OBSERVATIONS IN THE NORTHERN REGION 309 



The seasonal variation in the quaUty of the phytoplankton of the Northern Region 

 would seem to be best exemplified by the repeated series of observations carried out 

 between o and 20° E during the season 1938-9, with some work carried out in the same 

 area during the previous autumn. This seems to give a better representation of the 

 sequence than the consideration of material collected in different seasons. Obser- 

 vations in other parts of the northern zone tallied extremely well with this series, 

 however, and it was this that led to the possibility of recognizing the biogeographical 

 zonation used in this paper. It will be realized that it is impracticable to give all the 

 data for the whole of the Northern Region in detail. The figures for the 1938-9 series 

 are given in summarized form in Table 3. This gives the mean percentage at mean dates 

 for each category of microplankton included in the 'qualitative counts' in ordinary 

 type, and the number of stations at which each was observed is given as a fraction of the 

 total number of observations available. 



It is readily seen that Group I, oceanic pennate diatoms with Distephamis speculum, 

 was important at all times except in autumn, and to a lesser extent during the post- 

 maximal decrease period. Fragilariopsis antarctica was most important in the early 

 part of the main increase, but formed a considerable proportion of all microplankton 

 present at all seasons except late autumn. Nitzschia seriata was most important at the 

 peak of the main increase and subsequently through late summer and autumn. 



The larger diatom species of Group II were most important during late summer and 

 autumn, when large local concentrations of Chaetoceros criophilum, Rhizosolenia alata 

 and Rh. hebetata semispina phase were encountered. Corethron criophilum, the most 

 important member of this oceanic group, was present in moderate proportion at all 

 seasons, most important during the early part of the main increase and during the post- 

 maximal decrease. 



The position of the neritic and ice-edge forms (Group III) in the qualitative sequence 

 is very clearly brought out by the figures in the table. Chaetoceros sociale and Thalassio- 

 sira spp. were by far the most numerous in this truly oceanic area. With the rest of 

 the group they reached their maximum importance as the ice dispersed — immediately 

 before the peak of the main increase in the season studied. At other times they formed 

 a relatively insignificant proportion of the phytoplankton. 



The oceanic Chaetocerids (Group IV) were more evenly distributed throughout the 

 year, mainly owing to the ubiquity of the two leading members of the group in the 

 Northern Region— Chaetoceros atlanticum and Ch. dichaeta. Even with these, however, 

 the tendency to show maximum relative importance during the post-maximal summer 

 decrease and in autumn, characteristic of the group as a whole, was fairly clear. 



The small oceanic Discoidae (Group V) were quite unimportant except in the ex- 

 tremely scanty winter phytoplankton, and the same may be said of the non-holophytic 

 members of the microplankton that were included in the qualitative counts. 



The colonial green flagellate Phaeocystis brucei, whose numbers cannot be estimated 

 by our methods and which is clearly a first colonist when pack-ice melts and does not 

 long persist thereafter, is naturally of only local importance in the Northern Region, 



