Ice Algae-Produced Carbon Is Critical for Overwintering of Antarctic Krill Euphausia superba

Abstract

Antarctic krill Euphausia superba ("krill") constitute a fundamental food source for Antarctic seabirds and mammals, and a globally important fisheries resource. The future resilience of krill to climate change depends critically on the winter survival of young krill. To survive periods of extremely low production by pelagic algae during winter, krill are assumed to rely partly on carbon produced by ice algae. The true dependency on ice algae-produced carbon, however, is so far unquantified. This confounds predictions on the future resilience of krill stocks to sea ice decline. Fatty acid (FA) analysis, bulk stable isotope analysis (BSIA), and compound-specific stable isotope analysis (CSIA) of diatom-and dinoflagellate-associated marker FAs were applied to quantify the dependency of overwintering larval, juvenile, and adult krill on ice algae-produced carbon (α Ice ) during winter 2013 in the Weddell-Scotia Confluence Zone. Our results demonstrate that the majority of the carbon uptake of the overwintering larval and juvenile krill originated from ice algae (up to 88% of the carbon budget), and that the dependency on ice algal carbon decreased with ontogeny, reaching <56% of the carbon budget in adults. Spatio-temporal variability in the utilization of ice algal carbon was more pronounced in larvae and juvenile krill than in adults. Differences between α Ice estimates derived from short-vs. long-term FA-specific isotopic compositions suggested that ice algae-produced carbon gained importance as the winter progressed, and might become critical at the late winter-spring transition, before the phytoplankton bloom commences. Where the sea ice season shortens, reduced availability of ice algae might possibly not be compensated by surplus phytoplankton production during wintertime. Hence, sea ice decline could seriously endanger the winter survival of recruits, and subsequently overall biomass of krill.

Spatio-temporal variability in the winter diet of larval and juvenile Antarctic krill, Euphausia superba, in ice-covered waters

“…Samples of particulate organic matter (POM) from surface water and sea ice were collected to provide a baseline for BSIA. Measurements confirmed that the δ 13 C values were significantly higher in sea ice-derived carbon compared to carbon derived from the water column (t-test, t = 5.2, df = 10.9, p < 0.01; Kohlbach et al 2017). There also was no overlap between the δ 13 C values of sea-ice POM and water column phytoplankton POM, confirming that these can be recognized as distinct carbon sources (Kohlbach et al 2017).…”

“…This was confirmed by the δ 13 C values, which suggested that the AC0 krill had continuous access to sea ice-associated food sources (Kohlbach et al 2017).…”

“…The relatively low δ 15 N value suggests that AC0 krill from cohort 3 were feeding predominantly herbivorously in the past, while the other cohorts were feeding more omnivorously. This was based on the mostly low δ 15 N values in sea-ice and pelagic POM (Kohlbach et al 2017). Results show that compensating a lack of sea-ice resources with heterotrophic pelagic food sources, as seemed to be the case during 1 year in East Antarctica (Jia et al 2016), is not a general pattern in the Southern Ocean during winter.…”

“…This FA metabolizes rapidly, and is found to decrease when not replaced by new dietary input (Stübing et al 2003). The relatively large amount of the dinoflagellate-associated marker FA 22:6n-3 in the krill of cohort 3 could be a result of their relative high proportion of the phosphatidylcholine compared to other cohorts (Kohlbach et al 2017). However, FA 20:5n-3, also usually incorporated in PC, was lowest in the krill of cohort 3, strongly indicating that AC0 krill from cohort 3 had fed more extensively on dinoflagellates in the more distant past compared to the other cohort groups.…”

Review: the energetic value of zooplankton and nekton species of the Southern Ocean

“…The lipid content of E. superba and T. macrura showed a high local variability in several studies (Pond et al 1995; Hagen et al 1996; Mayzaud et al 1998; Färber-Lorda et al 2009a; Färber-Lorda and Mayzaud 2010; Ruck et al 2014; Kohlbach et al 2017). In E. superba lipid, but also protein content, was found to be highly variable within a single population during several seasons, and the variety within a season can be greater than between seasons (Torres et al 1994; Mayzaud et al 1998; Ruck et al 2014).…”