314 



DISCOVERY REPORTS 



At two stations in the South Georgia survey we had a phytoplankton concentration 

 considerably greater than at any other station— viz. Sts. WS 48 and 49. These were on 

 the "G" line running out from the west of the island. The distribution of the more 

 important species of Copepoda on this line is shown in Table LX. We see that even the 

 forms which usually show an indifference to the phytoplankton— Calamis acutus, 

 Rhincalanus gigas and Oithona frigida— here are taken in smaller numbers in the region 

 of high concentration. Drepanopus pectinatus alone appears to flourish under these con- 

 ditions. Perhaps if the concentration was still higher Drepanopus itself would be affected. 

 On the publication of Harvey's (1934) paper stressing the control of the phyto- 

 plankton by grazing I went over all the data again in order to see if I could find a 

 fallacy in this apparent close linkage between the vertical migration of the zooplankton 

 and its distribution in relation to phytoplankton. 1 Whilst all the horizontal nets were 

 fished at approximately comparable depths, and in the case of the vertical nets only the 

 top 250 m. of water were considered, I found that I had not taken into account the 

 possible effect of there being a greater depth of water below the ranges sampled at some 

 stations than at others. We have seen that the only common forms of zooplankton in our 

 survey which make little or no vertical migration are forms which are rarely found in 

 the upper phytoplankton zones. The forms which might control the phytoplankton by 



grazing are those which make vertical mi- . 



grations; thus the main grazing effect 

 would take place at night. Now as the ocean 

 currents approach the shallower regions 

 it might be expected that there would be 

 a gradually reduced number of animals in 

 the upper layers at night, because there is 

 a smaller body of water below from which 

 they can rise; the larger body of water 

 containing more animals in the daytime 

 is continually being carried away to one 

 side or the other. This principle is shown 

 in Fig. 168. It will be remembered that 

 more of our deep-water stations were 

 night stations than were our shallow- 

 water stations ; thus it appeared possible 

 that the conclusions drawn from the cor- 

 relations shown above might be fallacious, 

 since we saw that we had more night sta- 

 tions in the intermediate ranges of phosphate values than in others. In the above 

 Tables LIV-LIX I have put in the number of deep-water stations— those of a depth of 



1 We must recognize, as Harvey himself does, that his observations are extensive in time but not in space, 

 that bodies of water varying in their phyto- and zooplankton content move past ms line of observat.on, 

 and that the phytoplankton crop in the Plymouth area is likely to be very much less than that in the 

 Antarctic. 



DAYLIGHT 



NIGHT 



Fig. 168. 



