3i» 



DISCOVERY REPORTS 



stations, we see in both the shallow- and deep-water series a confirmation of the general 

 principle that the larger numbers of macroplankton organisms and Copepoda of 

 Group A are found in the zones of intermediate or higher phosphate values than in the 

 lower phosphate values. The Copepoda in Group C, and Antarctomysis maxima, again 

 show in general a higher concentration in the regions of lower phosphate values. The 

 factor of depth cannot be the cause for the general sorting out of the vertically migrating 

 plankton either away from or towards the higher concentrations of phytoplankton. It 

 would appear that there must be some automatic mechanism at work, in which the 

 extent of vertical migration is modified according to the density of the phytoplankton in 

 the upper layers. The evidence for such a modification in the vertical migration will be 

 reviewed in the next section. 



We have referred to the possibility that one, or perhaps two or three, outstandingly 

 abundant species may be largely controlling the phytoplankton by grazing, and that the 

 remainder of the vertically migrating zooplankton species are being influenced in their 

 distribution by that of the phytoplankton so controlled. It was the publication of 

 Harvey's (1934) paper which led me to this consideration. In the account of the natural 

 history of Euphausia superba (pp. 208 et seq.) we have collected evidence to show that this 

 species is ecologically probably the most important zooplankton organism in the 

 Antarctic. In our own survey we see that it outnumbers all other species of macro- 

 Table LXIV 



Showing the zooplankton organisms, expressed as numbers per 50 m. haul, taken in the top 250 m. {or less according 

 to depth of sounding) by the N 70 V nets at each night station; the stations being arranged in order of 

 decreasing soundings from deep to shallow water 



* Bottom not reached. 



