336 



DISCOVERY REPORTS 



laboratory experiments on cultures of Nitzschia closteriiim by Ketchum (1939) suggest 

 that at the higher concentration of phosphate, the proportionate intake relative to that 

 of nitrate may be higher. This raises the whole question of the ratio of nitrogen to 

 phosphorus present in sea water and in the plankton. Harvey (1928, p. 48) first drew 

 attention to the apparent constancy of this ratio in widely diflFerent seas and suggested 

 that in the main the relative requirements of the plankton (as a whole) for the two 

 elements would be found to be in the same proportion. This idea was subsequently 

 elaborated by Redfield (1934) and Cooper (1937). In sea water the general agreement 

 was close, but analyses of plankton gave more variable ratios. Consistent variations in 

 particular sea areas gave rise to Cooper's concept of the ' anomaly of the nitrate-phos- 

 phate ratio '. The variable ratios obtained in analyses of plankton are doubtless due to 

 specific differences in the proportions of the two elements required by different classes 

 of organisms — the resultant ratio in the sea water being the summation of the effect of 

 the biological 'take-out' over a given period of the seasonal cycle. It is to be expected, 

 therefore, that the anomaly of the nitrate-phosphate ratio in a given sea area will vary 

 with time according to the seasonal sequence of dominant forms in the phytoplankton, 

 as well as with the rate of regeneration and replacement by circulation of water masses. 

 Where one group of phytoplankton organisms predominates over the whole of a given 

 period — diatoms in Antarctic and boreale waters or (say) Coccolithophores in tropic 

 seas — the anomaly may be found to vary accordingly. Direct evidence of differing 

 requirements of the two elements on the part of phytoplankton organisms of different 

 classes is furnished by some of Pearsall's work in fresh waters (1932). 



With these considerations in view, it would appear that if the ratio of nitrate-phos- 

 phorus consumed, over the period of the main increase, in the South Georgia area 

 could be shown to be fairly close to that obtained in the English Channel, it would 

 follow that the crop calculated from consumption of the two elements should vary in 

 the same proportions in the two areas, and hence the ratio of phosphorus to units of 

 plant pigments present should be similar in both. 



Unfortunately, minimal nitrate figures for South Georgia are not available, but from 

 analyses in closely adjacent waters it seems safe to conclude that the nitrate content 

 there must fall at least to some 300 mg. per m.^ 



The relevant figures are shown in the following table, in which Cooper's (1938) 

 correction for salt error in phosphate analyses has been made which bring down the 

 ideal ratio N : P from 20 : i to 1 5 : i expressed in mg. atoms, or from 9 : i to 6-7 : i 

 by weight. 



