334 



DISCOVERY REPORTS 



figure works out at around 1650 metric tons wet weight of phytoplankton per sq. km. 

 of sea surface. For silica, the apparent production is very much less, yielding a theoretical 

 crop of some 115 metric tons only — less than one-twelfth of that calculated from con- 

 sumption of other nutrient materials. It is true, of course, that some phytoplankton 

 organisms do not require silica, but diatoms are definitely the dominant group in the 

 English Channel, so that as Cooper has convincingly shown (1933, pp. 695-7, 744) it 

 is highly probable that owing to a comparatively rapid mechanism of resolution silica 

 takes part several times over in the main diatom increase. 



Three series of observations over suitable periods from the northern part of the 

 Antarctic zone have been selected for comparison of the minimum theoretical crop 

 deduced from consumption of phosphorous and of silica, including one from the 

 neritic South Georgia area. The figures, with those from the English Channel for 

 comparison are given in Table 13. 



Table 13 



* Cooper, 1938, p. 187. 



t Cooper, 1933, p. 743. 



From the table it is at once apparent that silica is consumed on a very much larger 

 scale in the far south, and that the consumption most nearly parallels the phosphate 

 reduction over the shortest period studied, as one would expect if silica is redissolved 

 and used over again during the same plant cycle. Even over the shortest period, how- 

 ever, calculated production, on the basis of phosphate reduction, is sufficiently greater 

 than that calculated from silicate reduction, to make it practically certain that even here 

 silica must have been used at least twice over. 



Factors which would naturally lead to a relatively great ' take out ' of silica in our 

 southern areas are : (a) loss of silica to the 50-0 m. layer through rapid sinking of faecal 

 pellets of zooplankton herbivores, accentuated by the considerable diurnal vertical 



