211 



ZOOPLANKTON IN THE SOUTHERN OCEAN 

 With the development of spring and then summer, the level of greatest concentration gets nearer the 

 surface. In the Antarctic it is between 250 and 100 m. in October, and between 100 and 50 m. in 

 November and January; and by late summer and autumn it lies between 50 m. and the surface. A 

 similar sequence of vertical movement is also evident in the sub-Antarctic, although the winter 

 concentration in deep water does not appear to be so marked as that in the Antarctic. 



2- 





o 

 > 



1- 



SUB-ANTARCTIC 



IOo 



I 



T 



m 



N 



IX 



x xi xn 



1 



Fig. 



vi vn VTTT 



MONTH 



8. Seasonal variation in the mean concentration of zooplankton in the 100-50 m. and 50-0 m. horizons of sub- Antarctic 

 and Antarctic surface water. The numbers on each curve refer to the number of observations in each month. 



From Fig. 7 and Table 3 it is also clear that the largest seasonal fluctuations in plankton density 

 take place in the surface waters, which of course are the seat of phytoplankton production. The 

 degree of seasonal variation is better illustrated in Fig. 8, where the mean monthly volumes of the 

 50-0 and 100-50 m. hauls have been plotted graphically. The highest summer mean volume is about 

 seven times as great as the lowest winter mean volume in the sub-Antarctic, and about six times as 

 great in the Antarctic at the 50-0 m. level. In both zones there are two main periods of increase, the 

 first in spring reaching its maximum at the 100-50 m. level and the second in autumn at the 50-0 m. 



