coasts where plankton-algse were plentiful. Out in the 

 open sea, on the other hand, where there are not such 

 marked differences in salinity, temperature, and density be- 



Allen and 



Nelson. 



Plankton 

 extends 

 deeper, but 

 is less 



abundant, in 

 the open sea 

 than in 

 coastal areas. 



