CH. XIl] THE CIRCULATION OF NITROGEN 295 



marine plants by oceanic currents, or of the manner of utilisation 

 of these under different phj^sical conditions. We must remember 

 that animal and plant life is more luxuriant in the warmer land 

 regions than in the colder ones. There must therefore be a greater 

 drainage into the sea of the products of decomposition and 

 metabolism from the tropical and temperate land areas than from 

 those in the polar regions. If we could shew that a large propor- 

 tion of this material, originating on the land in the warmer 

 countries, were transported by oceanic currents into the seas of the 

 colder regions then it might be possible to account for the wealth 

 of life in the latter areas, without assuming the activity of 

 denitrifying bacteria. An hypothesis of such a nature has 

 recently been formulated by Nathansohn^. It is known that there 

 are considerable vertical movements of the water of the oceans, 

 that is currents may well up from the sea bottom, and conversely 

 the water of the surface layers may fall down to the bottom, as for 

 instance, when it becomes strongly cooled and sinks by convection. 

 Again, we have seen that planktonic organisms living near the 

 surface of the sea die and their bodies fall to the bottom, there 

 slowly to putrefy. Therefore there must be a greater proportion of 

 inorganic nitrogen compounds and carbon dioxide in these bottom 

 waters than at the surface ; and by the continual precipitation of 

 dead organic matter to the bottom, the surface layers must become 

 impoverished of their dissolved nitrogen food-salts. The latter, 

 and carbon dioxide, cannot be utilised at the sea bottom to a 

 great extent because plant life is scarce, or absent there, owing to 

 the deficiency, or complete absence, of light. Now if an upward 

 current of water transports these food materials from the bottom to 

 the surface, the waters of the latter layers at once become enriched 

 and there will arise an increase in the production of plants. 

 Nathansohn shews that such upwelling of bottom water occurs in 

 many parts of the sea, and that wherever it does occur there is 

 an increased abundance of plant life. Obviously this upwelling 

 bottom water will be colder than that normally present at the 

 surface of the sea into which it emerges. Nathansohn works out 



^ " tjber die Bedeutung vertikaler Wasserbewegungen f. d. Produktion des 

 Planktons," Abhandl. Math.-PJiys. Klasse konigl. Sachs. Gesell. Wissenschaft. 

 Leipzig, Bd. xxix. 1906. 



