Abstract
AbstractThe Southern Ocean provides strong contrasts in rates and directions of change in temperature and sea ice between its sectors, but it is unknown how these affect plankton species that are distributed right around Antarctica. Here, we quantify the changing circumpolar distributions of Antarctic krill, based on the CHINARE 2013/14 circum‐Antarctic expedition, plus independent analyses of compiled abundance data (KRILLBASE: 1926–2016). In the 1920s–1930s, average krill densities in the Atlantic‐Bellingshausen sector were eight times those in the other sectors. More recently, however, the concentration factor has dropped to only about twofold. This reflects a rebalancing broadly commensurate with climatic forcing: krill densities declined in the Atlantic‐Bellingshausen sector which has warmed and lost sea ice, densities may have increased in the Ross‐Pacific sector which showed the opposite climatic trend, while densities showed no significant changes in the more stable Lazarev‐Indian sectors. Such changes would impact circumpolar food webs, so to better define these we examined circumpolar trends of isotopic values in krill and other zooplankton based on the CHINARE cruise and a literature meta‐analysis. Krill δ15N values ranged significantly between sectors from 2.21‰ (Indian) to 3.59‰ (Ross‐Pacific), about half a trophic level lower than another key euphausiid, Thysanoessa macrura. These isoscapes form a baseline for interpreting the reliance of predators on euphausiids, within the varying food webs around the continent. Overall, we suggest that the Indo‐Pacific sector has acted as a refuge for the circumpolar krill stock while conditions for them deteriorated rapidly in the Atlantic sector.
Highlights
Euphausia superba provide one exception to the general paucity of long-term, large scale plankton data at the poles. The fact that this is a key species in Southern Ocean food webs, is important in elemental cycling (Schmidt et al 2016), supports an array of predators and is commercially exploited, have all contributed to the intensive sampling of this species for over a century
Based on circumpolar data mainly assembled in the Discovery era of the 1920s and 1930s, the current paradigm is that the Atlantic sector (0–90W) supports nearly three-quarters of the global krill stock (Atkinson et al 2008)
This has led to much recent research effort within the SW Atlantic sector, but relative neglect of how this key species is faring over the rest of its circumpolar range
Summary
Euphausia superba (hereafter “krill”) provide one exception to the general paucity of long-term, large scale plankton data at the poles The fact that this is a key species in Southern Ocean food webs, is important in elemental cycling (Schmidt et al 2016), supports an array of predators and is commercially exploited, have all contributed to the intensive sampling of this species for over a century Based on circumpolar data mainly assembled in the Discovery era of the 1920s and 1930s, the current paradigm is that the Atlantic sector (0–90W) supports nearly three-quarters of the global krill stock (Atkinson et al 2008) Within this sector, the Antarctic Peninsula waters form a juxtaposition of key spawning grounds (Perry et al 2019), locally intense fishing effort (Watters et al 2020) and exceptionally fast warming (Meredith and King 2005). A further complication is that sequential overharvesting of seals, whales and fish last century has perturbed the balance of the food web, so the changes in krill and zooplankton may include readjustments to changing top down control as well as to climate (Ainley et al 2015; Murphy et al 2016)
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