340 DISCOVERY REPORTS 



SUMMARY 



The aim of this work was to provide a picture of the major differences in phyto- 

 plankton distribution at different times of the year throughout the whole of the 

 Antarctic zone of the southern ocean. In deahng with such a vast area it is impossible 

 to do more than consider the larger qualitative and quantitative differences at as many 

 stations as practicable, and then to study the changes throughout the year in single 

 areas where conditions seem typical, so that one can distinguish between the effects of 

 the probable seasonal variation and inherent distributional differences. 



The principal method employed was estimation of the pigment content of catches 

 from 50 to o m. vertical hauls with a net fitted with a meter recording the volume of 

 water fihered. The results are expressed in arbitrary colour units per m.^ (Harvey, 

 i934«). The relative abundance of the leading forms was determined by counts from 

 the same hauls. Evidence from centrifuged water samples and other sources has also 

 been briefly considered with a full survey of previous work bearing on the problem in 



hand. 



The limitations of our methods are fully discussed in relation to recent advances in 

 phytoplankton technique. Nielsen's sedimentation method has many disadvantages for 

 long-range work of this type. It is shown that loss of nannoplankton forms through the 

 nets is probably less serious in the Antarctic than in any other large sea area. 



A division of the Antarctic zone into biogeographical regions or areas, designed to 

 facilitate the presentation of these results and the problems presented by them, is 

 described. It is based mainly on two fundamental environmental considerations, degree 

 of neritic influence, and the northward extent of the Antarctic surface water in the 

 longitudes concerned. The degree to which the Antarctic surface waters extend towards 

 the equator involves corresponding differences in the duration and intensity of the light 

 available for photosynthesis. The division is also in part arbitrary— unavoidably so— 

 for it is obvious that in nature conditions will merge gradually, while in practice it 

 becomes essential to draw boundaries somewhere if the descriptions are to be reduced 

 to manageable proportions. 



The divisions are: 



The Northern Region: between the Antarctic convergence and a line 330 miles south 

 of it, all round the world with the exceptions of special areas between 30 and 110° W; 

 and 150° W and 170° E. 



The Intermediate Region: between the southern limit of the above and the Antarctic 

 circle, all round the world with the exclusion of the same complicated areas. 



The Southern Region: all seas south of the Antarctic circle, excluding immediate 



coastal areas. 



These three regions may be regarded as providing essentially oceanic habitats, apart 



from the influence of pack-ice. 



The special areas include those where neritic influence is strong, or where the 

 Antarctic convergence is situated considerably to the south of its mean latitude (53° S). 



