Stepping stones towards Antarctica: Switch to southern spawning grounds explains an abrupt range shift in krill

Summary

Antarctic krill showed an unexpected range shift of approximately 1000 kilometers southwestward during the 1990s-2010s. This occurred during a period when surface ocean warming had slowed, rather than during the preceding 70 years of rapid warming. This sudden shift was driven by changes in breeding success at different locations rather than simply following temperature changes. The Southern Annular Mode (a major climate pattern) became increasingly positive, reducing reproductive success in the traditional northern breeding grounds around the Scotia Arc while new southern breeding areas near the Western Antarctic Peninsula became established. This mechanism of sequential "stepping stone" breeding areas explains how polar species can achieve rapid range shifts that are disconnected from surface warming trends, with major implications for predicting future species distributions and planning marine protected areas.

Maps showing changing distributions of (a) krill larvae and (b) adult krill over the last 90 years. Note the different color scales, which were chosen to highlight the main population centers (bright colors) in each time period. This approach masks major changes in overall population density between eras, which are better shown in Figure 5. Dots represent sampling locations in each era. Coordinates are shown on the top left map, with regional abbreviations: MB (Marguerite Bay), SG (South Georgia), SO (South Orkney Islands).

Key Findings

Krill shifted ~1000 kilometers southwestward during a warming hiatus (1990s-2010s), not during 70 years of prior rapid warming when distribution centers remained stable despite 0.5-1.0°C ocean warming.

The Southern Annular Mode became increasingly positive, reducing larval feeding success and reproductive success in traditional Scotia Arc breeding grounds rather than direct temperature effects driving the shift.

A new southern breeding area became established along the Western Antarctic Peninsula, enabling range shifts through sequential spawning hotspots rather than continuous population movement.

The range shift occurred at ~500 kilometers per decade, which was faster than most observed migration rates for marine species.

This mechanism shows polar species can achieve rapid range shifts disconnected from surface warming trends, challenging conventional climate-based distribution models and requiring new approaches for marine protected area planning.

Abstract

Poleward range shifts are a global-scale response to warming, but these vary greatly among taxa and are hard to predict for individual species, localized regions or over shorter (years to decadal) timescales. Moving poleward might be easier in the Arctic than in the Southern Ocean, where evidence for range shifts is sparse and contradictory. Here, we compiled a database of larval Antarctic krill, Euphausia superba and, together with an adult database, it showed how their range shift is out of step with the pace of warming. During a 70-year period of rapid warming (1920s–1990s), distribution centres of both larvae and adults in the SW Atlantic sector remained fixed, despite warming by 0.5–1.0°C and losing sea ice. This was followed by a hiatus in surface warming and ice loss, yet during this period the distributions of krill life stages shifted greatly, by ~1000 km, to the south-west. Understanding the mechanism of such step changes is essential, since they herald system reorganizations that are hard to predict with current modelling approaches. We propose that the abrupt shift was driven by climatic controls acting on localized recruitment hotspots, superimposed on thermal niche conservatism. During the warming hiatus, the Southern Annular Mode index continued to become increasingly positive and, likely through reduced feeding success for larvae, this led to a precipitous decline in recruitment from the main reproduction hotspot along the southern Scotia Arc. This cut replenishment to the northern portion of the krill stock, as evidenced by declining density and swarm frequency. Concomitantly, a new, southern reproduction area developed after the 1990s, reinforcing the range shift despite the lack of surface warming. New spawning hotspots may provide the stepping stones needed for range shifts into polar regions, so planning of climate-ready marine protected areas should include these key areas of future habitat.

Published in

Global Change Biology

2021

Authors

Atkinson, A., Hill, S.L., Reiss, C.S., Pakhomov, E.A., Beaugrand, G., Tarling, G.A., Yang, G., Steinberg, D.K., Schmidt, K., Edwards, M., Rombolá, E., Perry, F.A.

Institutions

Plymouth Marine LaboratoryBritish Antarctic SurveySouth West Fisheries Science Centre, NOAA FisheriesDepartment of Earth, Ocean and Atmospheric Sciences, University of British ColumbiaHakai InstituteLaboratoire d'Océanologie et de GéosciencesKey Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of SciencesVirginia Institute of Marine ScienceSchool of Geography, Earth and Environmental Sciences, University of PlymouthInstituto Antártico ArgentinoConsejo Nacional de Investigaciones Científicas y TécnicasMarine Biological Association

Methods

DataBiological sampling Field

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