378 DEPTHS OF THE OCEAN 



tween the surface water and the deep water, the pelagic 

 algse extended deeper ; at 50 metres, for instance, the quantity 

 was still near the maximum, and even as deep as 100 metres 

 or more the number was considerable. This, at any rate, 

 was what we found in the case of the diatoms that abounded at 

 our first stations off the Irish coast-banks and in the Bay of 

 Biscay, and this too was what Schimper discovered in the 

 Antarctic. It is also a regular rule that plankton is far more 

 plentiful along the coasts than in the open sea, and, judging 

 from investigations hitherto made, the proportion between what 

 is produced in a typical coastal area and what is developed in 

 typical oceanic water-masses would be more accurately expressed 

 by 100 : I than by 2:1. For this the best explanation which I 

 can give is that the open sea generally suffers from a want of 

 one or more nutritive substances required by the plants, for 

 though these are brought down to the sea in comparatively 

 large quantities by the rivers, they are almost entirely consumed 

 by the plant life of the coastal areas. 



This is why the abundant plant life of the coastal seas is 

 confined to the surface-layers, since the water-masses lying 

 below remain separated, and consequently cut off from the 

 plentiful supply of nutritive substances which regulate the 

 augmentation of plants. But out in the open sea there is 

 another important source of nutriment to be taken into account. 

 Nathansohn has pointed out that pelagic animals are constantly 

 taking nutritive matter down into deep water, and that for 

 the time being it is accordingly withdrawn from the plants, 

 even though the metabolism of the animals and the action 

 of bacteria liberate it once more in inorganic form. These 

 nutritive substances may rise to the surface-layers again by 

 diffusion, but the process will require a long time. They may 

 also accompany the ascending water-masses where off-shore 

 winds bring about up-welling, in cyclonic current systems, and 

 where the surface-layers, becoming chilled, sink and make 

 Vertical room for warmer layers from below. Wherever vertical 



circulation takes place, and it is assisted in its action by storms 

 and waves, the temperature and salinity will be extremely 

 uniform from the surface down to a depth where the water- 

 masses have such a high salinity that their greater density sets 

 a limit to circulation. Conversely uniformity in temperature 

 and salinity may be taken as a sign that vertical circulation has 

 just taken place. This was the condition of affairs at our 

 stations to the south-west of Ireland (see Fig. 252), where we 



circulation. 



