DISTRIBUTION OF ANTARCTIC MACROPLANKTON 137 



probable that the cold-water species generally develop earlier in the summer than the 

 warm-water species and become replaced by the latter as time goes on. 



It is difficult to say what connection may exist between the plankton population and 

 the movements of the pack-ice. It is generally understood that the ice-edge reaches its 

 most northerly limit at the end of winter or in the spring. This is also the time at which 

 the plankton contains the maximum proportion of cold-water species. The distribution of 

 certain species, also, seems to be limited to regions within the range of the pack-ice, for 

 the northern limit of Diphyes antarctica, Easirus and some other species typical of the 

 coldest water coincides, at least approximately, with the northern limit of the pack-ice. 



It is worth mentioning, as a matter of separate interest, that a curious red colouring 

 has been noticed in several species taken off the ice-edge in certain places, particularly 

 in the eastern part of the Weddell Sea and to the south-west of Bouvet Island. The 

 catches here included large numbers of Calanus propinquus in most of which the 

 antennae were of a bright red colour. The red-banded variety of Tomopteris carpenteri 

 mentioned on p. 99 also occurred in some of these samples, and a small red Amphipod, 

 at present unidentified, was not uncommon. The red antennae of the copepods often 

 imparted a striking reddish appearance to the whole sample, especially when combined 

 with other red species. 



PLANKTON COMMUNITIES 



Certain contrasts between the plankton of different regions have already been 

 established. It has been seen for instance that certain groups of species are typical of 

 the colder and others of the warmer waters. The chart shown in Fig. 21 (p. 105) 

 indicates that the northern limit of some species and the southern limit of others lie in 

 a belt running from the South Shetlands to South Georgia which coincides roughly with 

 the junction of the Bellingshausen and Weddell Sea water. It has further been seen that 

 in the coastal waters of the South Shetlands and South Orkney region the plankton has 

 a characteristic which appears to be constant, namely the persistent scarcity of nearly 

 all the species with which this paper is concerned (see Fig. 23, p. 1 10). On geographical 

 grounds three main water masses can be distinguished. It will be seen from Fig. 1 that 

 Graham Land and the South Shetlands divide the Weddell Sea from the Bellings- 

 hausen Sea in the higher latitudes, and leave a continuous outer belt of Antarctic water 

 in the lower latitudes, which flows from west to east immediately south of the con- 

 vergence. 



In this section the plankton of the Weddell Sea, the Bellingshausen Sea, the outer belt 

 and the area of scarce plankton around the South Orkney and South Shetland Islands 

 are considered separately, but it will be remembered that between the plankton popula- 

 tions of the Weddell Sea and the outer belt there is a comparatively broad transition zone. 

 The populations of these four areas are compared and an attempt is made further to 

 subdivide them according to the nature of the populations. To do this it is necessary to 

 examine separately all the important lines of stations. 



