C-23 



SUMMARY ECOSYSTEM DESCRIPTION 



The term krill refers to several species of zoo- 

 plankton in the Antarctic ecosystem. The dominant species, 

 and the only one of commercial interest, is Euphausia 

 superba . Krill is sometimes used as a synonym for this 

 species. E. superba is rarely found north of the Antarctic 

 Convergence, but is abundant immediately south. Some fish, 

 bird, and seal species exhibit the same distribution pattern, 

 circumpolar south of the Convergence. All waters south of 

 the Antarctic Convergence belong in the Antarctic ecosystem. 

 The Antarctic marine food web is centered around krill. 



The Antarctic ecosystem is unusual ecologically since 

 one species, krill, provides the major food resource for 

 fish, squid, birds, crabeater seals, and five baleen whales. 

 The timing of whale migrations, the fluctuation of the 

 pack ice, and the relationship of seal distribution to pack 

 ice type provide for some partitioning of the krill resource. 

 The partition taken by fish and cephalopods is not known, 

 although both groups are thought to be significant preda- 

 tors on krill. 



Krill play a key role in the nutrient dynamics of the 

 Antarctic marine ecosystem along with other zooplankton and 

 phytoplankton. They facilitate nutrient regeneration in the 

 euphotic zone during the growing season. Harvesting of 

 krill, fish, and whales removes nutrients from the Antarctic 

 ecosystem. Nutrients are brought into the system by exten- 

 sive upwelling. 



The biology of the Antarctic ecosystem is closely 

 regulated by the extreme seasonal variations in availability 

 of light and in areas of sea ice. The circumpolar current 

 unifies the system through transport of nutrients and 

 plankton. 



Krill have a circumpolar distribution as do most 

 birds and mammals. The distribution of fish and squid 

 species is less well known. Whales and at least one fish 

 species migrate across the Convergence. Squid are probably 

 most abundant near the Convergence, and may cross it. 



