-22- 



gyre circulation pattern in the Weddell Sea extending from 

 the Antarctic peninsula to almost 30°E longitude (Foster, 

 in press) which seems to coincide with some species distri- 

 butions (Everson, 1977) . There are other gyres in the 

 Bellingshausen and Ross Seas. .Plankton are transported by 

 water currents. 



3. Ice 



Seasonal variation in the ice cover characterizes the 

 southern ocean. Large icebergs form from calving of the 

 tongues of glaciers. Since bergs extend deep into the 

 water, their movements are determined by patterns of 

 water currents. The much more extensive sea ice which 

 covers enormous areas of ocean surface is formed by the 

 freezing of sea water and is known as pack ice. It averages 

 about 1 meter in thickness, and its movements are determined 

 by wind patterns. 



The pack ice extends northward from the continent with 

 a maximum areal extent of approximately 25.5 x 10" km'^ in 

 the winter and a minimum of about half that or 13 x 10 km 

 in the summer (Mackintosh and Brown, 1956). Thus the Antarctic 

 continent appears to pulsate. The total extent of the pack 

 ice varies from year to year. The outer edge of the pack ice 

 is always several degrees of latitude inside the Antarctic 

 Convergence (Mackintosh, 1972). 



The pack ice provides habitat for seals and penguins. 

 Most of the pack ice melts during the summer and so has a 

 maximum age of one year. Some multi-year ice is found in the 

 Weddell Sea where gyre circulation prevents it from moving 

 northward. Areas of pack ice in February and March (late 

 summer in the southern hemisphere) in the eastern Ross Sea 

 and along much of the coast of the Antarctic continent presum- 

 ably are population centers for pelagic seals (Gilbert and 

 Erickson, 1977) . 



As the pack ice expands and contracts, it covers and un- 

 covers krill and other nonmigrating populations. There is 

 intense biological activity at the edge of the pack ice 

 where primary productivity is high. Herbivores are active 

 and feeding on plankton is intense (Andriashev, 1968). 



Whale migration routes are related to pack ice distri- 

 bution. In the summer whales are found in open water, with 

 the most southern portions of the stocks approaching the 

 edge of the pack ice. 



