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.
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We thank the captain and crew of RV Polarstern expedition WISKY (ANTXXIX-7) as well as our helicopter teams for their excellent support with work at sea, R. Schlicht for statistical consultation and B. Raymond for technical contribution to present results. This work was funded by the PACES (Polar Regions and Coasts in a changing Earth System) programme (Topic 1, WP 5) of the Helmholtz Association. Additional funds were made available via the Helmholtz Virtual Institute ‘PolarTime’ (VH-VI-500: Biological timing in a changing marine environment—clocks and rhythms in polar pelagic organisms) and the Australian Government through Antarctic Science grant #4073 and the Antarctic Climate and Ecosystem Cooperative Research Centre. S.E.T. and E.J.M. were funded by the Natural Environment Research Council under British Antarctic Survey National Capability-Ecosystems. The surface velocity data were produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes (http://www.aviso.altimetry.fr/duacs/). TerraSAR-X images used to identify sampling sites were provided by German Space Agency (DLR) via the proposal “Investigation of the role of sea ice and snow properties on Antarctic krill distribution and condition in winter/spring”. We thank T. Busche (DLR) and E. Schwarz (DLR) for organizing near-real time image delivery on board of Polarstern.
The authors declare no competing financial interests.
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Electronic supplementary material
Supplementary Figures 1–11, Supplementary Tables 1–2, Supplementary Methods.
Patchiness and behaviour of larvae under sea ice during the day in the pack-ice zone.
Patchiness and behaviour of larvae in the marginal ice zone during sunset, larvae starting to leave the ice to be dispersed in the water column.
Larval krill feeding on a horizontal ice floe (“terrace”).
Larval krill feeding on the under-side of sea ice, frozen overnight at the diving hole.
Larval krill dispersed in the water column during night in the pack-ice zone.
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Meyer, B., Freier, U., Grimm, V. et al. The winter pack-ice zone provides a sheltered but food-poor habitat for larval Antarctic krill. Nat Ecol Evol 1, 1853–1861 (2017). https://doi.org/10.1038/s41559-017-0368-3
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