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

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

The macrozooplankton and micronekton community of the Lazarev Sea (Southern Ocean) was investigated at 3 depth layers during austral summer, autumn and winter: (1) the surface layer (0–2 m); (2) the epipelagic layer (0–200 m); and (3) the deep layer (0–3000 m). Altogether, 132 species were identified. Species composition changed with depth from a euphausiid-dominated community in the surface layer, via a siphonophore-dominated community in the epipelagic layer, to a chaetognath-dominated community in the deep layer. The surface layer community predominantly changed along gradients of surface water temperature and sea ice parameters, whereas the epipelagic community mainly changed along hydrographical gradients. Although representing only 1% of the depth range of the epipelagic layer, mean per-area macrofauna densities in the surface layer ranged at 8% of corresponding epipelagic densities in summer, 6% in autumn, and 24% in winter. Seasonal shifts of these proportional densities in abundant species indicated different strategies in the use of the surface layer, including both hibernal downward and hibernal upward shift in the vertical distribution, as well as year-round surface layer use by Antarctic krill. These findings imply that the surface layer, especially when it is ice-covered, is an important functional node of the pelagic ecosystem that has been underestimated by conventional depth-integrated sampling in the past. The exposure of this key habitat to climate-driven forces most likely adds to the known susceptibility of Antarctic pelagic ecosystems to temperature rise and changing sea ice conditions

Earlier pupping in harbour seals, Phoca vitulina

The annual reproductive cycle of most seal species is characterized by a tight synchrony of births. Typically, timing of birth shows little inter-annual variation. Here, however we show that harbour seals Phoca vitulina from the Wadden Sea (southeast North Sea) have shortened their yearly cycle, moving parturition to earlier dates since the early 1970s. Between 1974 and 2009, the birth date of harbour seals shifted on average by −0.71 d yr−1, three and a half weeks (25 days) earlier, in the Dutch part of the Wadden Sea. Pup counts available for other parts of the Wadden Sea were analysed, showing a similar shift. To elucidate potential mechanism(s) for this shift in pupping phenology, possible changes in population demography, changes in maternal life-history traits and variations in environmental conditions were examined. It was deduced that the most likely mechanism was a shortening of embryonic diapause. We hypothesize that this could have been facilitated by an improved forage base, e.g. increase of small fishes, attributable to overfishing of large predator fishes and size-selective fisheries

Matrix of pairwise population differentiation values (Φ_st) between populations of <i>Montastraea cavernosa</i> at Saba Bank and in the wider Caribbean and Gulf of Mexico.

<p>Significant values (p < 0.05) are provided in bold.</p

Frequency and distribution of haplotypes in populations of <i>Montastraea cavernosa</i> (A) and <i>Xestospongia muta</i> (B) in the wider Caribbean region.

<p>Haplotype frequencies provided as pie-chart per location, number of samples in brackets. Haplotype network of ITS (A) and I3-M11 (B), baes on all sequences collected throughout the wider Caribbean region. Size of circle reflects the number of individuals with a specific haplotype. Each line represents the genetic distance between haplotypes. Specific haplotype colors match those presented in the pie-charts. White circles (ITS) are haplotypes private to a certain location.</p

Main characteristics of <i>Montastraea cavernosa</i> and <i>Xestospongia muta</i> population at the 11 Saba Bank sites.

<p>All values for densities and signs of stress are based on image analysis of 50 m² transect surveys at each site and do not correspond to the samples collected for genetic analyses. (n) number of <i>M</i>. <i>cavernosa</i> and <i>X</i>. <i>muta</i>; densities per square meter; percentage of <i>M</i>. <i>cavernosa</i> colonies with tissue loss and overgrowth; and percentage of <i>X</i>. <i>muta</i> with cyclic bleaching or Sponge Orange Band Disease (SOB [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0155969#pone.0155969.ref049" target="_blank">49</a>]). No disease was detected in any of the recorded <i>M</i>. <i>cavernosa</i> colonies.</p

Bleaching in Xestospongia muta.

<p>(A) Cyclic bleaching (blue arrows). (B) Close up of cyclic bleaching close up.</p

Sample locations on Saba Bank (SB01-SB11) and around Saba Island (SI01 and SI02).

<p>Included is a scheme of Saba Bank’s position with respect to nearby islands and in the wider Caribbean region (indicated by red square in overview).</p