2i8 DISCOVERY REPORTS 



standing crop does not occur in the highest latitudes or at the lowest temperatures but in waters 

 between 50 and 55 S (Fig. 10) and at a temperature of 4-0 C (Fig. 12). 



This decrease might seem remarkable in view of the abundance of life in the colder Antarctic 

 waters, but it will be remembered that Euphausia superba above 20 mm. in length are excluded from 

 the volume measurements simply because, for all practical purposes, they are not sampled by the 

 N70V. Except for the northerly region of the Weddell drift between 6o° and 30 W, this species is 

 abundant only in high latitudes and it has no counterpart farther north. If the volume of organic 

 matter which it represents, as an element in the plankton, could properly be superimposed on the 

 histograms in Figs. 10 and 12, there would probably be further increase up to the highest latitudes 



900 700 500 300 

 MILES NORTH 



~~ 1 > 1 r 



IOO O IOO 



Ate 



300 500 700 900 

 MILES SOUTH 



MOO 



Fig. 13. The total volume at five series of stations in the meridian of o° plotted according to the distance (in nautical miles) 

 of the station north or south of the Antarctic Convergence. The serial numbers of the stations are shown. 



and coldest waters. As an example of the remarkable concentrations of krill that do occur in those 

 latitudes one can refer to observations made at sea; at St. WS 540 (57 55' S and mean temp. — o-23°C) 

 in the Weddell Drift, for instance, a horizontal tow with aim. net (N 100H) in 36 sec. caught 13 litres 

 (approximately) of krill. This represents a volume of plankton far greater than that of any other 

 sample measured in this work. 



While the importance of krill should not be underestimated in relation to the total standing crop 

 of plankton in high latitude waters, it is also possible that the maximum shown in Fig. 10 between 

 50 and 55 S is due to the northerly drift of Antarctic surface water. 



The Antarctic Convergence was crossed on forty-one occasions by the present series of observa- 

 tions, and 53 20' S — the average of its position on each occasion — lies within the area of maximum 



