Andrew John Constable ^1* Jess Melbourne-Thomas ² Monica M. C. Muelbert ³ Anne B Hollowed ⁴

• ¹ Centre for Marine Socioecology, Hobart, Australia
• ² CSIRO Environment, Hobart, Australia
• ³ Federal University of Rio Grande, Rio Grande, Rio Grande do Sul, Brazil
• ⁴ University of Washington, Seattle, Washington, United States

Southern Ocean ecosystems are unique, diverse, and globally valued (Stoeckl et al. 2024, Murphy et al. 2021, Roberts et al. 2021) but are also very vulnerable to climate-driven habitat changes (Swadling et al. 2023, Cavanagh et al. 2021, Pinkerton et al. 2021, Morley et al. 2020) and to other human impacts (Grant et al. 2021). Indeed, recent years have seen rapid and dramatic changes, particularly in Antarctic sea ice environments (Purich and Doddridge 2023), with implications for species and marine ecosystems. Managers of Southern Ocean ecosystems, together with national and international research agencies, need robust and regular assessments of status and change of these systems in order to protect ecosystem services, to identify options for mitigating impacts, and to understand the likelihood of different future trajectories for these systems (Constable et al. 2014;Press 2021). Regular assessments facilitate the setting and review of priority research activities to enhance future assessments and to provide science directed towards needs of policy makers. MEASO has been a bridge across many science communities (Constable et al. 2014) including: working groups of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), SCAR and its science groups, the global program of IMBeR in which ICED is the regional Antarctic program, SCOR and the SOOS, and many different groups within the IPCC and Intergovernmental Panel on Biodiversity and Ecosystem Services (IPBES). It was formally initiated in 2018 at an International Conference in Hobart, Australia which included a one-day Policy Forum for framing the policy objectives of the assessment. A planning workshop to progress the first MEASO was held in Woking in the United Kingdom in 2019. At this workshop, MEASO participants agreed: to a systematic and integrated ecological, sociological, and policy framework for considering Southern Ocean ecosystems and supporting management advice (Figure 1a); to identify ecoregions that capture variation in ecosystem attributes around the Southern Ocean including their physical and topographical differences (Figure 1b); and to use these areas and frameworks to enable assessments of dynamics and change at ecologically relevant spatial scales within those areas.MEASO areas were intended to reflect regions within which the dynamics of sea ice, ocean and benthic habitats combined remained similar across the region, from east to west. Connectivity arising from the large-scale currents and gyres makes it difficult to define more-or-less isolated marine regions. While the MEASO areas are similar to existing areas designed for particular disciplines and for management of fisheries in CCAMLR, they do not match because of the intention in MEASO to reflect ecological and ecosystem properties (i.e., across many disciplines) within an area. For this reason, they are often larger than areas designed to coordinate field research activities across nations operating in a given area, such as those adopted by the Southern Ocean Observing System (https://www. soos.aq/activities/rwg). The activities and outputs of MEASO have been guided by an International Steering Committee and supported by research support staff (see Table 1). Information and knowledge synthesis and management for MEASO was undertaken through the Southern Ocean Knowledge and Information wiki (SOKI). The aim of the MEASO Steering Committee was to not only deliver a synthesis of the science (background, general understanding, the assessment of status, change and causes, and future science priorities) but also to make it accessible to non-scientists, including summaries for policymakers. A MEASO summary for policymakers (Constable et al. 2023) was developed by members of the Steering Committee and the lead authors of the core papers as a synthesis of key findings from papers in the Research Topic, in plain language and developed for policy-and decision-makers in managing Southern Ocean ecosystems. Assessment statements in the summary for policymakers are assigned confidence levels (as per the IPCC reporting process) and crossreferenced to the original source in the Research Topic. Infographics from the original papers were adapted and presented as part of the summary for policymakers. The text of the key findings is being translated to increase accessibility (https://soos.aq/partnerships/measo-2023/measo-2023-translations; at time of publication this included Dutch, French, Portuguese and Spanish). Key messages from the MEASO Research Topic and the summary for policymakers are:• Southern Ocean ecosystems are an integral part of the Earth System, including being valued highly by people in many parts of the world. • Changes in Southern Ocean ecosystems will impact ecosystem services, including cultural services, and have impacts throughout the world's oceans and climate system, and vice versa. • Southern Ocean ecosystems are being impacted now by climate change. Moreover, sufficient evidence and tools are available for designing strategies to safeguard these ecosystems and to facilitate their resilience to future climate change and ocean acidification.• Long-term maintenance of Southern Ocean ecosystems, particularly polar-adapted Antarctic species and coastal systems, can only be achieved by urgent global action to curb climate change and ocean acidification.Meeting the challenge into the future MEASO and the outcomes of this research topic provide an integrated framework, including a spatial partition at ecologically relevant scales, for assessing long term trends of Southern Ocean ecosystems. Regular updates and syntheses of status and prognoses for the future are needed for adjusting management strategies and, particularly, for providing advice globally on the future of this important region. Thus, their regularity would best align with, at least, the assessment review cycles of the IPCC. The outcomes of this first MEASO shows that a timely second assessment could be achieved by 2028, drawing on the research from the UN Decade of the Oceans and integrating new initiatives in science, policy, management, and civil society.While knowledge of physical and chemical systems is well advanced in the Southern Ocean, studies of the implications of physical-chemical change on ecological systems needs to be substantially enhanced to have equivalent coverage across all MEASO areas and to satisfy the global demand for prognoses for ecosystems as a whole, from protists to top predators, in this era of rapidly changing environments. This can be readily achieved by investing in (i) sustained, year-round and ocean-wide scientific monitoring and assessment of the health of these biological systems, and (ii) developing coupled bio-physical, end-to-end models of Southern Ocean ecosystems suitable for assessing what future habitat changes and human impacts will mean to different ecosystems, communities and species.