The subantarctic zone is critical for anthropogenic carbon dioxide (CO2) uptake. Yet ocean biogeochemical models struggle to reproduce the observed summer drawdown of CO2 across this vast region, indicating that our mechanistic understanding of the carbon cycle remains incomplete. Poor representation of phytoplankton blooms, zooplankton grazing, sinking particles, and/or bacterial heterotrophy are all potential explanations. Here, we explore these processes at the Southern Ocean Time Series (SOTS) site using WOMBAT-mid, a one-dimensional ocean biogeochemical model. WOMBAT-mid was optimized using a surrogate machine-learning workflow to reproduce the seasonal cycle of chlorophyll, nutrients and air–sea CO2 flux at SOTS. The optimization revealed which parameters, and therefore which ecological and biogeochemical processes, exerted the strongest control on simulated CO2 uptake. Reproducing the observed CO2 seasonality required adjustments to phytoplankton bloom timing, zooplankton grazing, detrital sinking, iron resupply and microbial-loop efficiency, indicating that no single process can, in isolation, resolve the seasonality of the subantarctic carbon cycle. However, the strongest control involved heterotrophic bacteria - specifically, the extent to which bacteria partially oxidize the organic matter they consume. This suggests that subantarctic CO2 drawdown is strongly affected by the microbial pathways that determine when organic carbon is returned to CO2.

Cryogenic gypsum (CaSO₄·2H₂O) is an elusive mineral precipitating in sea ice brines, increasingly hypothesised as a possible ballasting agent for polar phytoplankton, released during ice melt and tentatively linked to enhanced vertical carbon export via the biological pump. Yet its abundance, distribution, and ecological role remain poorly constrained, hampered by complex brine precipitation dynamics and laborious detection methods. While the Arctic picture is only beginning to take shape, no comparable assessment yet exists for Antarctic sea ice, where the vast seasonal pulse of ice formation and melt could exert a substantial, unaccounted-for influence on Southern Ocean carbon export.

Here, we present the theoretical foundations of cryogenic mineral formation, alongside their proposed ballasting role, together with our first results aimed at detecting these minerals optically. Drawing inspiration from planetary remote sensing, we bridge scales down to the micrometer by adapting shortwave-infrared hyperspectral imaging, paired with autonomous, field-deployable computing, to reveal gypsum in extracted ice core sections and filtered melt samples. We report preliminary findings from controlled precipitation experiments in the laboratory and from ice cores analysed shipboard, laying the groundwork for rapid, non-destructive cryogenic mineral detection in a rapidly transforming Antarctic sea-ice.

Phytoplankton primary producers and zooplankton grazers form a network that is critical to the Southern Ocean ecosystem: they sustain higher trophic levels, support commercial fisheries, and drive the biogeochemical cycling of carbon and nutrients, including trace metals. However, despite their importance, the response of carbon and trace metal cycling by Southern Ocean phytoplankton and zooplankton to ocean warming across different nutrient regimes remains poorly quantified. We addressed this knowledge gap under controlled laboratory conditions using two study organisms: the Southern Ocean diatom Chaetoceros flexuosus and Antarctic krill (Euphausia superba), exposed to three warming scenarios (0.5, 3, and 6 °C) and two nutrient regimes (iron-limited and iron-replete). First, we characterised the mechanisms that control C. flexuosus and Antarctic krill carbon and trace metal content under varying temperature and iron availability, using Antarctic krill proteomics and C. flexuosus growth rate as physiological indicators. Second, we determined the impact of warming and iron availability on the release of carbon and trace metals during Antarctic krill grazing on C. flexuosus. These experiments shed light on the mechanisms underpinning the roles of phytoplankton and zooplankton in carbon and nutrient cycling in the present-day Southern Ocean, and how these roles may change under future climate scenarios.

Climate change is projected to reduce global NPP in the future with cascading effects for marine ecosystems, carbon export and fisheries. A key part of this link is mediated by the response of marine zooplankton, who transfer resources to higher trophic levels or recycle them for re-use by primary producers. Zooplankton Fe:C stoichiometry varies considerably across species and regions. Yet its influence on projections of NPP, iron and carbon cycling under future climate change remains unknown. Here, we use the PISCES-v2 ocean biogeochemical model to quantify this sensitivity through a control and two perturbation experiments conducted under RCP8.5 high emissions scenario. We find that the control model projects a global decline in NPP and grazing, which is in line with prior work. Raising zooplankton Fe:C ratios dampens the climate-driven decline in NPP by 20-42% and zooplankton grazing by 15-42%. Higher zooplankton Fe:C ratios also alter both the mean state and climate response of biological Fe cycling, reducing Fe recycling between 13-22% at baseline but reversing its climate response from a decline to an increase. Iron export increases between 15-23% at baseline, but declines less under warming. These results suggest that altering the zooplankton Fe:C stoichiometry is an important but overlooked source of uncertainty in projections of global NPP under climate change.

Antarctic krill (Euphausia superba) embryos develop while sinking from surface spawning areas down to 1000m depth, a non-feeding period that represents a critical bottleneck for recruitment. As the Southern Ocean warms and absorbs increasing CO₂, embryos may face higher energetic demands before hatching, with potential consequences for residual energy, larval condition, and survival. We tested how temperature and pCO₂ affect the costs of early development by rearing embryos across three temperatures: 0.5, 1.5 and 2.5 °C, and three pCO₂ levels: 400, 600 and 800 ppm. Using advances in microrespirometry, we measured oxygen consumption of individual embryos across four developmental stages: early gastrula, late gastrula, limb bud and nauplius I, alongside development time and hatching success. Our results indicate that temperature is the dominant driver of embryo performance: metabolic rates increased across ontogeny and with warming, development time decreased, and hatching success was highest at 0.5 °C. By contrast, CO₂ effects were weaker, more variable, and often stage- or temperature-dependent. Overall, mild warming appears to accelerate development but increase total energetic expenditure, potentially reducing energy reserves available upon hatching.

The Shackleton Ice Shelf and Denman Glacier represent two contrasting ice-edge environments along East Antarctica. The Shackleton region is influenced by extensive seasonal sea-ice retreat and polynya activity, while the Denman system is fed by one of Antarctica’s deepest glaciers, where meltwater outflow and fast-ice cover shape the surface ocean. These differing physical settings are expected to drive distinct patterns of biological productivity. During the Denman Marine Voyage (March–May 2025) aboard RSV Nuyina, zooplankton samples were collected from both regions using the ship’s Wet Well intake system. Zooplankton community composition and size structure have been analysed. Preliminary results showed that samples from the west of the Shackleton Ice Shelf contain a higher presence of larger zooplankton, including lipid-rich calanoid and cyclopoid copepods, whereas the east near Denman Glacier is dominated by appendicularians and smaller cyclopoid copepods. The Shackleton region further exhibits a productive but strongly size-structured zooplankton community. Concurrent oceanographic (e.g., fluorescence, turbidity, salinity, temperature) and biogeochemical (e.g., iron concentration) data will be used to investigate environmental controls on surface zooplankton production. This study aims to identify key drivers of surface zooplankton dynamics across contrasting ice-edge systems and improve predictions of zooplankton community responses to ongoing climate change in East Antarctic.