338 



DISCOVERY REPORTS 



Table 14 



sufficient to show that there must be a huge loss in our southern localities. From the 

 reasons already given it seems probable that this is mainly due to grazing, as in the 

 English Channel, but it must not be overlooked that actual death and sinking of diatoms 

 may account for some of it — in the far south we are considering only the 50-0 m. layer 

 in deep seas, while in the English Channel it is possible to consider the conditions 

 throughout the whole water column (72-0 m.). The extensive deposition of diatom ooze 

 and diatomaceous mud is not necessarily proof of the sinking of diatoms during the 

 period of the main increase, however. The forms that remain intact or as recognizable 

 fragments in the bottom deposits are precisely those which retain their structure in the 

 stomachs of plankton animals and in bird guano. Less strongly silicified forms, known 

 to be exceedingly numerous in the plankton, are very rarely recognizable in the bottom 

 deposits. It is quite probable that most of the diatom remains in the bottom deposits of 

 deep waters have passed through the stomachs of several animals on their way down. 

 Even the observation that chlorophyll granules are present in some deposits (Neaverson, 

 1934, p. 299) does not detract from this argument, for it is now known that when the 

 phytoplankton is abundant the zooplankton herbivores tend to feed far in excess of 

 their requirements, and to excrete many diatoms in a very partially digested condition 

 (Harvey et al. 1935, p. 425, confirmed by direct observation in the Antarctic zone). 

 Later in the year actual sinking may be important, but over the period of the main 

 increase, grazing is probably responsible for nearly all the loss of crop in the far south. 

 To return to the table, we now see that taking the average standing crop/calculated 

 minimum crop as our standard of comparison, it would seem that the relative intensity 

 of grazing must be from three to five times as great in our Northern Region as it is in 

 the English Channel, while in the South Georgia area it is very slightly greater. In 

 actual fact some years have shown a much greater average standing crop at South 

 Georgia during the main increase than 1936. This year was selected for comparison 



