DISCOVERY REPORTS 



the phytoplankton are excreted. If this suggestion be correct, it follows that regeneration 

 of phosphate from the faeces of the zooplankton in situ is likely to be smaller than that 

 of silicate which appears to be very rapid, as the evidence at St. 1543 indicates. It must 

 be emphasized that the above statement is strictly confined to regeneration in situ from 

 faecal remains of zooplankton and does not include decomposition and regeneration of 

 nutrient salts from dead zooplankton. Cooper (1935, p. 200) added samples of animal 

 and plant plankton to sea-water in glass vessels and found that the breakdown of the 

 zooplankton was very rapid and that more phosphate was set free than had originally 

 been added as plankton. The balance was produced from dissolved organic phosphorus 

 compounds in the sea water. The breakdown of phytoplankton showed a short time lag 

 and only a part of the added phosphorus was set free as phosphate. 



In support of the argument that the zooplankton retains part of the phosphate and 

 rejects the siliceous skeletons of the phytoplankton, it will be noted that vertical 

 sections of phosphate and silicate contents in the Southern Ocean and north of it differ 

 very considerably. Thus in the South Atlantic Ocean for example, phosphate is at a 

 maximum in the region 3 8-43 ° S whereas silicate is always maximal in the Antarctic 

 zone. The phosphate maximum occurs north of the Antarctic convergence because 

 plankton is carried out of the Antarctic zone into the Antarctic intermediate current, and 

 in this current maximum decomposition and regeneration of phosphate occur in the 

 above latitudes. On the other hand the maximum concentrations and the greatest mor- 

 tality of phytoplankton occur within the Antarctic zone or close to the Antarctic 

 convergence, with the consequence that maximum silicate is found in this zone. The 

 statement that maximum regeneration of phosphate in the South Atlantic Ocean 

 occurs in the regions 38-43 S in the west and 41 ° S in the east is supported by 

 the fact that the phosphate content of the bottom water is maximal in these regions 

 (Plates IV and V); this tends to suggest that decomposition occurs in two stages, one 

 of which takes place in the Antarctic intermediate current and the other in the great 

 depths of the bottom water. The former is undoubtedly the seat of the greater regenera- 

 tion but the phosphate regenerated in the bottom water must travel farther north before 

 it completes the phosphate cycle by being transferred by mixing to the bottom of the 

 south-going warm deep water. 



In view of the possible application of the results obtained from such an enclosed 

 area as the English Channel to the open sea, it is interesting to note that Cooper (1933, 

 pp. 741-4) found that the minimum production of plankton calculated from chemical 

 data of changes in the contents of carbon dioxide, oxygen, phosphate, nitrate and 

 silicate in the English Channel agreed very well amongst themselves with the single 

 exception of silicate. Cooper suggested that silicate seemed to go through the life cycle 

 of plankton several times in one season. This work (loc. cit. p. 697 and also part III) 

 strongly suggests that regeneration of silica can proceed rapidly at the surface and 

 near the bottom in a depth of 70 m. 



The surface area of most plankton diatom skeletons is large compared with the volume 

 of the individual and offers a good opportunity of re-solution in sea water of moderately 



