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

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

<i>Euphausia superba</i>.

<p>Comparison of the size distributions of postlarval krill in the surface layer and the 0–200 m layer in summer (2007/2008), autumn (2004) and winter (2006). <i>p</i>: Kolmogorov-Smirnov test significance.</p

<i>Thysanoessa macrura</i>.

<p>Density of postlarval krill [ind. m⁻²] at the open surface and under ice (0–2) m in summer (2007/2008), autumn (2004) and winter (2006). Mean values significantly different from each other (ANOVA <i>p</i><0.05) were printed in bold. ice = under-ice SUIT hauls; ow = open water SUIT hauls; <i>n</i> = number of samples.</p

Under-ice versus open water comparison of geometric mean densities of postlarval <i>Thysanoessa macrura</i> in summer (2007/2008).

<p>(<b>A</b>) <i>Thysanoessa macrura</i> from the 0–2 m layer, and (<b>B</b>) from the 0–200 m layer. Error bars denote value ranges. Bold red bars indicate 25% to 75% percentile ranges. ice = under-ice SUIT hauls; ow = open water SUIT hauls.</p

<i>Euphausia superba</i>.

<p>Day/night comparisons of the surface layer density of (<b>A</b>) postlarval krill at three locations in summer (2007/2008), (<b>B</b>) postlarval krill, and (<b>C</b>) furcilia larvae at two locations in winter (2006). Scaling of y-axis differs. Denotations above bars are location codes. d = daytime, night = night-time; ice = under-ice, ow = open water SUIT hauls.</p

<i>Euphausia superba</i>.

<p>Density of postlarval krill and furcilia larvae [ind. m⁻²] at the open surface and under ice (0–2 m) in summer (2007/2008), autumn (2004) and winter (2006). Geometric mean values significantly different from each other (ANOVA <i>p</i><0.05) were printed in bold. ice = under-ice SUIT hauls; ow = open water SUIT hauls; <i>n</i> = number of samples.</p

Under-ice versus open water comparison of geometric mean densities of postlarval <i>Euphausia superba</i> in (A–B) summer (2007/2008), (C–D) autumn (2004), and (E–F) winter (2006).

<p>(<b>A, C, E</b>) <i>Euphausia superba</i> from the 0–2 m layer, and (<b>B, D, F</b>) from the 0–200 m layer. Error bars denote value ranges. Bold red bars indicate 25% to 75% percentile ranges. ice = under-ice, ow = open water SUIT hauls.</p

<i>Euphausia superba</i>.

<p>Generalized Additive Models of the density of postlarval krill. Effect of additive smoothing functions of (<b>A</b>) proportional ice coverage during SUIT hauls, and (<b>B</b>) mixed layer depth in summer (2007/2008), (<b>C</b>) salinity in the 0–200 m depth layer in autumn (2004), and (<b>D</b>) temperature in the mixed layer in winter (2006) on the fitted density of postlarval krill. Dashed lines show 95% confidence intervals of smoothers.</p