ZOOPLANKTON IN THE SOUTHERN OCEAN 217 



a surface temperature of 0-77° C. Any comparison of results from these two stations on a latitudinal 

 basis would thus be misleading. 



Variations in the 100-0 m. volumes are being considered, and the mean temperature of this layer 

 at the time of sampling suggests itself as a more realistic scale on which to compare volumes, since 

 it gives a more direct indication of the type of water from which the plankton came. 



The observations previously presented in Figs. 10 and 11, have therefore been replotted in Fig. 12, 

 based on Table 6, according to arbitrary temperature groups. The ten Indian Ocean stations which, 

 although they cover a latitudinal range of nearly 30 , only cover a temperature range of 12° C, have 

 been included in the winter observations. It will be noted that the temperature groups from 29-0 to 

 1-5° C are in intervals of 2-5° C, while the colder temperature groups are equivalent to an interval of 

 1-5° C. This adjustment seemed necessary owing to the great number of measurements from Antarctic 

 waters which cover a relatively small temperature range (4-0 to - 2-0° C, approximately) compared to 

 sub-Antarctic waters (1 4-0-4-0° C approximately). The average volumes and the number of observa- 

 tions in each temperature group are given in Table 6. 



Table 6. The mean volume of plankton in the whole water column (1000-0 m.) and in the upper 100 m. 

 ( 1 00-0 m.) between arbitrary temperature limits in winter and summer. Numbers of observations are 

 given in brackets 



[Note. The temperatures are the mean temperatures for the 100-0 m. layer at each station.] 



From Fig. 12 it is seen that as the temperature of the water decreases the quantity of plankton in 

 the whole water column and, except in winter, in the upper 100 m. of water increases. In summer a 

 maximum is reached at temperatures between 4-0° and 1-5° C which is about six times the volume at 

 temperatures between 21-5° and 19-0° C. There is a decrease in volume in waters colder than 1-5° C, 

 but not to as low a value as the warmest temperature groups. 



The winter 1 000-0 m. volumes, which include the observations in 90° E, show that the colder 

 waters are still the richest, although the maximum now occurs at temperatures between 1-5° and o° C, 

 owing no doubt to the seasonal cooling of the water. Apart from this there seems little seasonal change 

 in total volume in waters colder than 4 C. In the warmer temperature groups, however, the total 

 volume appears to be less in winter. 



Comparison of Fig. 10 with Fig. 12 shows that the outstanding feature of each is that the maximum 



4-2 



