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Understanding the vulnerability of marine calcifiers to ocean acidification is a critical issue, especially in the Southern Ocean (SO), which is likely to be the one of the first, and most severely affected regions. Since the industrial revolution, ~30% of anthropogenic CO 2 has been absorbed by the global oceans. Average surface seawater pH levels have already decreased by 0.1 and are projected to decline by ~0.3 by the year 2100. This process, known as ocean acidification (OA), is shallowing the saturation horizon, which is the depth below which calcium carbonate (CaCO 3 ) dissolves, likely increasing the vulnerability of many resident marine calcifiers to dissolution. The negative impact of OA may be seen first in species depositing more soluble CaCO 3 mineral phases such as aragonite and high-Mg calcite (HMC). Ocean warming could further exacerbate the effects of OA in these particular species. Here we combine a review and a quantitative meta-analysis to provide an overview of the current state of knowledge about skeletal mineralogy of major taxonomic groups of SO marine calcifiers and to make projections about how OA might affect a broad range of SO taxa. We consider a species' geographic range, skeletal mineralogy, biological traits, and potential strategies to overcome OA. The meta-analysis of studies investigating the effects of the OA on a range of biological responses such as shell state, development and growth rate illustrates that the response variation is largely dependent on mineralogical composition. Species-specific responses due to mineralogical composition indicate that taxa with calcitic, aragonitic, and HMC skeletons, could be at greater risk to expected future carbonate chemistry alterations, and low-Mg calcite (LMC) species could be mostly resilient to these changes. Environmental and biological control on the calcification process and/or Mg content in calcite, biological traits, and physiological processes are also expected to influence species-specific responses.

Intertidal estuarine habitats are inundated by seawater and uncovered with every tidal cycle, with potential exposure to both marine and atmospheric heatwaves. Little is known about the role of intertidal soft sediment ecosystems in the carbon cycle and how increasing extreme temperature events may affect carbon flux dynamics. Here we conducted a multi-day experiment simulating a low tide atmospheric heatwave at two estuary intertidal flats (sandy/muddy) to test the responses of macrobenthic biodiversity and fluxes of methane (CH 4 ) and carbon dioxide (CO 2 ). Results show heatwave simulation increases CO 2 uptake at the sandy site and causes a switch from efflux (source) to influx (sink) of CO 2 at the muddy site. Raw CH 4 fluxes are unchanged by the temperature treatment but effect sizes relative to controls are greatest in muddy sediments. We provide evidence for cumulative effects of heatwave duration on macrobenthic biodiversity and greenhouse gas fluxes and show that increasing muddiness (often associated with degradation) and increasing duration of heatwave events may change the carbon source/sink status of estuaries. Simulated heatwaves shifted carbon fluxes in estuarine flats, with stronger effects after longer heatwave durations. Findings reveal that degradation state will influence heatwave effects on carbon dynamics including changes in source/sink status.

Environmental stressors impact marine larval growth rates, quality and sizes. Larvae of the Antarctic bivalve, Laternula elliptica, were raised to the D-larvae stage under temperature and pH conditions representing ambient and end of century projections (-1.6°C to +0.4°C and pH 7.98 to 7.65). Previous observations using light microscopy suggested pH had no influence on larval abnormalities in this species. Detailed analysis of the shell using SEM showed that reduced pH is in fact a major stressor during development for this species, producing D-larvae with abnormal shapes, deformed shell edges and irregular hinges, cracked shell surfaces and even uncalcified larvae. Additionally, reduced pH increased pitting and cracking on shell surfaces. Thus, apparently normal larvae may be compromised at the ultrastructural level and these larvae would be in poor condition at settlement, reducing juvenile recruitment and overall survival. Elevated temperatures increased prodissoconch II sizes. However, the overall impacts on larval shell quality and integrity with concurrent ocean acidification would likely overshadow any beneficial results from warmer temperatures, limiting populations of this prevalent Antarctic species.

Transformational climate adaptation requires coordinated action across sectors and scales, enabled by governance arrangements that can address systemic barriers. Public-private partnerships (PPPs) may provide such a mechanism by aligning incentives, pooling resources and fostering collaboration between government and industry. In this paper, we evaluate a national PPP, The Aotearoa Circle (The Circle), and how it has catalysed climate adaptation in Aotearoa New Zealand’s seafood sector. Commissioned by The Circle, we conducted adaptation pathways planning, a decision-support process that identifies and sequences adaptation strategies and actions to reduce future climate risk, for two fisheries and two aquaculture industries. The process engaged company, government, research and civil society stakeholders to design suites of incremental and transformational strategies to reach long-term industry goals, and actions to implement them. We analysed these outputs to assess the PPP’s relevance in enabling implementation. Amongst the transformational pathways, 48% of actions required collaboration by public and private entities. Strategies were also identified to address systemic adaptation barriers across the sector (agile regulation, climate forecasting, fleet decarbonisation, oceans research). For these, the majority of actions required leadership by The Circle or public-private collaborations. Key informant interviews revealed challenges in implementation of immediate actions. While some were underway, shifts in national government policies had resulted in uneven progress amongst the industries. Interviewees emphasised the necessity for The Circle to convene stakeholders and sustain momentum, but resourcing was a constraint. Our results highlight The Circle’s core role in enabling transformational adaptation within industries and across the sector. Yet the PPP remains vulnerable to political cycles, variable public sector commitment and risk aversion. Strengthening PPP resourcing arrangements is necessary to action long-term adaptation, and to ensure ongoing, iterative pathways planning.

Global models predict that Antarctica has little suitable habitat for macroalgae and that Antarctic macroalgae therefore make a negligible contribution to global carbon fixation. However, coastal surveys are rare at southern polar latitudes (beyond 71° S), and here we report diverse and abundant macroalgal assemblages in un-navigated coastal habitats of the Ross Sea from 71.5°–74.5° S. We found extensive macroalgal assemblages living at depths >70 m and specimens of crustose coralline algae as deep as 125 m. Using global light modelling and published photosynthetic rates we estimate that Antarctic macroalgae may contribute between 0.9–2.8 % of global macroalgal carbon fixation. Combined, this suggests that Antarctic macroalgae may be a greater contributor to global carbon fixation and possibly sequestration than previously thought. The vulnerability of these coastal environments to climate change, especially shifting sea ice extent and persistence, could influence Southern Ocean carbon fixation and rates of long-term sequestration.

High Antarctic coastal marine environments are comparatively pristine with strong environmental gradients, which make them important places to investigate biodiversity relationships. Defining how different environmental features contribute to shifts in beta-diversity is especially important as these shifts reflect both spatio-temporal variations in species richness and the degree of ecological separation between local and regional species pools. We used complementary techniques (species accumulation models, multivariate variance partitioning and generalized linear models) to assess how the roles of productivity, bio-physical habitat heterogeneity and connectivity change with spatial scales from metres to 100's of km. Our results demonstrated that the relative importance of specific processes influencing species accumulation and beta-diversity changed with increasing spatial scale, and that patterns were never driven by only one factor. Bio-physical habitat heterogeneity had a strong influence on beta-diversity at scales <290 km, while the effects of productivity were low and significant only at scales >40 km. Our analysis supports the emphasis on the analysis of diversity relationships across multiple spatial scales and highlights the unequal connectivity of individual sites to the regional species pool. This has important implications for resilience to habitat loss and community homogenisation, especially for Antarctic benthic communities where rates of recovery from disturbance are slow, there is a high ratio of poor-dispersing and brooding species, and high biogenic habitat heterogeneity and spatio-temporal variability in primary production make the system vulnerable to disturbance. Consequently, large areas need to be included within marine protected areas for effective management and conservation of these special ecosystems in the face of increasing anthropogenic disturbance.

Antarctic sea ice and the cold waters surrounding the continent are key elements of the global climate system, influencing heat redistribution, oceanic circulation and the absorption of carbon dioxide from the atmosphere. However, the Southern Ocean is predicted to warm by 1—6°C over the next century, altering sea ice extent, thickness and permanence. To better understand the connections between coastal sea ice conditions and the functioning of Antarctica's unique marine benthic ecosystems, we performed manipulative experiments on the seafloor at two southwestern Ross Sea sites with contrasting sea ice conditions. Benthic systems at both study sites were net heterotrophic during the study period (early November), with primary production most likely limited by light availability rather than nutrients. There was five times more fresh algal detrital material in benthic sediments at the site with the thinner, snow-free, annually formed sea ice, relative to the site with thicker, multiyear sea ice. This elevated quantity and quality of algal detrital matter corresponded with a significantly greater rate of sediment oxygen utilization by the benthos and an altered pathway of nitrogen regeneration (tighter coupling between nitrification and denitrification). Large benthic animals (brittle stars, Ophionotus victoriae) enhanced the efflux of dissolved inorganic nutrients from the sediment to the water column and played a greater role in nutrient regeneration at the site with more food. Although changes in sea ice characteristics in the Western Ross Sea are difficult to predict at present, large benthic organisms can be expected to have an expanded role in mediating the effects of elevated coastal productivity and detritus supply on ecosystem dynamics in this part of Antarctica.

Quantifying the structural complexity provided by biogenic habitat structures is important in ecology, conservation and management, and yet remains a challenging task, particularly in deep sea and polar environments, that current photogrammetry tools can alleviate. In this study, we demonstrate how small remotely operated vehicles and compact underwater GoPro® action cameras can be easily integrated into coastal Antarctic surveys to quantify structural complexity of under‐ice benthos via underwater photogrammetry. Forty‐four pairs of 1 m2 quadrats at 1 cm resolution, each comprising an orthomosaic and three‐dimensional reconstructions, were analyzed to describe relationships between benthic cover and structural complexity metrics. The study case provided insights into a unique biogenic habitat, highlighting the role of integrating structural complexity metrics in Antarctic benthic surveys. Although no clear relationships between structural complexity and biodiversity were found, high cover of live reef‐building polychaetes was associated with higher levels of structural complexity, particularly fractal dimension (D). Further, broken biogenic structures, product of disturbance events retain habitat structural complexity known to be associated with larvae settlement and biogenic reef growth. This suggests that D can be used as a metric for detecting subtle changes in biogenic structural complexity. We build from available open‐source code, a reproducible scientific workflow that is expected to facilitate the acquisition and analysis of structural complexity metrics. The workflow presented aims to encourage and accelerate the use of photogrammetry tools for benthic studies aiming to quantify biogenic structural complexity across depths and latitudes. La complejidad estructural provista por organismos bénticos es un elemento importante, pero difícil de cuantificar, en ecología, conservación y manejo de recursos marinos. El desarrollo de herramientas como la fotogrametría digital ha demostrado múltiples ventajas para el monitoreo de arrecifes someros y tropicales. Sin embargo, las aplicaciones en el océano profundo y latitudes polares son escasas. En este estudio, demostramos como vehículos submarinos operados remotamente y cámaras de acción GoPro® pueden ser fácilmente integradas a estudios costeros y polares. Se analizaron 44 pares de cuadrantes de 1 m2 a 1 cm por píxel de resolución, cada par comprendiendo un ortomosaico y un modelo tri‐dimensional (3D), para describir relaciones entre cobertura béntica y métricas de complejidad estructural. Nuestro caso de estudio provee perspectivas únicas a un arrecife biogénico único, resaltando el rol de integrar métricas de complejidad estructural en los esfuerzos de monitoreo Antárticas. Aunque no encontramos relaciones claras con biodiversidad epi‐béntica, alta cobertura de poliquetos formadores de arrecifes estuvo asociada a un incremento en métricas de complejidad estructural, particularmente dimensión fractal (D). No obstante, demostramos como escombros de estructuras biogénicas, producto de disturbios naturales, preservan complejidad estructural que incrementa el sustrato disponible para asentamiento y ha sido asociada con el incremento en la extensión de arrecifes de poliquetos. Nuestros resultados sugieren que D puede ser utilizada como una métrica para estudiar y detectar cambios en estructuras biogénicas. En este trabajo construimos, a partir de código de acceso abierto, un flujo de trabajo científico reproducible que puede facilitar la adquisición y análisis de métricas de complejidad estructural. Este estudio abre posibilidades para futuras investigaciones sobre la complejidad provista por múltiples estructuras biogénicas y organismos formadores de hábitats en diferentes latitudes y profundidades. Quantifying the structural complexity provided by biogenic habitat structures, particularly in deep sea and polar environments, is important in ecology, conservation and management, but yet remains a challenging task that current photogrammetry tools can alleviate. In this study, we demonstrate how remotely operated vehicles and compact underwater GoPro® action cameras can be easily integrated into coastal Antarctic surveys to quantify structural complexity of under‐ice benthos via underwater photogrammetry. Forty‐four pairs of 1 m2 quadrats at 1 cm resolution, each comprising an orthomosaic and a three‐dimensional model, were analyzed, respectively, for benthic cover and structural complexity metrics. We build from available open‐source code a reproducible scientific workflow that is expected to facilitate, encourage and accelerate the use of photogrammetry tools to quantify biogenic structural complexity across depths and latitudes.

The Ross Sea, Antarctica, is amongst the least human-impacted marine environments, and the site of the world’s largest Marine Protected Area. We present research on two components of the Ross Sea benthic fauna: mega-epifauna, and macro-infauna, sampled using video and multicore, respectively, on the continental shelf and in previously unsampled habitats on the northern continental slope and abyssal plain. We describe physical habitat characteristics and community composition, in terms of faunal diversity, abundance, and functional traits, and compare similarities within and between habitats. We also examine relationships between faunal distributions and ice cover and productivity, using summaries of satellite-derived data over the decade prior to our sampling. Clear differences in seafloor characteristics and communities were noted between environments. Seafloor substrates were more diverse on the Slope and Abyss, while taxa were generally more diverse on the Shelf. Mega-epifauna were predominantly suspension feeders across the Shelf and Slope, with deposit feeder-grazers found in higher or equal abundances in the Abyss. In contrast, suspension feeders were the least common macro-infaunal feeding type on the Shelf and Slope. Concordance between the mega-epifauna and macro-infauna data suggests that non-destructive video sampling of mega-epifauna can be used to indicate likely composition of macro-infauna, at larger spatial scales, at least. Primary productivity, seabed organic flux, and sea ice concentrations, and their variability over time, were important structuring factors for both community types. This illustrates the importance of better understanding bentho-pelagic coupling and incorporating this in biogeographic and process-distribution models, to enable meaningful predictions of how these ecosystems may be impacted by projected environmental changes. This study has enhanced our understanding of the distributions and functions of seabed habitats and fauna inside and outside the Ross Sea MPA boundaries, expanding the baseline dataset against which the success of the MPA, as well as variability and change in benthic communities can be evaluated longer term.

Several forms of calcifying scleractinian corals provide important habitat complexity in the deep-sea and are consistently associated with a high biodiversity of fish and other invertebrates. How these corals may respond to the future predicted environmental conditions of ocean acidification is poorly understood, but any detrimental effects on these marine calcifiers will have wider impacts on the ecosystem. Colonies of Solenosmilia variabilis , a protected deep-sea coral commonly occurring throughout the New Zealand region, were collected during a cruise in March 2014 from the Louisville Seamount Chain. Over a 12-month period, samples were maintained in temperature controlled (∼3.5 °C) continuous flow-through tanks at a seawater pH that reflects the region’s current conditions (7.88) and an end-of-century scenario (7.65). Impacts on coral growth and the intensity of colour saturation (as a proxy for the coenenchyme tissue that covers the coral exoskeleton and links the coral polyps) were measured bimonthly. In addition, respiration rate was measured after a mid-term (six months) and long-term (12 months) exposure period. Growth rates were highly variable, ranging from 0.53 to 3.068 mm year −1 and showed no detectable difference between the treatment and control colonies. Respiration rates also varied independently of pH and ranged from 0.065 to 1.756 µmol O 2 g protein −1 h −1 . A significant change in colour was observed in the treatment group over time, indicating a loss of coenenchyme. This loss was greatest after 10 months at 5.28% and could indicate a reallocation of energy with physiological processes (e.g.  growth and respiration) being maintained at the expense of coenenchyme production. This research illustrates important first steps to assessing and understanding the sensitivity of deep-sea corals to ocean acidification.