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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.

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.

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.

Ocean surface temperatures and the frequency and intensity of marine heatwaves are increasing worldwide. Understanding how marine organisms respond and adapt to heat pulses and the rapidly changing climate is crucial for predicting responses of valued species and ecosystems to global warming. Here, we carried out an in situ experiment to investigate sublethal responses to heat spikes of a functionally important intertidal bivalve, the venerid clam Austrovenus stutchburyi . We describe changes in metabolic responses under two warming scenarios (five days and seven days) at two sites (muddy and sandy). Tidal flat warming during every low tide for five days affected the abundance of multiple functional metabolites within this species. The metabolic response was related to pathways such as metabolic energetics, amino acid and lipid metabolism, and accumulation of stress-related metabolites. There was some recovery after cooler weather during the final two days of the experiment. The degree of change was greater in muddy versus sandy sediments. Our findings provide new evidence of the metabolomic response of these important bivalve to heat stress, which could be used for resource managers when implementing strategies to mitigate the impacts of climate change on valuable marine resources.

We report striking discoveries of numerous seafloor seeps of climate-reactive fluid and gases in the coastal Ross Sea, indicating this process may be a common phenomenon in the region. We establish the recent emergence of many of these seep features, based on their discovery in areas routinely surveyed for decades with no previous seep presence. Additionally, we highlight impacts to the local benthic ecosystem correlated to seep presence and discuss potential broader implications. With these discoveries, our understanding of Antarctic seafloor seeps shifts from them being rare phenomenon to seemingly widespread, and an important question is raised about the driver of seep emergence in the region. While the origin and underlying mechanisms of these emerging seep systems remains unknown, similar processes in the paleo-record and the Arctic have been attributed to climate-driven cryospheric change. Such a mechanism may be widespread around the Antarctic Continent, with concerning positive feedbacks that are currently undetermined. Future, internationally coordinated research is required to uncover the causative mechanisms of the seep emergence reported here and reveal potential sensitivities to contemporary climate change and implications for surrounding ecosystems. Researchers reveal widespread, newly formed seafloor seeps along Antarctica’s Ross Sea coast. Methane-rich flows alter local ecosystems and may influence warming. The drivers remain unknown, warranting coordinated study.

Little is known of the reproductive traits and dispersal potential of many deep-sea corals, and in-aquarium spawning has been observed for very few species globally. Here, we document the first known observation of larval release by Goniocorella dumosa (Alcock 1902), a habitat-forming deep-sea scleractinian stony coral found in the New Zealand region. In contrast to previous understanding that G . dumosa were broadcast spawners, colonies of G . dumosa released large (approx. 1.1 mm × 0.8 mm) free-swimming planula larvae. Further investigation confirmed that this species is a brooder, with up to 10 mature larvae found in single mature polyps, and is the only known brooding deep-sea scleractinian branching coral that produces swimming larvae. Mature corals were collected from the Chatham Rise (400 m depth, 43º 22.13 S, 179° 27.09 E), to the east of New Zealand, in June 2020. We describe the observed larval behaviour, settlement and post-larval growth and development of G . dumosa , held in an aquarium, from September to December 2020. The more limited dispersal potential for larvae from a brooding species compared to a broadcast spawning coral has significant implications for both population connectivity and for the potential recovery of this species from disturbance by human activities. This in turn could influence management and protection strategies for G. dumosa and their habitat.

Marine ecosystem monitoring requires observations of its attributes at different spatial and temporal scales that traditional sampling methods (e.g., RGB imaging, sediment cores) struggle to efficiently provide. Proximal optical sensing methods can fill this observational gap by providing observations of, and tracking changes in, the functional features of marine ecosystems non-invasively. Underwater hyperspectral imaging (UHI) employed in proximity to the seafloor has shown a further potential to monitor pigmentation in benthic and sympagic phototrophic organisms at small spatial scales (mm–cm) and for the identification of minerals and taxa through their finely resolved spectral signatures. Despite the increasing number of studies applying UHI, a review of its applications, capabilities, and challenges for seafloor ecosystem research is overdue. In this review, we first detail how the limited band availability inherent to standard underwater cameras has led to a data analysis “bottleneck” in seafloor ecosystem research, in part due to the widespread implementation of underwater imaging platforms (e.g., remotely operated vehicles, time-lapse stations, towed cameras) that can acquire large image datasets. We discuss how hyperspectral technology brings unique opportunities to address the known limitations of RGB cameras for surveying marine environments. The review concludes by comparing how different studies harness the capacities of hyperspectral imaging, the types of methods required to validate observations, and the current challenges for accurate and replicable UHI research.

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.

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.

Elevated atmospheric CO 2 concentrations are contributing to ocean acidification (reduced seawater pH and carbonate concentrations), with potentially major ramifications for marine ecosystems and their functioning. Using a novel in situ experiment we examined impacts of reduced seawater pH on Antarctic sea ice-associated microalgal communities, key primary producers and contributors to food webs. pH levels projected for the following decades-to-end of century (7.86, 7.75, 7.61), and ambient levels (7.99), were maintained for 15 d in under-ice incubation chambers. Light, temperature and dissolved oxygen within the chambers were logged to track diurnal variation, with pH, O 2 , salinity and nutrients assessed daily. Uptake of CO 2 occurred in all treatments, with pH levels significantly elevated in the two extreme treatments. At the lowest pH, despite the utilisation of CO 2 by the productive microalgae, pH did not return to ambient levels and carbonate saturation states remained low; a potential concern for organisms utilising this under-ice habitat. However, microalgal community biomass and composition were not significantly affected and only modest productivity increases were noted, suggesting subtle or slightly positive effects on under-ice algae. This in situ information enables assessment of the influence of future ocean acidification on under-ice community characteristics in a key coastal Antarctic habitat.