304 DISCOVERY REPORTS 



depths during the night. It rarely swims into the zone of actual rich phytoplankton in 

 the upper 50 m., but living below the phytoplankton will likely benefit by the falling 

 rain of food without either bringing about a reduction of the phytoplankton above or 

 from the opposite point of view suffering exclusion from the concentrations well above it. 

 Now, whilst we saw on p. 230 that the zooplankton communities of the regions 

 occupied by different water masses and having different phytoplankton concentrations 

 were in general very much the same, we saw that certain forms, e.g. Euphausia superba, 

 Limachia helicina and Salpa fusiformis, were more characteristic of the Weddell Sea 

 water than of the Bellingshausen Sea water and the area of mixture (Group II) on 

 the west of the island. We know too that there was a greater phytoplankton production 

 and a greater reduction in phosphates on the western side of the island than on the 

 eastern side. It might be that the general correlations between zooplankton and reduced 

 phosphate content, and so phytoplankton, could be accounted for simply by the differing 

 content of these water masses. Indeed, as referred to on p. 277, this at first seems likely ; 

 but we have reserved a consideration of this point until the present juncture, because 

 it is possible to show that within each of the groups of stations concerned, in spite of the 

 patchiness of the plankton and the small number of stations averaged, there is in 

 general the same sorting out of the zooplankton in relation to phosphate values. This 

 is shown in Table LVI. The groups of stations II, III and IV are the groups of stations 

 having a characteristic phytoplankton; they are discussed on pp. 69 to 72 and again in 

 relation to zooplankton on p. 230. 



We will now turn to the results of the N 70 V and N 70 H nets, the nets used for 

 the capture of the smaller zooplankton species. We will take the N 70 V nets first. These 

 nets it will be remembered were fished vertically, when the depth allowed it, in a series 

 of hauls from 1000 m. to the surface: 1000-750, 750-500, 500-250, 250-100, 100-50 

 and 50-0 m., the nets being closed at the end of each range of depth. We will consider 

 only the hauls for the top 250 m. and express the numbers of organisms as an average 

 per 50 m. haul, because at some stations the depth of water did not allow of a full 

 250 m. range being taken. The stations are grouped and averaged for the same ranges 

 of phosphate values as used for the N 100 H nets and the results are tabulated in 

 Table LVI I. The organisms which occur in numbers sufficient for the calculations are 

 largely Copepoda. We may conveniently consider the other organisms first; they are 

 those shown in the first group in the table. The Foraminifera, the Radiolaria and the 

 polychaete Pelagobia longicirrata show an optimum concentration at the range of phos- 

 phate values of 90-100 mg. per m. 3 In the case of the chaetognath Eukrohnia hamata 

 no such optimum concentration is shown, but the numbers involved are very small ; 

 nevertheless we shall see later that this exception is possibly significant. 



The Copepoda at first sight appeared to present difficulties and to give results con- 

 flicting with those found for the macroplankton organisms in Table LIV. However, 

 we may divide them into three categories, which we will call Groups A, B and C. Those 

 in Group A appear to follow the general principle. Those in Group B, Calamis acatus, 

 Rhincalanus gigas and Oithona similis, show little or no correlation with the phosphate 



