The Association of Antarctic Krill Euphausia superba with the Under-Ice Habitat

Abstract

The association of Antarctic krill Euphausia superba with the under-ice habitat was investigated in the Lazarev Sea (Southern Ocean) during austral summer, autumn and winter. Data were obtained using novel Surface and Under Ice Trawls (SUIT), which sampled the 0–2 m surface layer both under sea ice and in open water. Average surface layer densities ranged between 0.8 individuals m−2 in summer and autumn, and 2.7 individuals m−2 in winter. In summer, under-ice densities of Antarctic krill were significantly higher than in open waters. In autumn, the opposite pattern was observed. Under winter sea ice, densities were often low, but repeatedly far exceeded summer and autumn maxima. Statistical models showed that during summer high densities of Antarctic krill in the 0–2 m layer were associated with high ice coverage and shallow mixed layer depths, among other factors. In autumn and winter, density was related to hydrographical parameters. Average under-ice densities from the 0–2 m layer were higher than corresponding values from the 0–200 m layer collected with Rectangular Midwater Trawls (RMT) in summer. In winter, under-ice densities far surpassed maximum 0–200 m densities on several occasions. This indicates that the importance of the ice-water interface layer may be under-estimated by the pelagic nets and sonars commonly used to estimate the population size of Antarctic krill for management purposes, due to their limited ability to sample this habitat. Our results provide evidence for an almost year-round association of Antarctic krill with the under-ice habitat, hundreds of kilometres into the ice-covered area of the Lazarev Sea. Local concentrations of postlarval Antarctic krill under winter sea ice suggest that sea ice biota are important for their winter survival. These findings emphasise the susceptibility of an ecological key species to changing sea ice habitats, suggesting potential ramifications on Antarctic ecosystems induced by climate change.

Seasonal changes in the vertical distribution and community structure of Antarctic macrozooplankton and micronekton

“…Also in ice-free water, under-representation of the surface community can occur, because the RMT samples the upper 10-15 m in the wake of the ship, where surface water is displaced by the forward moving vessel and propeller mixing. In contrast, the SUIT samples the surface layer away from the ship in comparatively undisturbed water (Flores et al, 2012). To date, no obligate neuston species have been described south of the Antarctic Polar Front.…”

“…2). This difference in community structure can in large parts be attributed to an overwhelming dominance of euphausiids in the surface layer, most likely caused by a close association of Antarctic krill with the underside of sea ice (Flores et al, 2012;Marschall, 1988;. Besides Antarctic krill, several other species are positively or negatively associated with sea ice, which thus acts as a key factor in structuring the surface layer community in the Lazarev Sea (Flores et al, 2011).…”

“…During this period, the bulk of primary production occurs within sea ice rather than in the water column (Arrigo and Thomas, 2004;Lizotte, 2001;McMinn et al, 2010), making the ice-water interface layer a potentially important site of energy transmission between sea ice biota and the pelagic food web. Indeed, the ice-water interface layer has been reported to be at least a seasonal key habitat for Antarctic krill (Daly, 1990;Flores et al, 2012;Marschall, 1988), and some copepod species (Hoshiai et al, 1987;Schnack-Schiel et al, 2008). However, the significance of this habitat for other macrofauna species remains largely unknown.…”

“…These studies, however, were either limited to one depth stratum (Flores et al, 2011;Hunt et al, 2011), or focused on few taxa (Flores et al, 2012;Kruse et al, 2010b;Meyer et al, 2010;Siegel, 2012). Earlier studies included only one season (Boysen-Ennen and Piatkowski, 1988;Efremenko, 1991;Pakhomov et al, 1994).…”

Advection in polar and sub-polar environments: Impacts on high latitude marine ecosystems

“…Regional decreases in krill stocks since the 1970s are thought to be associated with the cumulative effects of sea-ice reduction in the Scotia Sea and west of the Antarctic Peninsula (Atkinson et al, 2004;Flores et al, 2012;Saba et al, 2014). In the East Antarctic, variation in the distribution of krill south of the Southern Boundary of the Antarctic Circumpolar Current has been linked to fluctuations in regional ocean circulation and seaice conditions (Nicol et al, 2000).…”