LIMITING FACTORS 191 



the pelagic genus Chaetoceros in polar seas, for Marshall and Orr (1928, p. 337) have 

 found that genus to be peculiarly susceptible to systrophe. 



Mention of the discontinuity layers formed as the ice melts leads to the consideration 

 of a complex factor, the resultant of meteorological and ice conditions, which perhaps 

 plays the most important part of all in promoting phytoplankton production within the 

 area. This is the stabilization of the surface layers consequent upon the melting of the 

 ice and the cessation of the convection effects of the southern winter, which will tend to 

 keep the contained diatoms within the zone of optimum light intensity. Obviously this 

 factor can only exercise a beneficial effect in waters with a superabundance of nutrient 

 salts. Elsewhere it will tend to promote the rapid depletion of the surface layers, though 

 even then it is possible that the most marked production will take place after the spring 

 temperature overturn (cf. Marshall and Orr, 1930, p. 870). The possible importance of 

 stabihzation in the south has already been recognized by Gran (1932, pp. 351, 352) from 

 a few stations worked in about 57° S between the South Sandwich Islands and Bouvet 

 Island, and in about 61° S in the Weddell Sea, from the whaling factory 'Vikingen'. 

 He observed that extensive proliferation did not begin until the water layers became 

 stratified by the melting of the ice. Pelagic whaling factories are dependent on a certain 

 amount of shelter from the ice for working conditions, so that we may assume that the 

 adverse factor of instability was in this case due entirely to the prolongation of the con- 

 vection effect characteristic of winter conditions, and not to the violent storms apparently 

 responsible for a similar eflf'ect slightly farther north in the South Georgia area. Further 

 examples of the adverse effect of instability are given by the "desert areas" observed 

 over shoal water where constant vertical mixing takes place. It would seem that the 

 favourable influence of upwelling in bringing resting spores up into the photic zone is 

 nullified in these turbulent areas by downward movements carrying the organisms out 

 of the zone again too rapidly to permit of extensive proliferation. The chief of these 

 areas observed were at the western end of Bransfield Strait, to the east of King George 

 Island (South Shetlands), and to the north-west of South Georgia. 



Ice, important as an agent favouring temporary stabilization of the surface layers 

 in summer, may affect phytoplankton production in other ways. Its possibilities as a 

 carrier of resting spores have already been noted, together with the early references to 

 this fact as regards the polar ice of the northern hemisphere. Mr R. E. Priestley of the 

 Scott Expedition has noted (in correspondence) the presence of abundant living diatoms 

 on Antarctic sea-ice, and the examination of samples of ice diatoms collected during the 

 period dealt with by this paper revealed the presence of numerous plankton forms (e.g. 

 Corethron valdiviae) in addition to those which appear to be peculiar to this habitat 

 (e.g. Amphiprora spp.). It seems certain that by supporting diatoms and their spores in 

 a resting condition — notably those species that find their optimum in its immediate 

 vicinity, such as Fragilaria antarctica — the Antarctic pack-ice plays an important role in 

 maintaining the rich flora of the Southern Ocean. 



Another way in which ice may assist in promoting phytoplankton development, and 

 which also involves the effect of temperature, is in the degree of polymerization of the 



