Life in de polar oceans : the role of sea ice in the biology and ecology of marine species

Algae and other small organisms can be found living in the sea ice of the polar oceans, which in turn form a food source for animals living underneath the sea ice. In addition, the sea ice has other functions in the ecosystems, for example as a platform for reproduction or as a hiding place against predators. Knowledge on how organisms use the sea ice remains incomplete. Collecting biological samples from underneath the ice, necessary to unravel polar ecosystems and food chains, is very difficult. Such knowledge is, however, necessary for making nature protection and fisheries policies. In order to collect such samples a ‘Surface and Under Ice Trawl’ (SUIT) was developed, enabling sampling of the surface water directly underneath the ice. This fishing gear was used during this research on the importance of sea ice in the ecosystem of both the Arctic and the Antarctic. Antarctic krill are central in the ecosystem of the Southern Ocean. The winter is a harsh period for young Antarctic krill that were born in the preceding summer. This is because the young krill did not yet have the chance to build up reserves, preparing them for the lack of food during this season. The research showed that young krill living in ice-covered waters, accumulated at the under-ice surface. Here, they were feeding mainly on sea-ice algae and other organisms that were closely associated with the sea ice. Sea-ice resources are thus essential for the condition and survival of young krill living in waters that are covered with sea ice. In the Arctic ocean it is known that one- and two-year old polar cod reside closely to the sea-ice surface. Results showed that these fish also mainly feed on organisms that are closely associated with the sea ice. The research indicated that the sea ice provides sufficient resources for the polar cod to feed on, and that sea-ice decline, as a consequence of climate warming, can potentially have negative effects. Although key species such as Antarctic krill and polar cod form the core of this study, the energetic value of all species provide important knowledge for understanding the food chain of the polar oceans. Therefore, the energy density of as many species as possible was investigated, in order to understand the attractiveness of the sea-ice for sea birds, seals and whales.