Sea ice decline drives biogeographical shifts of key Calanus species in the central Arctic Ocean.

Elizaveta A Ershova,Ingrid Ellingsen,Neil S Banas,Ksenia N Kosobokova,Nicole Hildebrandt,Barbara Niehoff,Hans‐Juergen Hirche

doi:10.1111/gcb.15562

Abstract

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 life. Here, we analyze the regional and temporal changes in pan-Arctic distribution and population structure of the key zooplankton species Calanus glacialis and C. hyperboreus in relation to recent changes in ice conditions, based on historical (1993-1998) and recent (2007-2016) zooplankton collections and satellite-based sea ice observations. We found strong correlations between Calanus abundance/population structure and a number of sea ice parameters. These relationships were particularly strong for C. glacialis, with higher numbers being observed at locations with a lower ice concentration, a shorter distance to the ice edge, and more days of open water. Interestingly, early stages of C. hyperboreus followed the same trends, suggesting that these two species substantially overlap in their core distribution area in the Arctic Ocean. Calanus glacialis and C. hyperboreus have been historically classified as shelf versus basin species, yet we conclude that both species can inhabit a wide range of bottom depths and their distribution in the Arctic Ocean is largely shaped by sea ice dynamics. Our data suggest that the core distribution patterns of these key zooplankton are shifting northwards with retreating sea ice and changing climate conditions.

Highlights

Copepods of the genus Calanus commonly dominate the biomass of zooplankton in temperate and polar seas and serve as a critical link between microbial primary/secondary production and higher trophic levels, such as fish, birds and marine mammals (Dahl et al, 2003; Falk-Petersen et al, 2009)
In the Arctic Ocean, Calanus are represented by three species, two of which, C. glacialis and C. hyperboreus, are resident to the Arctic, and one, C. finmarchicus, is an expatriate advected with currents from the North Atlantic (Conover, 1988; Hirche & Kosobokova, 2007; Kosobokova, 2012; Wassmann et al, 2015)
Our results demonstrate that both species can inhabit waters with a wide range of bottom depths and that the observed trends in distribution of at least one of them, C. glacialis, is mainly driven not by bathymetry, but by sea ice dynamics

Summary

Introduction

Copepods of the genus Calanus commonly dominate the biomass of zooplankton in temperate and polar seas and serve as a critical link between microbial primary/secondary production and higher trophic levels, such as fish, birds and marine mammals (Dahl et al, 2003; Falk-Petersen et al, 2009). In the Arctic Ocean, Calanus are represented by three species, two of which, C. glacialis and C. hyperboreus, are resident to the Arctic, and one, C. finmarchicus, is an expatriate advected with currents from the North Atlantic (Conover, 1988; Hirche & Kosobokova, 2007; Kosobokova, 2012; Wassmann et al, 2015). The pronounced reduction in sea ice extent, together with increasing influence of Atlantic and Pacific waters advected into the Arctic (a process recently defined as borealization, Polyakov et al, 2020), has caused many documented northward shifts in distribution of sub-Arctic species across all trophic levels (see review in Polyakov et al, 2020). The changes in temperature and primary production patterns are affecting the phenology of organisms. The boreal C. finmarchicus reaches a later developmental stage in warmer years in the Fram Strait region through shifts in timing of reproduction and accelerated growth at higher temperatures (Weydmann et al, 2018)

Results

Discussion

Conclusion