OVERWINTERING DEVELOPMENT, GROWTH, AND FEEDING OF LARVAL EUPHAUSIA-SUPERBA IN THE ANTARCTIC MARGINAL ICE-ZONE

Seasonal extent of pack ice has enormous influence on the Antarctic ecosystem. An investigation of the marginal ice zone in winter took place in the Scotia-Weddell Seas, a major nursery ground for the Antarctic krill Euphausia superba. Krill larvae were abundant at the ice edge and on the undersurfaces of ice floes where the pack ice provided greater concentrations of food and a superior refuge compared to the water column. Larval development and growth, which is dependent on food supply, progressed steadily from June through August. In early June, furcilia stages F3-F5 were most abundant but, by August, F6s and juveniles predominated. Krill molted about every 20 d and growth rates (0.07 mm d-1) were similar to reported summer rates. Gut fullness indicated that 98% of the larvae were feeding both day and night. Ingestion rates based on gut pigments, however, were inadequate to meet respiratory requirements. Dietary analysis revealed that in addition to diatoms, larvae ingest protozoans and possibly detritus. If heterotrophic C is considered, larval krill feeding on sea ice biota could ingest sufficient C to support observed growth. Seasonal pack ice coverage over nursery grounds thus plays an important role in the overwintering of larval krill.