BIOGEOGRAPHICAL REGIONS AND AREAS 279 



have recently employed. For such work to be of value in considering the conditions in 

 large sea areas, it must be preceded by a large-scale survey by cruder methods such as 

 those employed by us. Without this it will be quite impossible to say whether any 

 series of more detailed observations, such as could be carried out within a reasonable 

 period of time, will be typical of conditions over a wider area or not. In the north 

 Atlantic and adjacent waters previous work may already provide a sufficient background ; 

 in other regions where the economic significance of the phytoplankton begins to be 

 realized, such as the Antarctic zone and the north Pacific, it does not. Moreover, the 

 precision methods now being elaborated do not lend themselves to the study of 

 fluctuations over wide areas, and it is just such differences as these that one desires to 

 study in attempting to link up plankton ecology with human economy. Gran has said 

 that a single ' absolute ' determination of phytoplankton would be about as valuable as a 

 single temperature determination carried to the third decimal place. The new methods 

 have got beyond the stage of being open to this kind of criticism, but still demand an 

 expenditure of time that precludes their use in our attempts to solve some of the most 

 important phytoplankton problems. The sea is wide and man has but a little time to 

 live. 



DIVISION OF THE ANTARCTIC ZONE INTO 

 BIOGEOGRAPHICAL REGIONS AND AREAS 



The Antarctic zone may be defined as the sea area covered by Antarctic surface 

 waters, as shown by the work of our hydrologists. Its northern limit may be taken 

 from Deacon's (1937) presentation of the probable average position of the Antarctic 

 convergence — where the Antarctic surface waters sink below the more saline but 

 warmer sub-Antarctic waters to the north. The mean latitude of the Antarctic con- 

 vergence is 53"^ S. Thus polar conditions of climate and hydrological environment 

 extend very much farther towards the equator than they do in most parts of the northern 

 hemisphere, and their distribution bears little relation to such purely mathematical 

 entities as the Antarctic circle. In general, the Antarctic surface waters extend some 

 thousand miles to the north of the coast Hne of the Antarctic continent. 



The area covered by Antarctic surface waters is very large — at least 12 million square 

 miles. In considering the conditions of existence of phytoplankton organisms in an 

 area of this size, it is obviously essential to adopt some scheme of subdivision, in order 

 to keep both the descriptions of observations, and discussion of their significance, 

 within reasonable proportions. Ideally, such a scheme should be based on the principal 

 changes in the conditions of existence, in practice a degree of arbitrariness will obviously 

 be unavoidable. In nature conditions will always merge more or less gradually, but in 

 practice boundaries must be drawn somewhere. This difficulty is very apparent in the 

 Antarctic zone where the gradient in water temperature, for example, is very slight. 



In the areas south of the three great oceans the latitude of the Antarctic convergence 

 approaches its mean fairly closely. Here a satisfactory division may be made by con- 

 sidering the interaction of two important factors known to exert a profound influence 



3-2 



