PHYTOPLANKTON OF EAST CUMBERLAND BAY 215 



tendency to sink unless conditions near the surface are ideal for them. They are frequent 

 among pack-ice, near which shallow discontinuity layers are set up in calm weather. 

 Father south in the Weddell Sea Lohmann (1928) gives January as the maximum 

 period for this genus. 



CONCLUSIONS 



It has been shown that the phytoplankton in East Cumberland Bay is very scanty, 

 and that a species of Fragilaria is the dominant neritic form. The principal factors 

 preventing extensive production are the rapid mass movements of the water brought 

 about by the wind, and the amount of fine inorganic particles in suspension brought 

 down by drainage from land. Early in the season prolonged sunshine appears to be 

 favourable to diatom growth, but will not promote any marked increase unless wind 

 conditions are also favourable. 



Meteorological conditions during the season studied were somewhat abnormal, but 

 it seems probable that in the ordinary course of events there would be : 



Firstly, a spring increase with Fragilaria sp. dominant, coincident with the first 

 periods of prolonged sunshine before thawing on the lower slopes is considerable ; and 

 secondly, a mid-season increase with Coscinodiscus stellaris dominant, dependent upon 

 a short period of calm weather. 



Following periods of prolonged northerly winds there is likely to be a considerable 

 invasion of outside forms, especially Chaetoceros criophilum and Corethron valdiviae 

 These are large species which frequently dominate the phytoplankton community in 

 the open sea east of South Georgia, and find their optimum conditions near the surface. 



It has been shown that the phosphate content of the water in these regions is always 

 very much higher than in corresponding latitudes in the northern hemisphere, so that 

 it is unlikely that the amount of nutrient salts available influences phytoplankton 

 periodicity in the south. In East Cumberland Bay, the concentration of phosphate fell 

 during the first half of the season, during which time the richest hauls of diatoms, com- 

 paratively speaking, were obtained. Since these diatoms were not at any time present 

 in sufficient quantity to warrant the assumption that this was due to their activity alone, 

 it is concluded that dilution of the inshore water consequent upon the thawing of 

 glaciers and snow-slopes was partly responsible for this decrease in phosphate. This 

 dilution causes at the same time a marked reduction in the salinity at the surface, so 

 that this eff'ect can be distinguished from the depletion in phosphate caused by the rich 

 phytoplankton development oiTshore, the influx of outside water being marked by a rise 

 in salinity. This view is also supported by the fact that the mean phosphate content at 

 the routine station position was lower than that found at corresponding depths in the 

 open sea offshore during the season studied. 



Gran (1929) and his co-workers, notably Mrs Ruud-Foyn, have held that off the 

 Norwegian coast thaw water is responsible for the onset of the great vernal diatom 

 increase by increasing the amount of nutrient salts present. This is precisely the reverse 

 of the effect observed in Cumberland Bay, where, however, conditions are very different. 



