3 20 DISCOVERY REPORTS 



plankton. We know that Euphausia superba is largely a phytoplankton feeder. Hart 

 (1934) referring to this species writes: 



The food of these Euphausians consists very largely, if not entirely, of diatoms and other phyto- 

 plankton organisms. Thus we have here one of the simplest food chains possible, the building up of 

 the vast body of the whale being only one stage removed from the organic fixation of the radiant 



energy of the sun by the diatoms Examination of the stomach contents of adult and post-larval 



E. superba has been made on a number of occasions, though this line of investigation has not been 

 followed up systematically as yet. Small diatoms appear to be ingested by some form of filtering 

 mechanism, and the more typically oceanic species are evidently digested rapidly: recognizable 

 fragments are rather rare even in the crop, the contents of which in krill from waters in which such 

 species are dominant, presenting the appearance of a green porridge. Two forms that occurred con- 

 stantly and remained clearly recognizable were Fragilaria antarctica and Thalassiosira antarctica. 

 Torn fragments of the large species Chaetoceros criophilum indicated that the adults are capable of 

 triturating and swallowing the larger forms in addition to possessing a filtering mechanism. In post- 

 larval forms entire examples of large Foraminifera (Globerigina sp.) were frequently found. Possibly 

 these are eaten for the sake of the contained calcium. 



In Fig. 169 are shown the numbers of Euphausia superba taken at the shallow- water 

 stations and at the deep-water stations. Each series of stations is arranged in order of 

 decreasing phosphate values, which are also shown as curves. Both day and night 

 stations are included, the latter being indicated by the letter "N". Whilst the number 

 of observations is few, considering the patchiness exhibited by this species, it is perhaps 

 noteworthy that in each series there is a general gradual decline in the number of 

 Euphausians taken as we pass from the higher to the lower phosphate values ; we have 

 seen that this reduction in phosphate values indicates an increase in phytoplankton 

 production (cf. Fig. 145). It does seem possible then that this outstanding species 

 Euphausia superba may be actually playing an important part in the control of phyto- 

 plankton production. 



Whilst the data concerning the horizontal and vertical distribution of the plankton, 

 together with the physical and chemical observations, are perhaps fuller than any 

 hitherto examined for an area of the size we are dealing with, it is nevertheless all too 

 scanty. From it we have been attempting to disentangle the two principles, the effect 

 of the zooplankton on the phytoplankton by grazing and the effect of the phytoplankton 

 on the zooplankton by exclusion. Whilst the data are scanty, we are perhaps unusually 

 fortunate in our material. Phytoplankton production is exceptionally heavy in the area. 

 If exclusion is a real effect we have seen that it must be due to a movement of the 

 zooplankton and not simply to mortality, and that such movement must be brought 

 about through vertical migration ; in this area extensive vertical migration is a striking 

 feature in the behaviour of the majority of plankton animals. We have in the area too 

 one herbivorous species, Euphausia superba, which stands out above all others in eco- 

 logical importance. From an examination of the data it appears firstly that, whilst all 

 herbivorous animals naturally take their part in reducing the phytoplankton, the phyto- 

 plankton itself has a marked influence on the distribution of the vertically migrating 

 zooplankton, and secondly that one or two such outstandingly important herbivorous 

 animals such as Euphausia superba may by their grazing have an influence on phyto- 



