| Summary: | © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Melle, W., Klevjer, T., Drinkwater, K. F., Strand, E., Naustvoll, L. J., Wiebe, P. H., Aksnes, D. L., Knutsen, T., Sundby, S., Slotte, A., Dupont, N., Salvanes, A. G. V., Korneliussen, R., & Huse, G. Structure and functioning of four North Atlantic ecosystems - a comparative study. Deep-Sea Research Part II: Topical Studies in Oceanography, 180, (2020): 104838, doi:10.1016/j.dsr2.2020.104838. The epi- and mesopelagic ecosystems of four sub-polar ocean basins, the Labrador, Irminger, Iceland and Norwegian seas, were surveyed during two legs from Bergen, Norway, to Nuuk, Greenland, and back to Bergen. The survey was conducted from 1 May to 14 June, and major results were published in five papers (Drinkwater et al., Naustvoll et al., Strand et al., Melle et al., this issue, and Klevjer et al., this issue a, this issue b). In the present paper, the structures of the ecosystem are reviewed, and aspects of the functioning of the ecosystems examined, focusing on a comparison of trophic relationships in the four basins. In many ways, the ecosystems are similar, which is not surprising since they are located at similar latitudes and share many hydrographic characteristics, like input of both warm and saline Atlantic water, as well as cold and less saline Arctic water. Literature review suggests that total annual primary production is intermediate in the eastern basins and peaks in the Labrador Sea, while the Irminger Sea is the most oligotrophic sea. This was not reflected in the measurements of different trophic levels taken during the cruise. The potential new production was estimated to be higher in the Irminger Sea than in the eastern basins, and while the biomass of mesozooplankton was similar across basins, the biomass of mesopelagic micronekton was about one order of magnitude higher in the western basins, and peaked in the Irminger Sea, where literature ...
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