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DISCOVERY REPORTS 



may behave in the opposite manner to those in the Plymouth area. It appears possible 

 that the relative quantities of phytoplankton might be an explanation in both cases. 

 Normally phytoplankton is richer in May and June than in July, August and September. 

 If vertical migration bears a relation to phytoplankton, this might explain why a number 

 of animals are found higher in the water in the late summer than in early summer. 

 Turning to the Clyde results we see that a similar explanation might hold. In 1932 

 Nicholls' vertical hauls at his Stations I and II, at which he observed the greater 

 abundance of Calanus in the upper layers in May and June, "were divided at the 

 30 metre line, the object being to simplify counting by confining the diatoms to one 

 part of the catch." He then adds, "As it happened in this year diatoms were seldom 

 rich enough to cause any difficulty of this nature ". In 1933 Marshall, Nicholls and Orr 

 divided their hauls at the 10 metre line; they also record the numbers of diatoms in 

 20 cc. of sea water at the surface and at 30 m. By comparing their Table II and Table IV 

 it is seen that there was a marked falling off in the number of diatoms at the time of the 

 increase in the number of Calanus at the surface in April and May. 1 



Herdman (1907) in describing his plankton investigations off the Isle of Man to 

 the British Association laid emphasis on the vertical relationship between the zoo- 

 and the phytoplankton, as follows: 



One or two broad features of the collections made were obvious. In the earlier part of the time, 

 up to about the middle of April, diatoms were abundant, and nearly all the gatherings had a greenish 

 tinge. During that period the plants were more abundant in the bottom waters, and the animals at 



1 I have brought together the two sets of figures from their two tables as follows : 



