342 DISCOVERY REPORTS 



all Other seasons form a negligible proportion of the phytoplankton. This evidently 

 shows close correlation with the pack-ice, these mainly meroplanktonic forms flourishmg 

 in the open ocean only for a short time after the pack begins to disperse. Small pennate 

 forms (Group I) form the basis of the population in the Northern Region, as in most 

 other parts of the Antarctic zone. Autumn seems to be the only time when they are 

 numerically unimportant. At this time the rather heterogeneous collection of ' large 

 species' in Group II take up the running with Group IV (oceanic Chaetocerids). 

 Group II are also important during the period of post-maximal decrease in late summer. 

 The oceanic Discoidae (Group V), always present in small numbers, reach their greatest 

 relative importance in the scanty winter phytoplankton when the holozoic constituents 

 of the microplankton also become prominent in the small samples obtainable. 



In the Intermediate Region no appreciable increase was observed until the end of 

 November, and the maximum appears to be reached about the middle of January. The 

 post-maximal decrease is more gradual and less marked, and a slight autumnal increase 

 appears to take place in May. Thus the whole cycle is later than in the Northern Region, 

 as we expected from earlier less conclusive evidence. The Intermediate Region was 

 relatively richer in the large diatom species (Group II) than the Northern Region. The 

 other outstanding qualitative feature was the dominance of Phaeocystis briicei in the 

 period immediately following the break up of the pack. The relative importance 

 of the diatom groups varied with the seasons in very much the same way as in the 

 Northern Region. Neritic/ice-edge forms (Group III) showed the same peak early 

 in the season. The post-maximal preponderance of Group IV (oceanic Chaetocerids) 

 and Group II is even more marked and would almost certainly be found during the 

 slight autumnal increase also, though qualitative data from the May observations 

 are lacking. 



In the Southern Region it is impossible to obtain adequate data for all seasons. It is 

 only on rare occasions that our ships have been able to penetrate to it in spring and 

 autumn. The evidence suggests that production must be altogether negligible during 

 winter, but that a small increase in phytoplankton takes place from November onwards 

 wherever there is sufficient open water. The main increase begins in January, when 

 there are always considerable areas free from pack, and rises steadily to a peak late in 

 February. Early in March the diminution is slight but the phytoplankton must 

 decrease very rapidly later in that month as new ice is formed. In this region Group II 

 species are even more important, but not at the height of the main increase. Two of 

 them, Corelhron criophilum and Chaetoceros criophilum, are particularly prominent in 

 January and again in March. Our single autumnal observation shows these, together 

 with Dactyliosolen antarcticus and Thalassiothrix antarctica, strongly dominant. Group 

 III is again most important early on, as was Phaeocystis brucei, which is not so all-per- 

 vading here as during the early part of the main increase in the Intermediate Region. 

 Oceanic Chaetocerids (Group IV) were scarce in January but prominent in February 

 and March. 



Observations in the South Georgia area show in striking fashion the enormous 



