MINIMUM CROP CALCULATIONS 337 



From this it would seem that the effects observed in short-period cuhure experiments 

 by Ketchum do not apply to these mixed diatom populations over longer intervals. 



From the ratios obtained we see that in the English Channel the relative con- 

 sumption of NO3/N is less than that at South Georgia, but this is to be expected, for 

 we know that the nitrate content of the southern waters is considerably greater than 

 that in the English Channel, which makes it probable that other forms of available 

 nitrogen are available in greater relative quantity in the English Channel. Additional 

 significance is given to this point by Harvey's recent demonstration that ammonium 

 compounds may be absorbed in preference to nitrate in mixed diatom cultures (Harvey, 



1940, p. 119). 



Using Cooper's revised ratio (1938, p. 179) of N : P= 15 : i mg. atoms or 67 : i by 

 weight, and the figure relating phosphorus to plant pigments given by Harvey et al, 

 we get the ratios 



o-o8 mg. P : 0-536 mg. N : 1000 units of pigments, 



and from this the theoretical minimum production in the two areas may be calculated 

 thus, on the basis of observed consumption of the two elements : 



E 1 : 15-5 mg. P consumed, then from the above total production should be 



-^1 X 1 000 = some 194,000 units per m*. 

 o-o8 



88 mg. N consumed, then total production should be 



88 

 , X 1000 = some 164,000 units per m.' 



0-536 

 South Georgia area: 39-3 mg. P consumed, then as before total production should be 



^^ X 1 000 = some 490,000 units per m.^ 

 o-o8 



250 mg. N consumed, then total production should be 



^^ X 1000 = some 466,000 units per m.^ 

 0-536 



Bearing in mind the fact that figures from consumption of nitrate will always be too 

 small, because the plants can utilize other sources of nitrogen, it would appear that the 

 agreement is sufficiently close to warrant the assumption that the N : P ratio m the 

 phytoplankton populations of the two areas is much the same. 



If the relation o-o8 mg. P per 1000 units of plant pigments may be applied to dis- 

 cussions of crop in the northern part of the Antarctic zone without much risk of error, 

 then we can proceed to consider the observed standing crop as a fraction of the crop 

 calculated from the minimum take out in three southern areas where figures are 

 available over periods suitable for comparison with those studied by Harvey et al, and 

 to discuss the implications of the apparent loss of crop. 



Observed reduction of phosphate and observed average standing crop are alone 



