158 DISCOVERY REPORTS 



and South Georgia and falls sharply to about 59° 30' S in 70 W, rounding the Horn at 

 below 6o° S. 



The sub-Antarctic Zone is limited to the south by the Antarctic convergence and to 

 the north by the subtropical convergence; the latter is found in about 34 S in the 

 meridian of Greenwich and falls gently to the east and to the west, until it reaches 

 44 S in the longitude of 30° E and 44 30' S in the longitude of 50 W, where it 

 probably turns northwards again. 



The subtropical Zone is less sharply defined ; the convergence between it and the 

 tropical zone originates probably in about 28 S in longitude 30 W, and extends 

 obliquely through 30 of latitude to a position just north of the line at approximately 

 io° E. (Fig. 1.) 



The zonal convergences are sufficiently well-marked to be logged with precision, and 

 are due to the meeting of large areas of surface water of different temperature and 

 salinity, the denser water sinking below the lighter. The geographical positions of the 

 convergences and the consequent width of the zones tend to vary from season to season. 



According to Dr T. J. Hart, it is reasonable to suppose that the striking changes of 

 chemical and physical factors met within comparatively short distances of a con- 

 vergence, together with the violence of the vertical mixing that ensues, are sufficient 

 to keep the algal floras of the respective zones fairly separate, although a certain amount 

 of overlapping is to be expected, particularly where the convergence is ill-defined. 



The following table is compiled from data obtained by the 'Discovery II' during a 

 passage up the middle of the South Atlantic along the 30th W meridian. The observa- 

 tions extend from 57 36' S to 14 27J' N and show the great changes which are met 

 with in passing through the four zones mentioned above. 



In the table, data are given for two positions only, viz. at the surface of the water, and 

 at a depth of 100 m. 1 These two levels have been selected because they define the limits 

 of variation which operate on the diatom population. 



The general indications are that the salinity, temperature and^H of the surface waters 

 increase as we approach the equator, and the phosphate and nitrate concentrations, 

 which are very high in polar waters, decrease rapidly in the subtropical Zone, phos- 

 phate being entirely absent from the surface waters N of latitude 26 o6i' S. 



This absence of phosphate is continued north of the Line for surface waters, but a 

 considerable increase is noticed at the depth of 80-100 m. at a position 3 20' S of the 

 equator, and on reaching a depth of 1000 m. the phosphate content is two and a half 

 times more than that at 100 m. depth. A similar decrease is noticed for nitrate through 

 the subtropical and tropical Zones, particularly at the surface of the water, although 

 increase is again recorded at a depth of 100 m. The hydrological factors have been dealt 

 with fully by Deacon (1933), and utilized by Hart (1934) in considering the distribution, 

 abundance and constitution of the phytoplankton of the south-west Atlantic. 



1 The full data will be found in Discovery Reports, iv, pp. 150-165 (1932). 



