Changing circumpolar distributions and isoscapes of Antarctic krill: Indo‐Pacific habitat refuges counter long‐term degradation of the Atlantic sector

Abstract

The 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 δ 15 N 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.

Quality of Sea-ice Habitat Traversed by Antarctic Krill Larvae Influences Recruitment Success

“…Similarly, the Brans eld region may be less responsive to changes in sea-ice habitats, supporting previous suggestions that this region may provide a pocket of resilience to climate change 21 . Similar pockets of resilience may be occurring in other regions across the circumpolar range of krill 69,70 . All these considerations are highly relevant to informing strategies for the sustainable management of the krill shery in the context of climate change.…”

Future Risk for Southern Ocean Ecosystem Services Under Climate Change

“…Projections suggest that changes to the distribution and abundance of both krill and its predators are likely to be apparent by the end of the current century Piñones and Fedorov, 2016;Klein et al, 2018;Tulloch et al, 2019;Veytia et al, 2020). While there is some debate over the magnitude of recent changes in krill numerical density in the Scotia Sea region (Atkinson et al, 2004(Atkinson et al, , 2019Cox et al, 2018;Cox et al, 2019;Hill et al, 2019), evidence of climate-driven changes in productivity and distribution (Huang et al, 2011;Forcada and Hoffman, 2014;Atkinson et al, 2019;McMahon et al, 2019;Yang et al, 2020), suggest that change has already occurred. Some studies suggest that climate change may bring limited benefits to some life stages.…”

Status, Change, and Futures of Zooplankton in the Southern Ocean

“…Using data spanning 1926 to 2004, Atkinson et al (2008) estimated that 70% of the circumpolar krill stock was concentrated in the sector between 0 • and 90 • W (the southwest Atlantic); however, several independent data sources suggest that the circumpolar krill stock is now more evenly distributed with average densities in the Atlantic sector being about twice those seen in other sectors (Siegel and Watkins, 2016;Yang et al, 2020). At finer spatial scales (<1000 km), krill can be highly aggregated, with the majority of individuals occurring in large swarms (over >300 m 2 ) during summer (Murphy et al, 2004a,b;Thorpe et al, 2007;Tarling et al, 2009;Meyer et al, 2017).…”

“…In the intervening decade, understanding of the distribution, ecology, and response to change of these taxa has continued to advance concomitant with a greater diversity of research approaches. These advances have been facilitated in part by improved data availability as a result of new surveys and process studies (e.g., Meyer et al, 2017;Wallis, 2018;Wallis et al, 2019;Conroy et al, 2020;Yang et al, 2020), and extensive data rescue and compilation (e.g., Mackey et al, 2012;Atkinson et al, 2017;Tarling et al, 2018;Perry et al, 2019;Pinkerton et al, 2020;Takahashi et al, 2021). At the same time advances have been made in modelling across multiple taxa (e.g., Pinkerton et al, 2020) and of key species such as Antarctic krill (e.g., Constable and Kawaguchi, 2018;Veytia et al, 2020;Sylvester et al, 2021) and Thysanoessa macrura (e.g., Driscoll et al, 2015), and Salpa thompsoni (e.g., Henschke et al, 2018).…”