Sea-ice habitat minimizes grazing impact and predation risk for larval Antarctic krill

Carmen L David,Hauke Flores,Brian P V Hunt,Angelika Brandt,Giulia Castellani,Fokje L Schaafsma,Jan A Van Franeker,Benjamin A Lange,Evgeny A Pakhomov

doi:10.1007/s00300-021-02868-7

Abstract

Survival of larval Antarctic krill (Euphausia superba) during winter is largely dependent upon the presence of sea ice as it provides an important source of food and shelter. We hypothesized that sea ice provides additional benefits because it hosts fewer competitors and provides reduced predation risk for krill larvae than the water column. To test our hypothesis, zooplankton were sampled in the Weddell-Scotia Confluence Zone at the ice-water interface (0–2 m) and in the water column (0–500 m) during August–October 2013. Grazing by mesozooplankton, expressed as a percentage of the phytoplankton standing stock, was higher in the water column (1.97 ± 1.84%) than at the ice-water interface (0.08 ± 0.09%), due to a high abundance of pelagic copepods. Predation risk by carnivorous macrozooplankton, expressed as a percentage of the mesozooplankton standing stock, was significantly lower at the ice-water interface (0.83 ± 0.57%; main predators amphipods, siphonophores and ctenophores) than in the water column (4.72 ± 5.85%; main predators chaetognaths and medusae). These results emphasize the important role of sea ice as a suitable winter habitat for larval krill with fewer competitors and lower predation risk. These benefits should be taken into account when considering the response of Antarctic krill to projected declines in sea ice. Whether reduced sea-ice algal production may be compensated for by increased water column production remains unclear, but the shelter provided by sea ice would be significantly reduced or disappear, thus increasing the predation risk on krill larvae.

Highlights

Antarctic krill (Euphausia superba, Dana 1850, hereafter ‘krill’) is a key species in the Southern Ocean, supporting large populations of top predators and an important commercial fishery (McBride et al 2014; Krafft et al 2015)
For measurements in the water column, a represents values averaged over the 0–100 m depth layer, and b represents values integrated over the 0–250 m depth layer
At the ice-water interface the abundance of larval Antarctic krill was within the same range as the dominant herbivorous species, the ice-associated copepod S. longipes and the pelagic copepods C. propinquus and Ctenocalanus spp (David et al 2017)

Summary

Introduction

Antarctic krill (Euphausia superba, Dana 1850, hereafter ‘krill’) is a key species in the Southern Ocean, supporting large populations of top predators and an important commercial fishery (McBride et al 2014; Krafft et al 2015). The ice-water interface is considered to be a key overwintering habitat for krill (Meyer et al 2009; Flores et al 2012), providing shelter and ice algae as an important food source (Daly and Macaulay 1991; Quetin and Ross 1991; Meyer et al 2017; Schaafsma et al 2017). Other sea ice-derived resources such as protozoans, small copepods and detritus may offer an alternative food source during winter for krill larvae and other pelagic species dwelling at the ice-water interface (Daly 1990; Meyer 2012; Schmidt et al 2014; Schaafsma et al 2017)

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