ZOOPLANKTON IN THE SOUTHERN OCEAN 



209 



O* 2° 4° 6= e° 10 



I I I I I I 



100 



250 



90 E 



DEGREES SOUTH 

 12° 14° 16° 18" 20° 22" 24° 26' 28* 30" 



III' I I I I I 



OCTOBER 1951 



2891 



I 



A 



SCALE 



Fig. 6. Vertical distribution of the standing crop of zooplankton during October 195 1, in 90 E. 



AVERAGE MONTHLY VARIATIONS IN THE SUB-ANTARCTIC AND ANTARCTIC 

 Consideration of the individual lines of observations shows that it is reasonable to assume that in any 

 one month the vertical distribution of the standing crop of zooplankton throughout the Southern 

 ' Ocean will, on the whole, be the same. It is thus possible, by grouping observations from all areas 

 according to month, to gain a general idea of the variations that take place in the quantity of sub- 

 Antarctic and Antarctic zooplankton from month to month in an average year. Table 3 gives the 

 mean monthly volume of plankton per 50 m. haul, together with the number of observations, at all 

 depths down to 1000 m. in the sub-Antarctic and Antarctic zones. Results for the 1500-iooom. 

 horizon are not given owing to the inadequacy of the data ; nor, for the same reason, can mean values 

 be assessed from the few observations in sub-tropical waters. Fig. 7 is based on the data given in 

 Table 3, and as in the previous diagrams, concentration of plankton is equivalent to width, and quantity 

 to area of the block at each depth interval. 



It should be pointed out that in this and all subsequent diagrams involving average values, it has 

 been necessary to use observations from day and night stations, and possible variations in the vertical 

 distribution of the plankton due to the effects of diurnal migration have been ignored. This seemed 

 justifiable, at least in the case of sub- Antarctic and Antarctic data, because the results from those lines 

 in which day and night stations occur, particularly those in 8o° W, show no large or constant differences 

 in the relative vertical distribution of the quantities of plankton caught. 



This is not surprising since it has been shown (Mackintosh, 1937) that the three commonest 

 organisms sampled by the N 70 V at least in 8o° W, Rhincalanus gigas, Enkrohnia hamata and Calanus 

 acutus, accomplish no diurnal but only seasonal migrations. Moreover, these species in adult form 

 are relatively large, whereas the most common species showing diurnal variation in its vertical distri- 

 bution, Pleuromamma robusta, is a copepod of small individual size. Hardy & Gunther (1935) also 

 found Calanus simillimus and Metridia gerlachii to be diurnal migrants, but they are also small. Of 

 the larger forms the euphausiids Euphaiisia triacantha, E. vallentini and E. frigida are known to be 

 influenced by daylight (Hardy & Gunther, 1935 ; Mackintosh, 1937), but as adults they are all relatively 

 uncommon in N70V net hauls. Obviously the importance of such animals volumetrically, and hence 

 their relative importance as components in the total standing crop of plankton, is very variable, being 

 influenced by many factors, including latitude, longitude, stage in life history, etc. As far as can be 

 seen from the limited data available, however, diurnal variation seems to play only a minor part in 

 directing the gross vertical distribution of the plankton, at least in sub-Antarctic and Antarctic 

 plankton. In the tropics and sub-tropics diurnal variations are probably more important, as was 



3-2 



