38o DEPTHS OF THE OCEAN chap. 



carry out systematic quantitative plankton investigations all 

 through the winter, in combination with hydrographical re- 

 searches, in parts of the Atlantic like the sea round the Azores, 

 where the plankton is known to be scanty during the summer, 

 but where during the course of winter vertical circulation 

 might be expected to create different conditions of existence. 

 Whipple. In this connection it should be mentioned that the influence 



of vertical circulation upon the production of plankton-algse 

 in fresh water has long been known to biologists. It has been 

 pointed out by Whipple, who showed that the maxima of 

 diatoms in particular coincide with the seasons when vertical 

 circulation takes place, namely autumn and spring. And in 

 the sea, too, it seems that diatoms, with their power of rapid 

 augmentation, are the first to respond to improved conditions of 

 nourishment. 



Which of the essential nutritive substances are the chief 

 limiting factors in the sea, it is impossible to say as yet. Prob- 

 ably, however, nitrogen is the most important, and next to it, 

 perhaps, more especially in the case of diatoms, we may put 

 silicic acid. Brandt and Nathansohn have both discussed 

 the occurrence of these substances, but we need further and 

 more conclusive information than what we now possess. 

 Nathansohn has likewise considered the possibility of carbonic 

 acid occurring "in minimum." This seems paradoxical, of 

 course, since there are comparatively large quantities of it in 

 sea-water. Still the greater part is combined in the form of 

 carbonates, and only a very small portion is set free by dis- 

 sociation at any given moment, so as to become available for 

 the plants. How much there is in this form will depend on the 

 alkalinity of the sea-water and on the temperature. When 

 the free carbonic acid is used up by the plants, fresh quantities 

 will gradually be absorbed from the atmosphere, though this 

 may take place so slowly that there need not necessarily be any 

 equilibrium between the carbonic acid tension in the atmosphere 

 and at the surface of the sea. It is accordingly quite conceiv- 

 able that the shortage may for a time be considerable enough 

 to stop the algae from assimilating carbonic acid. When the 

 temperature is high the quantity of free carbonic acid in the 

 sea-water will ceteris paribus be less than when it is low, and 

 this also may help to explain the relatively poor production in 

 warm seas. Variations in the tension of carbonic acid, how- 

 ever, have not as yet been sufficiently studied. 



The organic substances built up by pelagic algae unquestion- 



