Antarctic krill is a key species of important Southern Ocean food webs, yet how changes in ocean temperature and primary production may impact their habitat quality remains poorly understood. We provide a circumpolar assessment of the robustness of krill growth habitat to climate change by coupling an empirical krill growth model with projections from a weighted subset of IPCC Earth System Models. We find that 85% of the study area experienced only a moderate change in Relative Gross Growth Potential (RGGP) (± 20%) by 2100. However, a temporal shift in seasonal timings of habitat quality may cause disjunctions between krill's biological timings and the future environment. Regions likely to experience habitat quality decline or retreat are concentrated near the northern limits of krill distribution and in the Amundsen-Bellingshausen Sea region during autumn, meaning habitat will likely shift to higher latitudes in these areas.Antarctic krill (Euphausia superba, hereafter krill) are an ecologically 1-4 and commercially 5,6 important species in the Southern Ocean, and yet qualitative reviews hypothesize that krill may be vulnerable to projected oceanic warming as stenothermic crustaceans [7][8][9] . Previous projections have suggested that ocean warming will cause favourable krill habitat to contract 10,11 , resulting in possible declines in abundance and/or biomass. Currently, oceanic warming is manifesting more rapidly in regions of the Southern Ocean than the global average 12,13 , but its effect on observed krill abundance and distribution is a topic of debate [14][15][16][17][18] . Predicting the population response of krill to climate change, and the ecological impacts of these changes, is therefore important to conservation efforts and the management of the krill fishery 8 . Projecting future changes in krill habitat requires knowledge of environmental drivers of habitat quality. Existing empirically-based models describe krill growth as a function of two major factors: temperature and food concentration (typically phytoplankton approximated by primary production (PP) and/or chlorophyll-a) [19][20][21] . Thus, accurate estimates of these variables are required.Recent advances in computing capacities now allow climate models to support the reasonable representation of the carbon cycle within fully coupled Earth System Models (ESMs) 22 . However, it remains difficult for modelled PP fields to reproduce current observations with a resolution and accuracy sufficient for application to biological models 23,24 .Existing studies modelling future changes in krill habitat have so far explored future PP by manipulating (i.e. % increase/decrease) satellite chlorophyll-a observations. For example, projected changes in sea surface temperature (SST) and sea ice resulted in changes in spawning habitat ranging from +51 to -83%, depending on changes in summer chlorophyll-a of +25 to -50% 25 .Increases in SST also resulted in mid-latitude (~55-65⁰S) declines in adult krill habitat quality, and potentially biomass 10,11 ...

In the Southern Ocean, several zooplankton taxonomic groups, euphausiids, copepods, salps and pteropods, are notable because of their biomass and abundance and their roles in maintaining food webs and ecosystem structure and function, including the provision of globally important ecosystem services. These groups are consumers of microbes, primary and secondary producers, and are prey for fishes, cephalopods, seabirds, and marine mammals. In providing the link between microbes, primary production, and higher trophic levels these taxa influence energy flows, biological production and biomass, biogeochemical cycles, carbon flux and food web interactions thereby modulating the structure and functioning of ecosystems. Additionally, Antarctic krill (Euphausia superba) and various fish species are harvested by international fisheries. Global and local drivers of change are expected to affect the dynamics of key zooplankton species, which may have potentially profound and wide-ranging implications for Southern Ocean ecosystems and the services they provide. Here we assess the current understanding of the dominant metazoan zooplankton within the Southern Ocean, including Antarctic krill and other key euphausiid, copepod, salp and pteropod species. We provide a systematic overview of observed and potential future responses of these taxa to a changing Southern Ocean and the functional relationships by which drivers may impact them. To support future ecosystem assessments and conservation and management strategies, we also identify priorities for Southern Ocean zooplankton research.

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