TRANSITIONAL PHYTOPLANKTON 69 



The scanty evidence of an autumnal increase off South Georgia at present available 

 may be summarized as follows : 



Hardy (in press) states that phytoplankton catches in May 1927 were larger than 

 in March 1926, though still much smaller than those obtained in December and 

 January. Secondly, as regards the two main sources of the surface water in this area, a 

 moderately rich phytoplankton was observed in the southern half of Drake Passage in 

 April 1930, as described in the next section, with Chaetoceros atlauHciis, Nitzschia 

 seriata and Fragilaria antarctica the most numerous forms. Some of this old Bellings- 

 hausen Sea water would presumably reach the neighbourhood of South Georgia before 

 this phytoplankton was completely dissipated. On the Weddell Sea side Mangin's 

 analyses of the Scotia material indicate a slight autumnal increase with Chaetoceros 

 criophilum, Ch. atlanticus, Ch. dichaeta, Dactyliosolen antarcticiis and Nitzschia seriata 

 dominant ; and the later March stations of the ' William Scoresby ' in 193 1 , and the other 

 stations taken late in March and in the April of that year, indicated that phytoplankton 

 of this nature was to be found to the east and north of South Georgia. Of the autumn 

 forms, small Chaetocerids were the most important in both areas, as had already been 

 observed in the case of the Weddell Sea by Lohmann (1928, p. 16). Chaetoceros 

 atlanticus reaches its maximum farther north than Ch. dichaeta, which is a species 

 preferring very cold temperatures. It is therefore probable that round South Georgia 

 Ch. athnticiis will be much the more important of the two towards the latter end of 



most years. 



In abnormally mild open seasons, like 1929-30, it is probable that all the Corethron 

 valdiviae will go over to the spineless chain form. The latter would seem to propagate 

 vegetatively and it has also been found in sub-Antarctic surface water. There is probably 

 always a slight increase in dinoflagellates after mid-season when the diatoms are on the 

 wane, but except in abnormally mild seasons this will not be very noticeable, as the 

 diatoms remain completely dominant. 



The factors that may determine the succession indicated are discussed in the general 

 conclusions. 



THE TRANSITION FROM ANTARCTIC TO SUB- 

 ANTARCTIC PHYTOPLANKTON 



The material collected during the two seasons studied which bears on this part of the 

 problem is not very complete, as most of the lines of stations did not extend far into the 

 sub-Antarctic Zone. The majority of them also were worked towards the end of the 

 season; information on conditions at midsummer is unfortunately lacking as at this 

 time of year the research ships were engaged farther south. The effect of the Antarctic 

 convergence and the extent to which overlapping of species takes place is, however, 

 well brought out by the lines of stations about to be described, so that on this account 

 alone the material is of considerable value. 



