PELAGIC PLANT LIFE 371 



several of the necessary nutritive substances may be present in 

 such small quantities as to act as factors that limit the develop- 

 ment of the vegetation, then the more or less considerable 

 exchange taking place between the illumined surface -layers 

 and the vast water-masses of the deep is certain to produce a 

 great effect. All the forms of animal life inhabiting the sea 

 below 200 metres live solely upon organic substances which are 

 due to plants in the surface layers ; that is to say, they either 

 feed directly upon the plant-cells which sink downwards, or 

 upon the inanimate remains or excrements of the animals living 

 up above, or else upon other animals which, in their younger 

 stages, have inhabited the surface-layers and fed on the plants 

 they found there. A large proportion of the produce of the 

 surface-layers must thus be continually descending into the 

 deep sea, and these nutritive substances are therefore with- 

 drawn from their regular circulation in the photic zone. Down 

 in deep water, no doubt, the destructive metabolism of animals 

 will set free these nutritive substances, so that eventually 

 carbonic acid and ammonia will be produced ; still these gases 

 can only regain the photic zone by very slow degrees if 

 diffusion is their sole means of conveyance. If, however. Ascending 

 whole masses of water are brought up from the deep sea to <^""e"^s- 

 the surface, the nutritive substances contained in them will 

 once more enter into circulation, and cause an abundant plant 

 life to develop. Nathansohn has pointed out that marine areas 

 where such ascending currents occur, and where the surface- 

 layers are replaced by water from the deeper layers, are well 

 known to be extremely prolific, not merely in plankton, but 

 also in larger organisms. In anticyclonic systems like that 

 of the Sargasso Sea, on the other hand, where, conformably 

 to the laws of ocean-currents, the water-masses cannot ascend 

 from the deep sea, but where the surface -layers sink down- 

 wards, the plankton is much less plentiful than in any other 

 similar area where investigations have been made. Our 

 researches in the Atlantic during the summer of 19 10 have 

 done a great deal to settle this question. I shall first of all, 

 however, refer to a series of investigations which bring quite 

 another light to bear upon the question, and show what 

 difficulties we have to face. 



In 1907 Professor Nathansohn and I commenced to study Pelagic aigos 

 the Christiania fjord, and subsequently I continued these in- f/oS"^*'''"'' 

 vestigations alone. My previous observations had taught me 

 that the pelagic algae in this fjord attain their maximum between 



