ZOOPLANKTON IN THE SOUTHERN OCEAN 213 



seasonal variation in the total volume of plankton, particularly in the Antarctic, where the July (mid- 

 winter) volume is as great as that in April. A similar, but less marked, increase is also seen in the 

 sub-Antarctic. 



This rather unexpected result is due, without doubt, to the winter concentration of plankton in 

 the Warm Deep Current between 1000 and 500 m., which represents a depth horizon much deeper 

 than that of the Antarctic surface layer, which on an average extends to a depth of about 200 m. 



These and the previous results discussed in this section show that in the Antarctic and to a lesser 

 extent the sub-Antarctic there is an extensive winter vertical movement of plankton into the southerly 

 flowing warm deep current. In this way the plankton which has summered in the northerly drifting 

 Antarctic surface water is returned to the southward part of its environment. 



For this reason the Antarctic and to a lesser extent the sub-Antarctic winter standing crop of zoo- 

 plankton may be as large as the summer crop, though distributed through a greater depth of water. 

 It should be noted, however, that in summer the zooplankton is associated with a period of great and 

 rapid phytoplankton growth, and so the overall rate of grazing and zooplankton production is high. 

 But in winter, when phytoplankton production is at a minimum and the bulk of the zooplankton is 

 deep down and below the depleted euphotic zone, it is probable that zooplankton production is very 

 slow and possibly at a standstill. 



REGIONAL VARIATIONS IN THE STANDING CROP 



OF ZOOPLANKTON 



VARIATIONS WITH LATITUDE AND TEMPERATURE 

 For a thorough comparison of the relative quantities of plankton in different latitudes the ideal 

 material would include a series of meridional observations, extending from the equator to the ice-edge 

 and repeated through the seasons. Although no such complete observations exist it is at least possible, 

 by combining all the present data irrespective of longitude, to gain a very general idea of the major 

 variations. 



Fig. 10 shows the mean latitudinal variation at 5 intervals in the total quantity of plankton in the 

 whole water column sampled, i.e. 1 000-0 m. ; it shows also the variation in the quantity of plankton in 

 the upper 100 m. of water. The mean volumes and the number of observations in each group are 

 given in Table 5, which takes into account 2 171 of the total number of samples. 



It is convenient to consider the upper 100 m. separately, since it has been shown in the previous 

 section that the greatest seasonal changes in concentration occur in this layer. The values have been 

 arrived at by averaging the sum of the 50-0 and 100-50 m. volumes at each station. In order to allow 

 for these seasonal changes the data are treated in two seasonal groups : the summer group, November 

 to April, representing the period of major increase in surface plankton concentration ; and the winter 

 group, May to October, the period of minimal surface concentration. 



The 1000-0 m. values, as in the previous section, are found by summing the individual measure- 

 ments of all samples between 1000 m. and the surface at those stations where a complete series 

 exists. 



As there are only ten stations between o° and 30 S (Sts. 2886-2895) they have been omitted, and 

 the range of observations covers sub-tropic, sub- Antarctic and Antarctic waters. Fig. 10 shows that 

 except for the 100-0 m. winter volumes there is a gradual increase in average volume from low to high 

 latitude reaching a maximum between 50 and 55 S, which is about four times greater than the 

 average volume for 30-3 5 °. This is followed, as one approaches the ice-edge, by a reduction to a 

 volume between 65° and 70° S which is about half that between 50 and 55 S. 



