236 DISCOVERY REPORTS 



Soleniineae were abundant close inshore, present elsewhere in very moderate numbers, becoming 

 of some relative importance only in the very small minimal haul, and again at the outer end of the 

 line. Leptocylindrus danicus, one of the few neritic species in this essentially holoplanktonic group, 

 was mainly responsible for the inshore maximum. Various Rhizosolenia species, among the most 

 cosmopolitan of all plankton diatoms, were present at the minimal station ; while right offshore the 

 oceanic species Rhizosolenia simplex was the dominant form. 



'Pennatae' formed an unusually high proportion of the total diatom count at the inshore peak 

 station and were second only to Chaetoceros among the group totals at the shelf-edge peak. The small 

 'reversionary plankton forms' Nitzschia spp. and Asterionella japonica were chiefly responsible, with 

 a considerable proportion of the larger Thalassiothrix longissima at the shelf-edge. The latter usually 

 showed a more oceanic distribution than other members of the group in these surveys. 



WS970 WS969 WS968 



W5966 WS965 WS9M 

 I I I 



WS975 



« TOTAL DIATOMS 

 -» CHAETOCERACEAE 

 -' "PENNATAE" 

 ■> BIDDULPHIINEAE 

 o DISCINEAE 

 - SOLENIINEAE 



so ido 



SEA MILES FROM LAND 



Fig. 73. Estimated total diatoms, and diatom group totals, survey I. A. Mowe Point line, 4-5 March 1950. 



B. Northern Intermediate line, 5 March 1950. 



On the northern intermediate line, the estimated diatom totals were very moderate throughout, 

 with a slight maximum at the second and third stations seaward, diminishing to the outer end of the 

 line. 



From the group figures it appears that Discineae were relatively somewhat more important in the 

 small catches here than over the survey area generally. The panthalassic species Thalassiosira excentrica 

 and Planktoniella sol (oceanic) were both among the dominants at the three slightly richer stations at 

 the middle of the line. This may have been due to the mixing conditions thought to have obtained 

 within the ' Goniatdax-patch ' (see p. 251). 



Biddulphiineae were not observed in the routine analyses on this line and species of Chaetoceros 

 were present in small number at the two inshore stations only. A majority of this group proved to be 

 mainly neritic throughout the survey, but on the other lines they were nearly always dominant inshore ; 

 not the most poorly represented group, as here. Soleniineae were dominant inshore and at the peak 

 station, represented mainly by panthalassic cosmopolitan species such as Rhizosolenia hebetata and 

 R. alata ; the falling off in relative importance of this group towards the seaward end of the line was an 

 unusual feature thought to be bound up with the mixing conditions referred to above. 



On this northern intermediate line the ' Pennatae ' were the most important single group numerically, 

 with the minute Nitzschia delicatissima the most important individual species, joined by the large 



