OBSERVATIONS IN THE INTERMEDIATE REGION 313 



and March. The secondary minimum in April and sUght secondary autumnal increase 

 in May are well marked. It may be noted that the observations early in June indicate 

 that the descent to minimal winter values is less rapid than in the Northern Region, as 

 might be expected from the fact that the whole cycle is centred later in the year. 



The qualitative sequence in the Intermediate Region is shown in Table 5. Adequate 

 observations are available only for the period from the beginning of the main increase 

 through the post-maximal decrease period to the autumnal secondary maximum, so 

 that the major trends are not so clearly discernible as elsewhere. The relative importance 

 of Group I forms varies in very much the same way as in the Northern Region, if we 

 remember the later time of incidence of the main increase. While present in fairly high 

 proportions throughout the season, the group was most important in the early part of 

 the main increase, and during the post-maximal decrease. It was least important in 

 autumn. The only marked difference from the conditions observed in the Northern 

 Region was that Nitzschia seriata was more important in the earlier stages of the main 

 increase than it had been in that area, though reaching its maximum relative importance 

 in corresponding periods later in summer. 



The larger oceanic diatoms of Group II were of considerable importance in the early 

 part of the main increase in the Intermediate Region. Corethron criophilum and Chaeto- 

 ceros criophilum were much more prevalent than in the Northern Region at the corre- 

 sponding period. After the main increase the group as a whole showed a characteristic 

 rise in relative importance during the first part of the post-maximal decrease period. 

 Maximum relative importance of the group was attained in April — during the secondary 

 autumnal increase. 



In the Intermediate Region the neritic and ice-edge diatom species were most 

 important up to the peak of the main increase, as we found in the Northern Region. 

 Chaetoceros sociale was still one of the most important species, but Fragilaria spp. with 

 other more definitely tychopelagic ice forms and Nitzschia closterium were present in 

 proportions appreciably greater than those found farther north. 



Oceanic Chaetocerids (Group IV) were most important during the post-maximal 

 decrease in late summer. Thus far they showed close agreement with the proportions of 

 the group found in the Northern Region, but were relatively scarcer in autumn. Among 

 individual species the small tenuicornis phase of Chaetoceros dichaeta was more important 

 than in the Northern Region and Chaetoceros atlanticum was not so common. 



The other categories of microplankton counted were very scarce in the Intermediate 

 Region and showed a slight tendency towards maximum relative importance before the 

 main increase and in autumn as one would expect. They were abundant at the isolated 

 winter observation, and there is little doubt that they would be found to form an 

 important part of the scanty winter plankton, as they do farther north, if it had been 

 possible to obtain more winter observations. 



Phaeocystis was important in the Intermediate Region in December and January — 

 up to the time of the peak of the main increase. In December it was present at five out 

 of seven stations, and dominant at two. In January when observations were much more 



