REGENERATION OF PHOSPHATE AND SILICATE m 



high />H such as is found in Antarctic waters. It has been noted by Stanbury (1931) 

 that skeletons of dead diatoms disappear rapidly in culture experiments. 



Solution of silicate occurs not only from dead diatom skeletons ; Bachrach and Lefevre 

 (1928, 1929) have observed the complete loss of the external skeletons from a number 

 of living plants reared on sterilized media even in the presence of silicate. The removal 

 of the siliceous skeleton may require weeks or only a few days. The defective diatoms 

 appear to live and reproduce quite normally although after a few generations they 

 become so shapeless that identification is impossible. Cooper (loc. cit. p. 697) quotes 

 the above work to show the rapidity with which the skeletons may dissolve in sea water 

 and the possible adaptability to silicate shortage of diatoms leading an otherwise 

 normal life. Dr Hart assures me that in the northern region of the Antarctic zone the 

 diatoms tend to develop less strongly silicified frustules immediately after the main 

 outburst of the phytoplankton. 



Thus the regeneration of silicate within the Antarctic zone appears to take place 

 quite rapidly from the rain of dead diatoms or of diatom skeletons excreted in situ 

 Phosphate also regenerates in situ but at a much slower rate ; the evidence of vertical 

 sections shows that it is regenerated also in quite a large degree by the decomposition 

 of plankton outside the Antarctic zone. Although to the best of my knowledge no in- 

 stance has been recorded amongst the Discovery data of masses of dead zooplankton being 

 caught inside the Antarctic zone, I think it most likely that not all the zooplankton 

 passes out of the zone, nor is it all consumed by fish, whales, etc. Thus it is probable 

 that an unknown proportion of Antarctic zooplankton may die within the Antarctic 

 zone and be decomposed there to return nutrient salts to the water masses present. 

 When the smaller animals in the zooplankton are consumed by the larger individuals 

 large quantities of phosphorus-containing products of digestive action will be liberated 

 to the sea water and later will produce phosphate available for the phytoplankton. The 

 excretion of faecal pellets from all the plankton will be another source of phosphorus- 

 containing material. 



On occasions it has been noted that the plankton hauls contained quantities of the 

 cast off shells of crustaceans ; this casting off of shells takes place often, particularly 

 when the crustacean is growing quickly. It would be expected that cast shells of 

 calcified species of crustaceans would enrich the surrounding sea water with calcium 

 phosphate. If this is correct a direct return of phosphate in situ will occur. 



PHOSPHATE AND SILICATE CONCENTRATION AS POSSIBLE 

 FACTORS LIMITING PLANKTON PRODUCTION 



In spring and summer the change in concentration of phytoplankton in the Antarctic 

 zone is particularly rapid just south of the Antarctic convergence and less so in the 

 arbitrarily defined regions to the south. Such a change must be brought about by a 

 change in one or both of two opposing factors, the rate of growth of phytoplankton 

 (depending on illumination and probably the concentration of nutrient salts) and the 



