LIMITING FACTORS 187 



Of the weather, which has been used as a general term to include a number of 

 meteorological factors, it is impossible to say much. The prevailing winds, in concert 

 with the earth's rotation, are responsible for the course and strength of the surface 

 currents, and variations in the meteorological system will be reflected in the yearly 

 fluctuations of these currents. There is an obvious relation between weather and ice 

 conditions, but it is difficult to say which is the prime mover. Prevalence of fogs will 

 hinder phytoplankton production by cutting down the light intensity, while the short 

 periods of fine calm weather experienced far to the south in the pack will promote it, as 

 shallow strongly marked discontinuity layers tend to be set up, keeping the contained 

 diatoms within the zone of optimum light intensity. Moreover, there is evidence that, 

 when melting, the southern sea-ice liberates resting spores in a suitable internal con- 

 dition for rapid growth. The continuous westerly gales of the northern part of the Ant- 

 arctic Zone probably exert a strongly unfavourable influence by incessant mixing of 

 the surface layers, but enough has been said to indicate how closely the more concrete 

 physical factors are bound up with the meteorology, about which our information is 

 necessarily scanty. 



The surface currents are of such obvious importance throughout this area that their 

 action has been dealt with at length in the body of this paper. The stimulus to growth 

 provided by the convergence of currents has already been mentioned (p. 181); among 

 their other important eff'ects the transportation into the South Georgia area of the after- 

 math of the main increase farther south, after production on the spot had practically 

 ceased, is possibly the most noteworthy. The efl'ect of the north-easterly drift out of the 

 Bellingshausen Sea into the Bransfield Strait, and through Drake Passage into the 

 Scotia Sea, has also been emphasized. It is very probable that the individuality of the 

 phytoplankton of the eastern Weddell Sea water is due to the fact that the whirl forms 

 a partially closed system^ ; but the most important eff'ect of the surface currents, acting 

 in concert with the winds, is undoubtedly the northerly movement of the pack-ice in 

 spring, a movement which is followed by such a tremendous production in the northern 

 part of the Antarctic Zone. 



It will have been noted that in addition to the typically oceanic species, the abundant 

 flora developing along the advancing ice-edge in late spring presents in many respects 

 a neritic facies. A similar oceanic development of neritic forms in the vicinity of ice has 

 long been known in the northern hemisphere (Gran, in Murray and Hjort, 1912, 

 p. 342). In the southern area it is most marked by the abundant occurrence of such 

 forms as Fragilaria antarctica, Thalassiothrix antarctica, Nitzschia seriata and Thalas- 

 siosira antarctica. It will be noted that the majority of these forms belong to the 

 Pennatae, as do the majority of littoral and ice forms. This is extremely interesting in 

 view of the phylogenetic speculations of Lloyd (1926, pp. 105-10). Having pointed out 

 that the majority of plankton diatoms belong to the Centricae, she refers to the above 

 genera, which almost alone among marine pennate forms are of importance in the 

 oceanic plankton, as " reversionary plankton forms ". Her views are thus strongly sup- 



^ Cp. Clowes, A. J., Nature, cxxxi, p. 189 (1933). 



24-2 



