322 DISCOVERY REPORTS 



was to be explained by the enormous numbers of the small copepod Drepanopus 

 pectinatus bringing about a reduction in the already declining phytoplankton entering 

 the coastal regions. Over two-and-a-quarter million of this copepod were taken in one 

 tow-net haul at St. WS 42. A consideration of water movements showed that this water 

 had flowed from an area where the phytoplankton, made up largely of the small diatom 

 Chaetoceros socialis, was exceptionally rich. In this region of dense phytoplankton the 

 small copepods Drepanopus were very abundant. It is noteworthy that this instance of 

 a likely reduction of dense phytoplankton by zooplankton should have involved the 

 small copepod, which our later studies have shown to be the one plankton animal in our 

 survey which flourishes in and is not excluded from the highest phytoplankton con- 

 centrations met with. 



VERTICAL MIGRATION CONSIDERED IN RELATION 

 TO PHYTOPLANKTON 



In the preceding section we have seen that, apart from the general effect which a 

 large number of herbivorous animals will have on the phytoplankton population by their 

 grazing (cf. Castracane, 1885, and Harvey, 1934), and particularly such an effect brought 

 about by the outstanding species Euphausia superba, there appears to be some principle 

 in operation which modifies the distribution of most of the animals in relation to that of 

 the phytoplankton. There is an accumulation of animals in the zone of intermediate 

 phytoplankton densities, a smaller number of animals in regions of poor phytoplankton, 

 possibly through a shortage of food, 1 but more particularly a smaller number of animals 

 in the regions of dense phytoplankton. It is not a hard and fast reduction ; sometimes 

 there may be good catches of a particular animal in rich phytoplankton zones and some- 

 times poor catches in the intermediate zones ; but the effect is seen as an average result 

 in several series of observations selected and treated in different ways. It appears to be 

 a principle applying to planktonic animals belonging to widely different groups, and to 

 be in operation in regions of the survey having different phytoplankton floras. We have 

 further seen that it appears to be closely linked with that common zooplankton phe- 

 nomenon — vertical migration. 



In vertical migration the plankton animals come up into the phytoplankton zone 

 during the night, and sink away from it during the day. We have suggested the hypo- 

 thesis that the extent or duration of this migration away from the upper layers varies in 

 proportion to the concentrations of the phytoplankton. According to this hypothesis, 

 the exclusion of animals from the dense phytoplankton zones, which we see revealed in 

 horizontal distribution, would be primarily brought about by a diurnal exclusion in the 

 vertical plane. The greater their vertical displacement in distance or the longer in time 

 the further would the animals travel horizontally away from the source of exclusion, 

 if the speed of the ocean currents varies with depth. This mechanism will be fully dis- 



1 This is further discussed on p. 351. 



