Biogeographical shifts in key Calanus species in the Arctic Ocean driven by sea-ice decline

In recent decades, the central Arctic Ocean has been experiencing dramatic decline in sea ice coverage, thickness and extent, which is expected to have a tremendous impact on all levels of Arctic marine ecosystems. Here, we analyze the regional and temporal changes in pan-Arctic distribution and phenology of the key zooplankton species Calanus glacialis in relation to recent changes in ice conditions, based on past (1993-1997) and present (2011-2016) zooplankton collections and satellite-based sea ice observations. We found that both abundance and population structure of C. glacialis are strongly related to sea ice parameters, with sea ice concentration during time of sampling, distance from ice edge and total ice covered days in the past 6 months being the most highly correlated factors. The presence of early life stages of this species, indicating recent reproduction, was observed in areas with a patchy ice cover and recent ice retreat, and was independent of depth and latitude. C. glacialis has been historically classified as a shelf species, yet we propose that this species is associated with the seasonal ice zone, rather than the shelf. As the ice continues to retreat from the central Arctic, we expect the distribution of this species to shift towards the deep basin, potentially at the expense of “resident” species C. hyperboreus, which will have a substantial impact on food web dynamics in this ecosystem.