2 8o DISCOVERY REPORTS 



check, and the second view first put forward by Pearcey in the same year, and suggested 

 again by Fish in 1925, that dense phytoplankton excludes animal life from the water. 

 This latter hypothesis I will refer to as that of animal exclusion. 



When the present section was in its early stages my colleague Mr C. E. Lucas, with 

 whom I discussed the problem, emphasized the importance of the former view. For a 

 number of reasons I rejected this hypothesis as a possible explanation of the apparent 

 relationship between the distribution of the phytoplankton concentrations and that of 

 Euphausia superba. A discussion of some of these reasons which strengthened my re- 

 jection of this hypothesis must be reserved until the data underlying them are de- 

 scribed, but some of them may be mentioned here. Whilst it is clear that a shortage of 

 herbivorous plankton animals must allow a greater multiplication of the phytoplankton 

 if the conditions favourable to their growth, i.e. nutritive salts, light, etc., are present, 

 it did not appear likely that such a shortage of herbivorous animals could account for the 

 formation of the well-defined patches of dense phytoplankton that are found both in 

 the Antarctic and the North Sea. Whilst on land herbivorous animals may keep slow- 

 growing vegetation in check by grazing, in the sea it appeared likely that if conditions 

 very favourable to the multiplication of the plants were present these unicellular 

 organisms, which can double their bulk and separate at each division, could increase 

 their numbers at a much greater rate than could be checked by the animals. The num- 

 bers of animals such as Euphausia superba, which takes two years to reach maturity and 

 reproduce, could not increase to maintain the same balance of zoo- to phytoplankton 

 that existed before the favourable conditions to plant multiplication set in. Whilst other 

 smaller animals might multiply more quickly and help to restore the balance, it did not 

 seem possible that Euphausia superba and the number of other plankton animals dealt 

 with in the sections which follow could have this power. Again, it has long been 

 recognized in temperate regions that the phytoplankton which is so abundant in spring 

 becomes reduced by the feeding of the increasing zooplankton, and more recently it has 

 been shown that this reduction is assisted by the retardation in the growth of the plants 

 by the reduction of the available nutritive salts. Such a statement refers to the phyto- 

 plankton as a whole ; actually within the general decline of total phytoplankton there 

 may be a succession of different species which flourish and wane in their own season, 

 some early and some late. To what extent the decline of the earlier ones is due to the 

 multiplication of the zooplankton we do not know; we have however seen on p. 42 the 

 sinking of vast numbers of dead Coscinodiscus. 



From the findings of Savage and myself in the North Sea, an account of which has 

 just been published, we believe that there is little doubt that the herring avoids dense 

 concentrations of phytoplankton. Now the herrings at this time are not feeding 

 (Hardy, 1924; Savage, 193 1) but collecting preparatory to spawning. Thus they do not 

 avoid the phytoplankton because of a lack of animal food therein, unless they do so 

 because of a habit established in the summer feeding season. It appears unlikely that 

 such a reaction should be maintained in the autumn, when it not only has no value in 

 relation to feeding but would prevent them from reaching their normal shoaling 



