Winter grazing by Calanus at the Arctic Polar Front

Mesozooplankton, especially copepods, are integral to the marine food web, with species such as Calanus finmarchicus, Calanus glacialis, and Calanus hyperboreus playing key roles in subarctic and arctic waters. The Polar Front, characterized by the confluence of Atlantic and Arctic water masses, creates unique hydrological conditions for the entire ecosystem, which are further influenced by significant seasonal changes. Winters in this region, characterized by polar night and complete darkness, traditionally induce a diapause period among Calanus, relying on lipid reserves for survival. Recent findings, however, suggest ongoing feeding activities during winter, challenging the diapause and metabolic inactivity paradigm. This study aims to characterize the distribution and winter-feeding behaviour of Calanus spp. across the Polar Front.
The distribution patterns show a majority of the Atlantic C. finmarchicus across the polar front with most of CIV and CV for Calanus spp. However, the Arctic C. glacialis is more abundant furthest north of the Front. The detection of faecal pellets across stations indicates low but ongoing feeding activity, even under polar night conditions. This includes event happening at depth but also through all water columns. It reveals interspecific and interstage differences in feeding activities and lipid content among Calanus spp., highlighting differential strategies for overwintering and plasticity in feeding events during winter. The ongoing feeding activity, despite high lipid reserves, suggests that Calanus spp. maintain a dynamic balance between diapause and active feeding, ensuring survival in winter conditions.