CH. XIl] THE CIRCULATION OF NITROGEN 283 



the case. We know of course that the coastal waters are less salt 

 than those further out at sea, the cause of this lower salinity- 

 being that the sea there receives a large addition of fresh water. 

 These coastal waters then ought to contain more dissolved 

 nitrogen, and there is no doubt that this could be demonstrated if 

 it were not the case that these compounds are utilised almost as 

 rapidly as they are added to the sea. 



Utilisation of land drainage by marine organisms. All 

 round the coast there is, in the littoral and laminarian zones, a 

 selvedge of plant life, in the form of the larger sea- weeds. Below 

 a depth of about 20 fathoms or so these larger algae begin to thin 

 out, and in relatively deep water, even while still within the limits 

 of the continental shelf, they practically cease to exist. On the 

 shallow sea bottom near the land other plant life is also relatively 

 abundant. There are immense numbers of diatoms living in the 

 sand and mud near low water mark ; and the algae there are some- 

 times also covered with deposits of these organisms. Pelagic proto- 

 phyta, diatoms, peridinians, &c., are also more abundant in the 

 shallow water near the shore. We also find that the invertebrate 

 and fish life is more abundant in this region. Now it is evident 

 that the greater density of plant life near the land is directly due 

 to the fact that there is a greater amount of the ultimate food 

 materials, nitrogen compounds and carbon dioxide, there, than far 

 away from the land. These plant organisms use the substances 

 mentioned as food-stuffs, building up starch and proteid from the 

 carbonic acid and nitrogenous drainage of the land. Probably very 

 little, or perhaps none, of this ever reaches the central oceanic 

 areas. These sea-regions are, no doubt, self-supporting, that is, 

 the inorganic food-stuffs of the plants there are derived from the 

 decomposition of the dead bodies of the organisms inhabiting 

 these areas. 



With this conversion of the nitrogenous land drainage into 

 plant substance, the upward or constructive metabolism of this 

 matter may be, for a time at least, arrested. If all the algae, or 

 protophyta, which feed upon the waste substances washed down 

 from the land were eaten by animals, then the nitrogen, which we 

 have traced from the land down into the sea, would pass by such 



