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A dominant Antarctic ecological paradigm suggests that winter sea ice is generally the main feeding ground for krill larvae. Observations from our winter cruise to the southwest Atlantic sector of the Southern Ocean contradict this view and present the first evidence that the pack-ice zone is a food-poor habitat for larval development. In contrast, the more open marginal ice zone provides a more favourable food environment for high larval krill growth rates. We found that complex under-ice habitats are, however, vital for larval krill when water column productivity is limited by light, by providing structures that offer protection from predators and to collect organic material released from the ice. The larvae feed on this sparse ice-associated food during the day. After sunset, they migrate into the water below the ice (upper 20 m) and drift away from the ice areas where they have previously fed. Model analyses indicate that this behaviour increases both food uptake in a patchy food environment and the likelihood of overwinter transport to areas where feeding conditions are more favourable in spring.

A dominant Antarctic ecological paradigm suggests that winter sea ice is generally the main feeding ground for krill larvae. Observations from our winter cruise to the southwest Atlantic sector of the Southern Ocean contradict this view and present the first evidence that the pack-ice zone is a food-poor habitat for larval development. In contrast, the more open marginal ice zone provides a more favourable food environment for high larval krill growth rates. We found that complex under-ice habitats are, however, vital for larval krill when water column productivity is limited by light, by providing structures that offer protec- tion from predators and to collect organic material released from the ice. The larvae feed on this sparse ice-associated food during the day. After sunset, they migrate into the water below the ice (upper 20 m) and drift away from the ice areas where they have previously fed. Model analyses indicate that this behaviour increases both food uptake in a patchy food environment and the likelihood of overwinter transport to areas where feeding conditions are more favourable in spring.

A dominant Antarctic ecological paradigm suggests that winter sea ice is generally the main feeding ground for krill larvae. Observations from our winter cruise to the southwest Atlantic sector of the Southern Ocean contradict this view and present the first evidence that the pack-ice zone is a food-poor habitat for larval development. In contrast, the more open marginal ice zone provides a more favourable food environment for high larval krill growth rates. We found that complex under-ice habitats are, however, vital for larval krill when water column productivity is limited by light, by providing structures that offer protec- tion from predators and to collect organic material released from the ice. The larvae feed on this sparse ice-associated food during the day. After sunset, they migrate into the water below the ice (upper 20 m) and drift away from the ice areas where they have previously fed. Model analyses indicate that this behaviour increases both food uptake in a patchy food environment and the likelihood of overwinter transport to areas where feeding conditions are more favourable in spring.

A dominant Antarctic ecological paradigm suggests that winter sea ice is generally the main feeding ground for krill larvae. Observations from our winter cruise to the southwest Atlantic sector of the Southern Ocean contradict this view and present the first evidence that the pack-ice zone is a food-poor habitat for larval development. In contrast, the more open marginal ice zone provides a more favourable food environment for high larval krill growth rates. We found that complex under-ice habitats are, however, vital for larval krill when water column productivity is limited by light, by providing structures that offer protection from predators and to collect organic material released from the ice. The larvae feed on this sparse ice-associated food during the day. After sunset, they migrate into the water below the ice (upper 20 m) and drift away from the ice areas where they have previously fed. Model analyses indicate that this behaviour increases both food uptake in a patchy food environment and the likelihood of overwinter transport to areas where feeding conditions are more favourable in spring.

A successful larval overwintering is a major factor determining population sizes of Antarctic krill, Euphausia superba (hereafter krill). A high population recruitment success of krill was linked to years with more extensive sea ice in the previous winter. However, the benefit of the winter sea ice cover for a successful larval development during winter remains unclear. The aim of the present study was to test the hypothesis that larval krill in pack ice regions are in better condition in terms of food supply and feeding activity than larvae from OW regions. Therefore, the condition of larval krill (furcilia (F) III–VI as well as juveniles) was investigated in open water (OW), the marginal ice zone (MIZ1+2) and the pack ice region (Ice Camp1+2) during late winter, in relation to Chlorophyll a (Chl a) and particulate organic carbon (POC) concentrations in the water column and in the sea ice. In addition, differences in the dietary intake of larval krill during the day and night were examined. Overall, krill larvae from pack ice regions were not in better condition than in OW. Although, krill larvae caught at Ice Camp1 had the largest body lengths (mean 15.69 mm) and highest dry weights (mean 4.59 mg), only the body lengths showed significant differences with larval krill from OW and stage composition was similar (few F6 and mostly juveniles). Larval krill of Ice Camp2 had the smallest body lengths (mean 7.96 mm) and dry weights (mean 0.50 mg) while stage composition ranged from F3-F6. The high amount of available food sources within the sea ice (Ice Camp1: 21.78 μg L-1 Chl a and 400.55 μg L-1 POC; Ice Camp2: 12.68 μg L-1 Chl a and 330.2 μg L-1 POC) was used only to a small extent by krill larvae, which is reflected by their lowest feeding activities and relatively empty stomachs. At Ice Camp2, larval krill was observed performing diel vertical migration (DVM). During the day larval krill was closely associated with the sea ice, whereas during the night they descended