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Abstract High‐resolution glider sampling in the southwestern Ross Sea revealed an extensive phytoplankton bloom in austral summer 2022–2023 that persisted for over one month and extended through the upper 100 m of the water column. The temporal mean euphotic‐zone chlorophyll concentration was 20.38.5 , six to nine times higher than average summer Ross Sea concentrations. The bloom was likely initially dominated byPhaeocystis, favored over diatoms due to low light and high iron availability. Our observations are consistent with an ice‐edge bloom likely fueled by iron supply and enhanced stratification from late sea‐ice melt during an anomalously high ice‐covered summer. Photoacclimation to particularly low light conditions might have enhanced Chl‐a fluorescence. In the Ross Sea, the most productive region in the Southern Ocean, understanding the drivers of this extreme bloom is crucial for predicting potential impacts of the changing climate on primary production rates and carbon sequestration. 

Abstract. Three biogeochemical glider surveys in the Ross Sea between 2010 and 2023 were combined and analysed to assess production–export stock and rate dynamics. As the most productive of any Antarctic continental shelf, the Ross Sea is a site of substantial physical and biogeochemical interest. While this region and its annual bloom have been characterised for decades, logistical constraints, such as ship time and sea ice cover, have prevented a comprehensive understanding of this region over long (> 1–2 months) timescales and at high spatiotemporal resolution. Here, we use high-resolution datasets from autonomous gliders in mass balance equations to calculate short-term (days to weeks) net community production via oxygen concentration, change in particulate organic carbon (POC) concentration over time, and POC export potential during the period of peak primary production in the region (November–February). Our results show an overall decoupling of net community production (NCP), driven by biologic changes in oxygen, from overall biomass concentration as well as changes in POC over time. NCP and carbon change vary between seasons and appear related to changes in ice concentration and stratification. Substantial spatiotemporal variability exists in all datasets, but high-resolution sampling reveals short-term variations that are likely masked in other studies. Our study reinforces the need for high-resolution sampling and supports previous classifications of the Ross Sea as a high-productivity (average NCP range −0.7 to 0.2 g C m−2 d−1), low-export (average changes in POC over time range −0.1 to 0.1 g C m−2 d−1) system during the productive austral spring and sheds additional light on the mechanisms controlling these processes. 

Solar-warmed surface waters subduct beneath Antarctica’s ice shelves as a result of wind forcing, but this process is poorly observed and its interannual variability is yet to be assessed. We observe a 50-meter-thick intrusion of warm surface water immediately beneath the Ross Ice Shelf. Temperature in the uppermost 5 meters decreases toward the ice base in near-perfect agreement with an exponential fit, consistent with the loss of heat to the overlying ice. Ekman forcing drives a heat transport into the cavity sufficient to contribute considerably to near-front melting; this transport has increased over the past four decades, driven by the increasing heat content of the ice-front polynya. Interannual variability of the heat transport is driven by zonal wind stress. These results provide a benchmark against which model performance may be assessed as we seek to reduce uncertainty around the contribution of basal melting to sea level rise.