6 DISCOVERY REPORTS 



covers the area dealt with in this paper. One important recent discovery is incorporated 

 in the chart, in which it will be seen that the convergence takes an S-shaped course 

 between the Falkland Islands and South Georgia. Phytoplankton samples from this 

 area seem to indicate that this may be a permanent feature. 



The sub-Antarctic surface water is a deep layer in which salinity and temperature 

 decrease gradually with depth. This fact, coupled with vertical mixing and promoted 

 by the almost continual bad weather of the latitudes in which it is found, renders the 

 formation of shallow discontinuity layers, such as occur in the Antarctic surface water, 

 impossible. Here we have an initial factor militating against a heavy phytoplankton 

 production, as it is obvious that stability in the upper layers will tend to promote 

 the growth of passively drifting holophytic organisms by keeping them within the zone 

 of optimum light intensity, while instability will have the reverse effect. 



The sub-Antarctic surface water receives a considerable amount of Antarctic surface 

 water by mixing at and to the north of the Antarctic convergence, and its salinity is also 

 reduced by the heavy precipitation. Its depth increases from about 200 m. at the 

 Antarctic convergence to about 1450 m. at its northern boundary, the sub-tropical 

 convergence, the distance in long. 30" W being about 550 miles. The temperature of 

 the sub-Antarctic surface water ranges from 5-5° C. in the south to 14° C. in the north 

 in summer, and from 3 to 11° C. in winter. The sub-tropical convergence is marked by 

 a sudden increase in surface temperature from about 11-5 to 15-5° C. in winter, and 

 from 14 to 18° C. in summer. 



The salinity of the sub-Antarctic surface water increases gradually as one proceeds 

 from south to north from about 34-00 to 34-50 "/^o- The sub-tropical convergence is 

 marked by a sudden increase to about 3 5-00 "/o^. Its phosphate content is high as com- 

 pared with that found in equivalent northern latitudes, being of the order of 60-80 mg. 

 P2O5 per m.^ The sources of this rich supply of phosphate are the mixing with Antarctic 

 surface water at the Antarctic convergence, and the vertical mixing caused by bad 

 weather. The nitrate content is correspondingly high — about 200 mg. per m.^ of nitrate 

 nitrogen in the upper layers — while from 3-5 to 5-5 mg. of nitrite nitrogen have been 

 found in the upper 100 m., showing that more nitrate is being generated. 



The average position of the sub-tropical convergence is shown in Fig. i . It will be 

 seen that in general it lies between 37 and 40" S across the Atlantic, reaching 42° S 

 south of the Brazil current, and 42° 30' S south of Cape Town, where it is forced south- 

 wards by the Agulhas current. The Falkland Islands' current is a local extension of sub- 

 Antarctic water, strongest close in to the Patagonian coast, occasionally reaching as far 

 north as the mouth of the River Plate. 



The sub-tropical convergence is on the whole very well marked, but considerable 

 mixing both in a northerly and a southerly direction occurs. Entire "cut-offs " of sub- 

 tropical water surrounded by sub-Antarctic water are occasionally found, especially 

 south of the Cape and south of the Brazil current, accompanied by rapid changes in 

 temperature and salinity. 



The sub-tropical surface water attains its greatest depth as a layer, about 500 m. in 



