Opportunistic feeding by Antarctic krill in the eastern Weddell Gyre

Abstract

Antarctic krill, Euphausia superba , is an important species in the Southern Ocean and is vulnerable to climate change. We studied the carbon sources for krill—specifically pelagic and ice‐associated (sympagic) particulate organic matter (POM), representing phytoplankton and sea‐ice algae, respectively—during March 2019 across three distinct areas of the Kong Håkon VII Sea (KHS) in the eastern Weddell Gyre. Analyses of δ 13 C and δ 15 N composition revealed large geographic differences in pelagic POM that were reflected in krill across life stages. At Astrid Ridge (the eastern study area), elevated pelagic POM δ 13 C suggested that sea‐ice algae seeding influenced phytoplankton development, in line with the more extensive sea‐ice cover in this area and with the dominance of typically ice‐associated pennate diatoms. This was also reflected in the elevated krill δ 13 C compositions at Astrid Ridge as compared to the other two areas. In contrast, in the western areas (Maud Rise and 6° East), lower δ 13 C values in both pelagic POM and krill suggested limited influence of sea‐ice algae on phytoplankton communities and krill feeding. Moreover, no significant difference in the δ 13 C and δ 15 N compositions was found among krill genders and life stages throughout the study area. These geographical, rather than ontogenetic, differences suggest that krill of all life stages feed opportunistically on the most available algal sources in early autumn in the KHS. Considering the ongoing reduction in the seasonal sea‐ice cover, continued research on the dietary plasticity of krill is essential to understand the resilience and health of krill stocks in a changing Southern Ocean.

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