366 RYTHEB [CHAP. 17 



occurs in the open leads, but a more important stimulus to the production of 

 the region as a whole is believed to be the light which penetrates the ice beneath 

 the surface pools of melt-water, which may act as lenses. Three in situ 14 C 

 measurements at Station "Alpha" in 1957 within an open lead revealed no 

 detectable photosynthesis on July 1 and values of 0.006 and 0.005 g carbon/m 2 / 

 day in late July and early August. At Station "Bravo" five in situ 14 C measure- 

 ments gave values ranging from undetectable levels in June to a maximum 

 rate of 0.024 g carbon/m 2 /day in July, averaging 0.01 g carbon/m 2 /day for the 

 period June 20-August 20. It was estimated by both Apollonio and English 

 that the annual production of organic matter in the Polar Sea is something less 

 than 1 g carbon/m 2 . 



E. The Antarctic 



No published measurements of primary production in the Southern Ocean 

 were available prior to the preparation of this chapter, an unfortunate dis- 

 crepancy for purposes of regional comparison. However, more than a decade of 

 intensive exploration on the part of the Discovery Committee of Great Britain 

 has made this one of the best-known oceans of the world, particularly with 

 respect to its biology, and has left little doubt that it is the world's most fertile 

 large oceanic region. The following brief discussion is based on publications 

 concerning the region's physical oceanography by Deacon (1933), its nutrient 

 chemistry by Clowes (1938) and Ruud (1930), and its phytoplankton by Hart 

 (1934, 1942) and Hasle (1956). 



The explanation for the high fertility of the Southern Ocean lies in its 

 physical circulation. Surface water around the entire Antarctic continent 

 spreads northward to about 50° where, at the Antarctic Convergence, it sinks 

 below the sub-Antarctic surface water. This cold, fresh layer then continues to 

 spread to the north beneath the surface until, at about 40°, intensive wind 

 action causes it to become mixed with the warmer, more saline waters above 

 and below it. Here, at the sub-tropical convergence, the mixed water sinks and 

 moves northward as the Antarctic Intermediate Water. There is also sinking 

 of cold water in locations near the Antarctic Continent, forming Antarctic 

 Bottom Water which also moves to the north. 



The loss of water from these two sources is compensated by the southerly 

 flow of water from intermediate depths (2000-3000 m) which rises to the surface 

 near the edge of the continent and thence spreads northward as the above- 

 mentioned surface drift. Water from such great depths is, of course, extremely 

 rich in nutrients. Where it reaches the surface, concentrations of nitrogen and 

 phosphorus are probably higher than in any other unpolluted surface waters 

 of the world. As this water moves away from the continent during summer, 

 plant growth reduces the concentrations of dissolved nutrients appreciably, but 

 even at the Antarctic Convergence the levels of these substances are still high 

 enough for production probably never to be seriously nutrient-limited in most 

 parts of the region . 



