PHYTO- AND ZOOPLANKTON INTERRELATIONS 281 



grounds. The phytoplankton in the case of the herring appears to have the exclusion 

 effect first suggested by Pearcey. 



For these reasons I set myself to investigate the hypothesis of animal exclusion, 

 particularly in relation to Euphausia superba, but also in relation to the animal plankton 

 in general. Since this work has gone into first proof an important preliminary paper has 

 been published by Harvey (1934) describing his investigations of the zoo- and phyto- 

 plankton relationships in the Plymouth area as the season advances. He favours the 

 view that the phytoplankton is controlled by the numbers of herbivorous plankton 

 animals present. Whilst for the reasons stated and those which follow I did not believe 

 that the dense production of phytoplankton in the Antarctic or the North Sea can be 

 due to a shortage of herbivorous animals in these areas, it is essential in the investigation 

 of this important problem that we should keep our minds open to either possibility. 

 As Mr Harvey has said in recent correspondence I have had with him, the conception 

 of regulation of phytoplankton by grazing and animals avoiding very heavy phyto- 

 plankton are by no means antagonistic. In the light of Mr Harvey's investigations I have 

 re-examined the data, and, as will be shown in the next section, have found some evi- 

 dence that the phytoplankton may indeed be in part controlled by Euphausia superba, 

 but that there is an indication that the remainder of the macroplankton organisms and 

 some of the Copepoda may be excluded. 



Another aspect of the problem must be borne in mind. Although I can recall no 

 reference to the subject in the literature, in discussion planktologists have often com- 

 mented on the possibility that the inverse correlation between phyto- and zooplankton may 

 not be a real one but an apparent one, due to the tow-net becoming clogged by dense 

 phytoplankton and so failing to capture more than asmall proportion of the animals present 

 on account of reduced filtration. This does not apply to the results about to be described. 



Throughout all the stations of our South Georgia survey two nets of widely different 

 mesh were used, one of 200 meshes to the inch for the capture of the phytoplankton, and 

 one with a very coarse meshing partly 4 mm. mesh but with the end section of 1 5 meshes to 

 the inch for the capture of the macroplankton including the Euphausians. This second net 

 was of too wide a mesh to be clogged at all with the phytoplankton, so that here we have a 

 parallel series of observations. 1 In Table LI I are listed the numbers in which the five 

 most abundant macroplankton species were taken by the N 100 H nets in the December- 

 January survey of 1926-7 at all the stations where phytoplankton observations were 

 taken; and the stations have been arranged, together with their corresponding zoo- 

 plankton numbers, in order of the abundance of phytoplankton expressed as millions 

 of cells per haul with the N 50 V net. 2 There are a number of difficulties that have 

 already been noted in the section on zooplankton distribution that we have to contend 



1 After the 1926-7 season the end of this net was changed to stramin on account of the fragile nature of 

 the 15 meshes to the inch silk which frequently wore out. Stramin has a much lower filtration and is at times 

 clogged by the dense phytoplankton. 



2 At only two stations, 139 and WS 26, was the N 50 V net not used. The zooplankton at these stations is 

 not included in the table, but will be included in subsequent tables when the correlations of zooplankton and 

 phosphate values are given. 



