ANTARCTIC SURFACE WATER 23 



in early September 1932 and the third week in January 1934; these may be regarded as 

 winter and summer values and are 126 and 89 mg. respectively. It may be remarked 

 here that an exceptionally low value of 59 mg. was found south of the position where 

 89 mg. was recorded in January 1934 ; this represents a withdrawal of more than 54 per 

 cent of the available phosphate. North-east of the Ross Sea the convergence intersects 

 the ridge which runs between Cape Adare and Easter Island, and a very high value of 

 143 mg. was found just south of the convergence in mid-September 1932; upwelling 

 is probably the cause of this high value. Farther east during the same cruise values of 

 1 10-120 mg. were established just south of the convergence. 



In 8o° W a routine section was made in 1934 and there are four sets of observations 

 just south of the convergence. These are 112 mg. in mid-March, 107 mg. in mid- 

 September, 103 mg. in late October and 96 mg. in mid-November. In March the 0-60 m. 

 layer was largely composed of sub-Antarctic water which was overlying Antarctic surface 

 water and regeneration in situ had been in full operation for some time past owing to 

 the earlier date of the phytoplankton maximum in the sub-Antarctic zone. The Sep- 

 tember, October and November figures show that in 8o° W in 1934 not much phosphate 

 had been utilized by the phytoplankton up to the date of the November station, and 

 that in this year at least the main outburst did not begin in this longitude until after 

 16 November. It has been our misfortune to be elsewhere at the time of the main out- 

 burst of phytoplankton in 8o° W, and thus no estimate can be given of the amount of 

 phosphate withdrawn from the surface. 



Thus as a short summary we can say that at the ice-edge in the South Atlantic sector 

 of the Southern Ocean the amount of phosphate withdrawn by the phytoplankton is of 

 the order of 41 mg. out of an available 128 mg. or approximately 32 per cent. Excep- 

 tionally as much as 44 per cent has been withdrawn, whilst north of the ice-edge as much 

 as 48 per cent has been utilized. 



In the South Pacific sector the withdrawal at the ice-edge amounted to about 30 per 

 cent of the available phosphate, with again greater amounts on occasions north of the 

 ice-edge. 



The data for the northern part of the Antarctic zone are far from complete, but the 

 evidence at present suggests that a withdrawal of 33 per cent of available phosphate 

 occurs during the phytoplankton season. 



It is suggested that when phytoplankton is consumed by zooplankton the latter 

 incorporates a proportion of the phosphorus content of the plants and excretes most 

 of the silicate content. The excreted silicate is considered to redissolve quickly and thus 

 regenerate in situ whilst there is a distinct lag in the phosphate regeneration. 



PHOSPHATE CONTENT OF THE o-IOO M. LAYER 



From the foregoing remarks on the phosphate content at the surface in the Antarctic 

 zone it will have been realized how the individual values obtained vary with the season 

 of the year. As a means to a better understanding of the phosphate content of the 

 Antarctic water the mean integral values for the 0-100 m. layer have been calculated. 



