REGENERATION OF PHOSPHATE AND SILICATE 107 



sea-water or in the digestive system of the zooplankton. The importance of the 

 rain of dead plankton on the regeneration of nutrient salts may be seen in the high 

 phosphate and silicate contents of the Antarctic bottom water. Also, in the sub- 

 Antarctic zone, the subsurface current has a higher nutrient salt content than the surface 

 current ; whereas it should have a lower content judging by its method of formation 

 and by its constituent water masses which consist of sub-Antarctic surface water (which 

 sinks at the subtropical convergence) and subtropical water. If there were no rain of 

 dead plankton which decomposes and regenerates nutrient salts, the subsurface water 

 would undoubtedly have lower contents than the surface water. 



Thus nutrient salts are carried out of the Antarctic zone either free or contained in 

 dead and living plankton which decomposes in deeper layers in one part of the 

 nutrient salt cycle ; but also decomposition and regeneration in situ occur within the 

 antarctic and other zones. The decomposition and regeneration within the Antarctic 

 zone are complicated by the fact that the locality of the maximum concentration of 

 phytoplankton changes throughout the season. Thus at the Antarctic convergence and 

 immediately south of it the maximum concentration of phytoplankton occurs in 

 November-December, but as the season progresses the loss of nutrient salts by the 

 position of the maximum density of phytoplankton moves southwards so that in late 

 February maximum concentration is found near the ice-edge. Thus as the season 

 progresses the northward movement of the plankton is lessened because the position of 

 the maximum is farther south and hence there is more chance for, say the diatoms, to 

 sink and decompose and regenerate silicate in the Antarctic zone, whereas in the 

 earlier part of the season the diatoms tend to be swept out of this zone, to decompose 

 at the convergence or north of it. In the western and eastern parts of the South 

 Atlantic Ocean vertical sections of phosphate content (Plates IV and V) show that 

 maximum regeneration occurs in the region 3 8-43 ° S in the western part of the ocean 

 and at 41 ° S in the eastern part. This is obviously the result of decomposition of 

 plankton carried out of the Antarctic zone by the surface current which sinks at the 

 Antarctic convergence. 



As an example of regeneration in situ some observations at St. 1543 in 66° 297' S, 

 42 26' E may be cited. This station was made in late February 1935 on the ice-edge 

 north of Enderby Land. Dr Hart, who has examined the phytoplankton catches at this 

 station, tells me that there is strong evidence that the volume of phytoplankton present, 

 though of a moderately high concentration, is but a shadow of its former density and that 

 adjacent stations a little to the west and on the same latitude had much greater concen- 

 trations. The phytoplankton catches contained a high proportion of euphausian faeces 

 consisting of diatom remains. Thus the phytoplankton conditions at this station were 

 ideal for regeneration in situ. The table on p. 108 gives some of the relevant physical 



and chemical data. 



As may be seen from the low temperature at 200 m. the surface layer is a thick one 

 for this latitude and contains two well-marked discontinuity layers, one between 10 and 

 20 m. and the other, the normal division between the surface and warm deep currents, 



14-2 



