SECT. 41 



CKOGRAPH1C VARIATIONS IN PRODUCTIVITY 



367 



The upwelled Antarctic surface water is fresher, warmer and hence less dense 

 than the water 50-100 m beneath it (at = 26.8-26.9 as against 27.4). Thus the 

 euphotic layer, in addition to its high fertility, is very stable. The high con- 

 centrations of phytoplankton found in these waters are limited to depths of 

 25-50 m. North of the Antarctic Convergence, on the other hand, the waters 

 are far less stable and the phytoplankton is an order of magnitude less dense 

 and is dispersed over a depth of 100-150 m (Hasle, 1956). 



JonFMAMJ J A SOND 



Fig. 9. Plant pigments (Harvey units/m 3 ) in the northern (A), intermediate (B) and 

 southern (C) regions of the Antarctic Ocean and in the English Channel (D). (After 

 Hart, 1942.) 



The generalized picture presented above is, of course, greatly oversimplified. 

 Topographic features of the continent and sea-floor influence the vertical 

 circulation, which, in turn, results in considerable geographical variability in 

 the degree of surface enrichment, the details of which cannot be treated here. 



Quantitative studies of the plant pigments of the region by Hart (1942) give 

 the best indication of its relative productivity (Figs. 9 and 10). His results are 



