266 DISCOVERY REPORTS 



on the expedient of filtering the catch through a no. 2 Whatman paper instead of the 

 usual silk disk. Where it is very abundant this organism causes serious clogging of all 

 fine-meshed nets, which may appear almost as if they had been treated with ' aeroplane 

 dope' after being used in such water. Fortunately such conditions are rare, and are 

 almost entirely confined to what I have termed the intermediate region of the Antarctic 

 zone, for a short period after the rapid recession of the ice-edge about mid-season. 

 The pigment extract from catches where this organism predominated gave a good 

 match with the colour standards. At those few stations where it was really abundant, 

 however, our results are obviously vitiated by the clogging of the net. I believe that 

 under such conditions P. briicei, with its disintegrating gelatinous colonies, would defeat 

 all methods of quantitative estimation, except perhaps some modification of that 

 recently introduced by Riley (1938). 



Large Dinoflagellates, which spoil the colour match by the browner colour of their 

 pigments, are fortunately very rare within the Antarctic zone (Hart, 1934, p. 181). It 

 became very evident, however, that there were considerable differences in the quality 

 of the pigments in some of the diatoms themselves though this did not interfere 

 with the colour matches. Thalassiosira spp. were found to need much longer digestion 

 with acetone before all their pigments were dissolved, just as Harvey had found with 

 members of the same genus in the northern hemisphere (1934(7, p. 770). This might 

 be due to the physical characters of the living frustules or of the protoplasts rather than 

 any diflference of the pigments themselves, but Biddulphia striata, a neritic species, 

 yielded a vast amount of rich green pigment in proportion to its bulk. The extracts 

 sometimes appeared dark 'hookers green' or almost black on the rare occasions when 

 this species predominated in the catches. This peculiarly rich pigment in B. striata was 

 first noted by our assistant, Mr W. F. Fry, who carried out the estimations under the 

 direction of Mr J. W. S. Marr during the fourth commission. I was subsequently able 

 to confirm it on two occasions during the fifth commission ; off the Balleney Islands in 

 summer and near South Georgia in the autumn. On suitable dilution, however, these 

 rich extracts gave a very exact match with the tint of the standards. 



At many stations during the winter months colour matches could not be obtained, 

 owing to extreme poverty of the phytoplankton and at times to the high proportion of 

 animals in the minute catches. It is extremely unlikely that our picture of the main 

 sequence of events is affected by this, for lack of light alone is almost certainly suflicient 

 to preclude the possibility of any considerable production during this period, by 

 organisms which might be missed by the net. 



In general, it may be said that the colour match obtained from mixed catches within 

 the Antarctic zone was very good, and the direct visual comparison probably ample for 

 determining the broader differences in quantity of the standing crop which we desired 

 to study (cf. Harvey, 1934(7, pp. 771-3). 



Our centrifuge counts were made during the third commission by a modification of 

 the methods employed by Gran (1929, p. 6) and Marshall (1933, p. 1 12). It took some 

 time to evolve a method that could be used successfully aboard ship, where so much 



