MINIMUM CROP CALCULATIONS 335 



migrations of the latter; (b) greater individual requirements of certain dominant 

 diatom species, such as the heavily silicified Fragilariopsis antarctica ; (c) greater silica 

 requirements of the phytoplankton community as a whole — a more purely diatomaceous 

 one than in the English Channel ; (d) the possibility of lower temperatures lessening the 

 rate of regeneration of silica. In deep seas (even in the South Georgia area, where the 

 surface layers are under neritic influence, the area with depths less than 200 m. is very 

 small and oceanic depths preponderate) there is also loss through death and sinking of the 

 diatoms themselves to be considered, though this is not likely to be so important over 

 the period of the main increase as later in the year. Lastly, the return of silica should 

 perhaps be regarded as due to replacement rather than to regeneration on the spot — 

 ' younger ' surface water continually passing into the northern parts of the Antarctic 

 zone from the south. The slower processes of oceanic circulation are thus involved. 



It is also to be remarked that in deep waters the effect of the stratification of the upper 

 layers in summer will effectively prevent immediate return from much of the re- 

 generation in situ. A complicating factor which must not be lost sight of is that the 

 silicate content of the northward flowing Antarctic surface water will be modified not 

 only by the production of phytoplankton in the Northern Region, but by the extent to 

 which production has proceeded in the higher latitudes through which it has passed, 

 and by the past history of the upwelling deep water that took part in the formation of 

 that surface water, and determined its initial content of nutrient materials. 



Speculative calculations on the lines of those made by Harvey et rt/. (1935, p. 430) 

 have proved interesting and profitable in considering the probable influence of the 

 grazing-down factor as a cause of the post-maximal decrease in the phytoplankton of the 

 more northerly parts of the Antarctic zone. From estimations of the phosphorous 

 content of the phytoplankton these workers were able to show that this was related to 

 the pigment content in the ratio o-o8 mg. P per 1000 units of plant pigments, so that 

 from the observed reduction of phosphate in the sea, the probable minimum crop 

 could be calculated. For the years they studied, 1933 and 1934, the calculated values 

 over the period of the main increase were 85,000 and between 75,000 and 100,000 units 

 per m.^' respectively. In the same two periods the average values of the actual standing 

 crop observed were 2500 and 1800 units per m.^ or only 2-9%, and between i-8 and 

 2-4% of the theoretical total crops. Harvey et al. have marshalled strong evidence in 

 favour of the view that by far the greater part of this huge loss is due to heavy grazing 

 of the phytoplankton by herbivorous zooplankton. They also sound the warning that 

 though the basic ratio o-o8 mg. P per 1000 units of pigments seems sound it may not 

 be applicable to mixed diatom populations in other localities. 



Before embarking on similar calculations for our southern results it is necessary to 

 consider the probability of error in applying this figure, for direct analyses of the 

 Antarctic phytoplankton are lacking. We know that prior to the main increase, in the 

 South Georgia area, the nutrient sah content is very much higher than in the corre- 

 sponding period in the Enghsh Channel, the figures are around 550 mg. N03( + N02)/N 

 and 164 mg. P2O5 per m.^ as against E i figures around 115 mg. NO3/N and 39 mg.^ 

 P2O5 (Cooper, 1933, p. 706, the phosphate figure being corrected for salt error). Recent 



