The intriguing co-distribution of the copepods Calanus hyperboreus and Calanus glacialis in the subsurface chlorophyll maximum of Arctic seas

Herbivorous zooplankton channel the energy from microalgal primary producers to primary carnivores and play many roles in the oceanic carbon cycle (Hobson et al., 2002; Wassmann et al., 2006; Darnis and Fortier, 2012). In Arctic seas, copepods dominate the zooplankton with the primarily herbivorous genus Calanus accounting for up to 80% of mesozooplankton biomass (Søreide et al., 2008; Darnis and Fortier, 2012). Among several adaptations to the extreme seasonality of the arctic pelagic ecosystem, C. hyperboreus and C. glacialis extract and accumulate large lipid reserves from ice microalgae and phytoplankton during a short grazing season in late spring and summer (Falk-Petersen et al., 2007; Søreide et al., 2010; Wassmann, 2011; Daase et al., 2013). By making the energy captured during the short microalgal bloom available to higher trophic levels over the rest of the year, this build-up of lipid reserves by copepods is a pivotal process in arctic ecosystems (e.g., Falk-Petersen et al., 2009). By feeding fish, marine mammals and seabirds, copepod lipids underpin much of the ecosystem services provided to northern communities (Darnis et al., 2012). Like many arctic zooplankton species, Calanus hyperboreus and C. glacialis complete a seasonal vertical migration (SVM) from their autumn-winter position at depth towards the productive surface layer in spring and summer (Dawson, 1978; Hirche, 1997; Ashjian et al., 2003; Darnis and Fortier, 2014). In ice-covered waters and in open waters during the period of day-night succession in spring and autumn, limited diel vertical migrations (DVM) superimpose on the SVM of the two species (e.g., Fortier et RESEARCH ARTICLE