HYPOTHESIS OF ANIMAL EXCLUSION 3II 



in all regions, and that it too may be excluded from areas of dense phytoplankton which 

 it cannot keep in check. As Harvey in writing to me has said, the two principles are not 

 antagonistic. 



We must now see if the division of the Copepoda into the three groups A, B and C, 

 having different types of correlation with the phosphate values, can be linked with any 

 phenomenon seen in our survey ; such a linkage might be expected to throw important 

 light on the problem of this correlation with phosphates and so with phytoplankton 

 production. As soon as it appeared to be possible that the zooplankton distribution was 

 being governed by the phytoplankton it became necessary to consider how they would 

 be displaced away from areas of dense phytoplankton production. The increase of 

 numbers in regions of moderate phytoplankton and their decrease in regions of high 

 production cannot be due to actual multiplication and mortality, for many of these 

 plankton forms must take a considerable period to reach maturity in these cold latitudes. 

 It has been shown that Eitphausia superba probably takes two years (Ruud, 1932). 1 

 Two possible methods were considered : firstly that of actual horizontal migration by 

 swimming, and secondly that of vertical migration away from the upper layers and 

 remaining down until carried by currents, different in speed or direction from the 

 surface currents, out of the region of exclusion. We had seen Euphausians in the 

 fjords swimming round and round in dense swarms for long periods (see p. 210), and 

 it seemed possible that if the necessity arose they might be able to swim in a straight 

 line away from a region of exclusion. At first when Euphausia alone was being considered 

 this view was favoured, but Dr Kemp, with whom I discussed this problem, was of the 

 opinion that this was not possible and that the second method must be the correct one. 

 The subsequent investigation of the large number of different species, the results of 

 which have been given above, show that he was right. The smaller members of the 

 zooplankton, the Copepoda, the Pteropoda, to say nothing of the Foraminifera, ob- 

 viously could not make such swimming migrations; yet our results suggest that they 

 have the power of distributing themselves horizontally in relation to the phytoplankton 

 production in the upper layers. The power of vertical migration which so many of these 

 organisms possess would seem to be their means of "navigation". They rise towards 

 or into the phytoplankton zone during the night and fall away from it during the day. 

 It is suggested that the greater the phytoplankton concentration the deeper, or possibly 

 for a longer time, will the zooplankton organisms penetrate into the layers below, and 

 since the surface currents differ in speed and sometimes in direction also from those 

 below, the organisms will be carried gradually away from the zone of high phytoplankton 

 concentration. Similarly, according to this hypothesis, if the concentration is less dense 

 the zooplankton organisms would not show such a marked migration away from the 

 upper layers and so would remain in that body of water. It would appear that they 

 may be travelling just as balloonists do when they make use of the air currents going 

 in different directions at different heights. The balloonist is aerially "planktonic" — a 

 passively drifting creature, but one who can migrate vertically up or down by liberating 

 1 And shown independently by Wheeler, but data not yet published, see p. 215. 



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