2,6 discovery reports 



Effect of animal exclusion 

 In his Hypothesis of Animal Exclusion, Hardy (1935, pp. 273-356 and 1936c) suggests that vertically 

 migrating plankton animals get carried away in patches from zones of dense phytoplankton into zones 

 where the phytoplankton is poor through becoming involved in subsurface currents travelling at 

 different speeds or in different directions from those of the currents on the surface. I quote from 



page 353- 



Thus we shall have a series of parallel belts of plankton at intervals at right angles to the current. Now this is a 



phenomenon which we have observed from the 'Discovery' on a number of occasions; particularly striking were 



parallel belts oi Pyrosoma stretching across the sea, and possibly the patches oi Euphausia superba, Parathemisto, etc., 



observed in the consecutive net series^ (Figs. 133 and 134), were in reality belts at right angles or making an angle 



with our direction of steaming. 



He adds that if the organisms migrated upwards at 2100 hr. and downwards at 0300 hr., and the 



water into which they penetrated under moderate phytoplankton was travelling 200 yards a day faster 



than the surface water, and that into which they penetrated under the heavier phytoplankton i mile 



a day faster than the surface, then the belts of plankton would be 150 yards across and | mile apart. 



Such a disposition in the patchy krill has in fact been observed in the sea. 



Professor Hardy, however, tells me that he did not imply that this hypothesis would explain the 

 swarms of krill we see so often on the surface, and it could, therefore, equally well be, I believe, that 

 the patchiness of this species revealed by his consecutive nets, so strikingly illustrated in his Fig. 134, 

 was simply due to their random hitting, or missing, such swarms, and he readily agrees that this must 

 in fact have been what happened. ^ 



We must not, however, overlook the possibility that the dynamics involved in exclusion might 

 serve to produce patchiness among the larvae, for they, unlike the adults, are passive enough and as 

 such would be readily susceptible to any subsurface movements that might be operating to displace 

 them away in patches from a supposed exclusion area. If patchiness does so arise, so early in the 

 life-history of this species, then it would seem to follow that the dense concentrations of krill we 

 see on the surface are concentrations of long standing, of a year or more in age. But even in the larvae 

 (p. 109, Figs. 9-11) such vertical movement as there is seems neither to be rhythmic enough nor, for 

 the vast majority of the population, to go deep enough, for exclusion dynamics to be effective. 



As Hardy remarks an inverse relationship between density of zoo- and phytoplankton has been 

 recognised for many years and there can be little doubt that the grazing of the animals on the 

 plants must in part at least be responsible for it. In the cyclonic movement claimed to have been 

 discovered by the Russians off Enderby Land, however (p. 47), Beklemishev gives an instance of 

 such a relationship which he states was due neither to the grazing of the zooplankton nor to animal 

 exclusion. In the centre of this supposed vortex it will be recalled a dense local accumulation of 

 E. superba, with patches 100 m. across, was found at the surface over a considerable area in which 

 the phytoplankton was poor, Beklemishev ascribing the poverty of the plants to the rapid upwelling 

 of diatom-poor water from below, and the local richness of the animals to the fact that they, too, had 

 been brought to the surface from some deeper level. He adds that this was a good example of the 

 inverse correlation between animal and plant plankton 'unconnected with the displacement of animal 

 by plant life postulated by some writers'. Or to quote him again (Beklemishev, i960) in a more 

 striking passage, 'The barren upwelled "new water" carries along the phytoplankton cells to the 



• Conducted at Stations 150 and WS 53 on the east side of South Georgia in January 1927. 



2 Many years after his Hypothesis appeared Hardy (1956, 1958) writes that his attempts to test its challenging ideas 

 experimentally have so far failed. Perhaps he may yet succeed. 



