68 DISCOVERY REPORTS 



of the phytoplankton. Consequently the seasonal variation in the strength of the surface 

 currents and the degree to which their influence extends into the South Georgia area 

 in different years will be reflected in the phytoplankton crop. It seems probable that as 

 regards the seasonal variation the flow will be strongest in spring and early summer, 

 when the pack-ice begins to melt and break up. This is particularly true of the western 

 Weddell Sea current, which, as we have seen, produces the heaviest phytoplankton. Of 

 the yearly fluctuations in the flow little can be said until more is known concerning the 

 hydrology of the currents, beyond the fact that they occur on a large scale with a corre- 

 spondingly marked eff'ect upon the phytoplankton. 



THE PROBABLE SEASONAL SUCCESSION 



The main diatom increase takes place in late spring, when almost all the species attain 

 their maximum. Some of these forms persist in more moderate numbers throughout 

 the summer, to an extent which varies from year to year ; a few almost certainly have a 

 small secondary autumnal maximum, though on this point further observations are 

 required. The great outburst of diatom growth in the spring is an almost universal 

 feature in cold temperate and higher latitudes, and this fact leads to the consideration of 

 another way in which the surface currents probably afl"ect the plankton population off 

 South Georgia. From the writings of Gran, and others who have studied the production 

 of marine phytoplankton in the northern hemisphere, it has been clearly shown that 

 the date of the main spring increase, in any comparatively small area, falls within certain 

 fairly narrow limits from year to year. Further, the higher the latitude, the later is the 

 time of this increase, until in very high latitudes there is just a single prolonged period 

 of rapid growth, in some cases after midsummer. Now the surface water round South 

 Georgia originates in high latitudes, as we haveseen,so that if the flow is prolonged, there 

 is the possibility of the production due to the main increase farther south being apparent 

 in comparatively rich hauls off South Georgia after the large-scale production in that 

 area has ceased. Something of this sort was evidently taking place during the latter half 

 of the 1930-1 season, when comparatively large hauls of Chaetoceros criophilum, many 

 in a dying condition, were obtained to the east of the island. 



The normal succession off South Georgia is thus probably somewhat as follows : 

 A big spring increase, with the dominants found on the November 1930 survey, the 

 following species remaining important up to mid-season in years in which much ice 

 drifts north and the currents are strong: Chaetoceros socialis, Ch. criophilum, Ch. 

 atlanticus, Corethron valdiviae, Fragilaria antarctica, Eucampia antarctica, Rhizosolenia 

 styliformis, and more constantly Nitzschia seriata. To these in years in which the in- 

 fluence of Bellingshausen Sea water predominates, a second wave of Thalassiothrix 

 antarctica should be added. A minimum appears to be reached in February-March. 



The larger forms encountered in some abundance at mid-season appear to be re- 

 newed by successive waves of production farther south, drifting up into the area mainly 

 by way of the eastern Weddell Sea current. 



