The current understanding of Arctic ecosystems is deeply rooted in the classical view of a bottom-up controlled system with strong physical forcing and seasonality in primary-production regimes. Consequently, the Arctic polar night is commonly disregarded as a time of year when biological activities are reduced to a minimum due to a reduced food supply. Here, based upon a multidisciplinary ecosystem-scale study from the polar night at 79°N, we present an entirely different view. Instead of an ecosystem that has entered a resting state, we document a system with high activity levels and biological interactions across most trophic levels. In some habitats, biological diversity and presence of juvenile stages were elevated in winter months compared to the more productive and sunlit periods. Ultimately, our results suggest a different perspective regarding ecosystem function that will be of importance for future environmental management and decision making, especially at a time when Arctic regions are experiencing accelerated environmental change [1].

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Copepods of the genera Calanus and Pseudocalanus are important components of Arctic marine ecosystems. Despite the key roles of these zooplankters, little is known about the organisms they interact with most intimately, their parasites and symbionts. We applied metabarcode sequencing to uncover eukaryotic parasites present within these two copepod genera from three areas around the high Arctic archipelago of Svalbard. Ten distinct parasite groups were observed: four different Apostome ciliates, four different dinoflagellates (Chytriodinium sp., Ellobiopsis sp., Thalassomyces sp., and Hematodinium sp.), a Paradinium sp., and a trematode. Apostome ciliates closely related to Pseudocollinia spp. were the most commonly observed parasite, with overall infection rates of 21.5% in Calanus and 12.5% in Pseudocalanus. Infection by these ciliates varied seasonally, with no infections observed in early winter, but infection rates exceeding 75% in spring. Host specificity varied between parasites, with significant differences in infection rate between the two host copepod genera for four parasites (two ciliates, Chytriodinium, and a trematode). The diverse assemblage of parasites observed in these copepods, and the frequency of infection, with over one in five copepod individuals infected, suggest parasites may be playing a greater role in Arctic plankton communities than generally acknowledged.