PLANKTON OT IHE QVLE W MAIM-; 485 



water is probably low except for a brief period in spring while the rivers are in ilood, 

 that the vernal flowerings of diatoms are briefest and vanish most completely 

 after their culmination. 



The case is quite otherwise on Georges Bank, where one diatom community or 

 another flourishes from late winter to midsummer, but where these flowerings are 

 local by contrast to the extensive vernal flowerings in the inner part of the gulf. 



The distance of the bank out from the land and the general distribution of salinity 

 in the gulf forbid the possibility that the nutrients on which its diatoms depend are 

 contributed directly by river water, while hydrography in general equally rules out any 

 possible updraught of nutrients from the ocean deeps, this not being an area of up- 

 welling. Neither can we suppose that the general surface outllow from the gulf 

 reaches the bank especially rich in dissolved foodstuffs, for it is only for a brief period 

 in the spring that the basin of the gulf to the north supports an abundant diatom 

 flora. 



Probably the rich animal population of the sea floor of the bank makes the bank 

 itself a richer source for nitrogen than its comparative barrenness in fixed plant 

 growth would suggest. The destruction of plankton that takes place along the meeting 

 zone of cool and warm waters just off its southern face also affords a rich potential 

 food supply for pelagic plants as well as animals, though to what extent the products 

 of this decomposition actually reach the shallows of the bank is a question. With 

 the comparatively active vertical circulation that prevails locally on the bank even 

 in midsummer, tending to sweep any organic debris from the bottom up to the upper 

 layers, whether in suspension or in solution, the bottom no doubt contributes a 

 greater store of assimilable nitrogenous compounds to the overlying sea water than 

 in the deeper parts of the gulf to the north. This applies also to phosphates going 

 into solution from the dead bodies of animals decomposing on the sea floor. Further- 

 more, the activity of vertical circulation on the bank, combined with low surface 

 temperatures of summer dependent thereon, makes its waters a favorable environ- 

 ment physically for the flotation of pelagic plants, and these factors combined 

 may well account for the summer flowerings there. There is also the interesting 

 possibility that small amounts of silica go into solution from the felspathic sands, 

 pebbles, and gravel that floor the bank (p. 475). 



The precise causes of the periodic rise and fall of the peridinian flora are even 

 more obscure than those that determine the diatom flowerings which they replace 

 in summer and autumn, partly because, being less spectacular, they have attracted 

 less attention, and partly because the peridinians as a whole are less obviously depend- 

 ent upon any one nutrient substance than are the diatoms on a sufficiency of silica. 

 And as I have pointed out (p. 478), the suggestion that the abundance of peridinians 

 depends upon the available supply of phosphates is not borne out.bj' the seasonal 

 succession of this group and of diatoms compared with recent analyses for the phos- 

 phate content of the water. Nor is it by any means certain that the seasonal fluc- 

 tuations of the peridinians mirror the fluctuations in the supply of any one food 

 substance in the water as closely as the diatom flowerings are supposed to do. 

 Since the group as a whole is more thermaphile than most of the diatoms character- 

 istic of the Gulf of Maine, with the three most abundant species of Ceratium following 



