Abundance and Distribution of Krill in the Ice Edge Zone of the Weddell Sea, Austral Spring 1983

The effect of the ice edge zone on the abundance and distribution of Antarctic krill was investigated by acoustic and net sampling methods in the northwestern Weddell Sea during austral spring, 1983. The dominant acoustic target was Euphausia superba which occurred over the entire study area in the upper 100 m of the water column, either in aggregations or loosely defined layers. The vertical depth distribution under the pack ice was similar to that of open water. Average krill biomass under the ice ranged from 1 to 68 g m−2 and in open water range from 10 to 100 g m−2. Another euphausiid, Thysanoessa macrura, appeared to be ubiquitous but was much less abundant than E. superba. T. macrura also formed aggregations. All stages were found in open water, but only adults and juveniles under the ice. A few adults and larvae of a third euphausiid, E. frigida, were found only in open water. Salps, which were abundant in open water, were not present under the pack ice. The pack ice directly influenced the abundance and distribution of young krill. Juvenile E. superba were much more abundant under the ice than in open water north of the ice edge. Numerous observations were made of juvenile krill feeding on the undersurface of ice floes where chlorophyll concentrations were much greater than in the water column below. These observations appear to indicate that the pack ice serves as a giant nursery area for young krill, providing a refugium from predators as well as a vast food resource that may be critical to the survival of overwintering juvenile E. superba