Karen J. Heywood

The size of phytoplankton is a key metric that influences organic carbon production in the ocean and its transfer efficiency through marine food webs. There is evidence that the size structure of phytoplankton in Antarctic waters differs markedly from that in tropical oceans. However, logistical challenges have left many Antarctic regions poorly sampled. Here, we explore a dataset collected during an austral summer research cruise to the rarely sampled northwestern Weddell Sea continental shelf and open waters near the eastern Antarctic Peninsula. The dataset includes discrete measurements of size-fractionated chlorophyll-a concentration (pico- 20 μm) obtained using in vitro fluorescence and sequential size-fractionated filtration (SFF), phytoplankton pigments measured by high-performance liquid chromatography (HPLC), and in situ radiometric (ocean-colour) observations. We find broad agreement between HPLC-based (using diagnostic pigment analysis) and SFF-based estimates of total and size-fractionated chlorophyll-a, but with systematic differences for some size classes depending on the technique used. Ocean-colour chlorophyll-a algorithms developed in other regions of the Southern Ocean perform well in the northwestern Weddell Sea and outperform standard global algorithms, which systematically underestimate chlorophyll-a. SFF measurements indicate that medium-sized phytoplankton (nanophytoplankton) dominate both shelf and open waters of the northwestern Weddell Sea. We fitted a simple three-component model to the SFF data describing the partitioning of chlorophyll-a among the three size classes as a function of total chlorophyll-a. Model parameters were similar to those derived from an independent dataset from the western Antarctic Peninsula, supporting the broader dominance of nanophytoplankton around the entire Antarctic Peninsula, but differed markedly from parameters derived from lower-latitude tropical and subtropical (50° N to 50° S) Atlantic waters, where picophytoplankton dominate. Finally, we combined satellite-derived estimates of total chlorophyll-a during the cruise period with the tuned model parameters to map the spatial distribution of phytoplankton size classes across the broader northwestern Weddell Sea region, which confirmed the widespread dominance of nanophytoplankton. These datasets provide a valuable baseline for quantifying the rate and fate of organic carbon production in this rapidly changing region.

The highly productive Amundsen Sea Polynya in West Antarctica is bounded in the south by rapidly‐melting ice shelves, including the Dotson Ice Shelf (DIS). We present a high‐resolution, full‐depth pH data set within 10 km of DIS using a Lab‐On‐Chip (LOC) spectrophotometric pH sensor on an ocean glider. The LOC performs well with a 0.01 difference in pH between sensor measurements and discrete samples. Over 19 days in January–February 2022, the pH sensor captured the variability resulting from summertime biological and physical processes. At depth, modified Circumpolar Deep Water entering the DIS cavity exhibited a relatively low pH (7.91). The outflow had a higher average pH (7.94) as a result of dilution by glacial meltwater, but its pH was 0.01 lower than that of surrounding water at the same density. The Antarctic Coastal Current transported unventilated surface water with a low pH emerging from below fast ice east of Bear Island, westward to the eastern side of DIS. Net primary production increased pH in the upper 50 m in front of DIS, whilst temperature change was responsible for 5 of the change in pH. This study provides the first direct measurements of pH in the Amundsen Sea capable of deducing the impact of glacial meltwater on pH. It demonstrates the value, particularly in remote regions, of LOC pH sensors that have the longevity and stability to measure ocean pH without sensor drift.

Typically, diatoms, haptophytes, and cryptophytes dominate phytoplankton biomass and abundance in the Southern Ocean, determining bloom dynamics and carbon transfer and responding to physical factors such as sea ice concentration and mixing. However, phytoplankton community composition remains understudied in many regions of the Southern Ocean. Here, we present pigment-based phytoplankton community-composition data from a summer cruise to the northwest Weddell Sea continental shelf. Our pigment and diagnostic pigment analysis results show a predominance of fucoxanthin and 19'-hexanolyoxyfucoxanthin, i.e., diatoms and haptophytes, throughout various oceanic regimes and water column depths, consistent with community compositions from similar cold continental shelf regions. These are rare pigment-based community composition data from this key region for deep-water formation and carbon transport. Taken together, the relative biomass of the three dominant phytoplankton groups (39.7 % diatoms, 30.2 % haptophytes, and 14.9 % cryptophytes) help to explain ( R ² = 0.37–0.82) the magnitude of carbon export to 150 m. While additional measurements are needed to verify these relationships, these findings highlight the need to consider frequently overlooked phytoplankton types, alongside diatoms, in Southern Ocean models of carbon export due to the key, but differing, roles these groups play in production-export dynamics.

The size of phytoplankton is a key metric that influences organic carbon production in the ocean and its transfer efficiency through marine food webs. There is evidence that the size structure of phytoplankton in Antarctic waters differs markedly from that in tropical oceans. However, logistical challenges have left many Antarctic regions poorly sampled. Here, we explore a dataset collected during an austral summer research cruise to the rarely sampled northwestern Weddell Sea continental shelf and open waters near the eastern Antarctic Peninsula. The dataset includes discrete measurements of size-fractionated chlorophyll-a concentration (pico- 20 μm) obtained using in vitro fluorescence and sequential size-fractionated filtration (SFF), phytoplankton pigments measured by high-performance liquid chromatography (HPLC), and in situ radiometric (ocean-colour) observations. We find broad agreement between HPLC-based (using diagnostic pigment analysis) and SFF-based estimates of total and size-fractionated chlorophyll-a, but with systematic differences for some size classes depending on the technique used. Ocean-colour chlorophyll-a algorithms developed in other regions of the Southern Ocean perform well in the northwestern Weddell Sea and outperform standard global algorithms, which systematically underestimate chlorophyll-a. SFF measurements indicate that medium-sized phytoplankton (nanophytoplankton) dominate both shelf and open waters of the northwestern Weddell Sea. We fitted a simple three-component model to the SFF data describing the partitioning of chlorophyll-a among the three size classes as a function of total chlorophyll-a. Model parameters were similar to those derived from an independent dataset from the western Antarctic Peninsula, supporting the broader dominance of nanophytoplankton around the entire Antarctic Peninsula, but differed markedly from parameters derived from lower-latitude tropical and subtropical (50° N to 50° S) Atlantic waters, where picophytoplankton dominate. Finally, we combined satellite-derived estimates of total chlorophyll-a during the cruise period with the tuned model parameters to map the spatial distribution of phytoplankton size classes across the broader northwestern Weddell Sea region, which confirmed the widespread dominance of nanophytoplankton. These datasets provide a valuable baseline for quantifying the rate and fate of organic carbon production in this rapidly changing region.

The size of phytoplankton is a key metric that influences organic carbon production in the ocean and its transfer efficiency through marine food webs. There is evidence that the size structure of phytoplankton in Antarctic waters differs markedly from that in tropical oceans. However, logistical challenges have left many Antarctic regions poorly sampled. Here, we explore a dataset collected during an austral summer research cruise to the rarely sampled northwestern Weddell Sea continental shelf and open waters near the eastern Antarctic Peninsula. The dataset includes discrete measurements of size-fractionated chlorophyll-a concentration (pico- 20 μm) obtained using in vitro fluorescence and sequential size-fractionated filtration (SFF), phytoplankton pigments measured by high-performance liquid chromatography (HPLC), and in situ radiometric (ocean-colour) observations. We find broad agreement between HPLC-based (using diagnostic pigment analysis) and SFF-based estimates of total and size-fractionated chlorophyll-a, but with systematic differences for some size classes depending on the technique used. Ocean-colour chlorophyll-a algorithms developed in other regions of the Southern Ocean perform well in the northwestern Weddell Sea and outperform standard global algorithms, which systematically underestimate chlorophyll-a. SFF measurements indicate that medium-sized phytoplankton (nanophytoplankton) dominate both shelf and open waters of the northwestern Weddell Sea. We fitted a simple three-component model to the SFF data describing the partitioning of chlorophyll-a among the three size classes as a function of total chlorophyll-a. Model parameters were similar to those derived from an independent dataset from the western Antarctic Peninsula, supporting the broader dominance of nanophytoplankton around the entire Antarctic Peninsula, but differed markedly from parameters derived from lower-latitude tropical and subtropical (50° N to 50° S) Atlantic waters, where picophytoplankton dominate. Finally, we combined satellite-derived estimates of total chlorophyll-a during the cruise period with the tuned model parameters to map the spatial distribution of phytoplankton size classes across the broader northwestern Weddell Sea region, which confirmed the widespread dominance of nanophytoplankton. These datasets provide a valuable baseline for quantifying the rate and fate of organic carbon production in this rapidly changing region.

Bathymetry (ocean-bottom topography) beneath ice shelves is challenging to observe yet is vitally important for modelling how ice sheets will evolve into the future. An alternative to direct observation of bathymetry is to invert airborne gravity data for the bathymetric signal. Appropriate gravity data can be collected via remote sensing above the ice shelf and be used to provide an initial estimate of sub-ice shelf bathymetry, typically at wavelengths of ∼5 km and above. However, lateral variations in density associated with the underlying geology can distort the gravity field biassing the results. We show that techniques which tie inversion results to known bathymetry and topography, although solving some of these issues, may be insufficient in the case of large and deep basins lacking centrally located tie points. Using new direct bathymetry observations of the Dotson and Crosson ice shelves as a case study, we show that gravity inversion for bathymetry can be improved by considering and removing a model of the gravity field due to crustal isostatic compensation prior to inversion. We finally present our updated and improved bathymetric model for the Dotson-Crosson and Thwaites Glacier Ice Shelf system and discuss where our method can be best applied in future.

The size of phytoplankton is a key metric that influences organic carbon production in the ocean and its transfer efficiency through marine food webs. There is evidence that the size structure of phytoplankton in Antarctic waters differs markedly from that in tropical oceans. However, logistical challenges have left many Antarctic regions poorly sampled. Here, we explore a dataset collected during an austral summer research cruise to the rarely sampled northwestern Weddell Sea continental shelf and open waters near the eastern Antarctic Peninsula. The dataset includes discrete measurements of size-fractionated chlorophyll-a concentration (pico- 20 μm) obtained using in vitro fluorescence and sequential size-fractionated filtration (SFF), phytoplankton pigments measured by high-performance liquid chromatography (HPLC), and in situ radiometric (ocean-colour) observations. We find broad agreement between HPLC-based (using diagnostic pigment analysis) and SFF-based estimates of total and size-fractionated chlorophyll-a, but with systematic differences for some size classes depending on the technique used. Ocean-colour chlorophyll-a algorithms developed in other regions of the Southern Ocean perform well in the northwestern Weddell Sea and outperform standard global algorithms, which systematically underestimate chlorophyll-a. SFF measurements indicate that medium-sized phytoplankton (nanophytoplankton) dominate both shelf and open waters of the northwestern Weddell Sea. We fitted a simple three-component model to the SFF data describing the partitioning of chlorophyll-a among the three size classes as a function of total chlorophyll-a. Model parameters were similar to those derived from an independent dataset from the western Antarctic Peninsula, supporting the broader dominance of nanophytoplankton around the entire Antarctic Peninsula, but differed markedly from parameters derived from lower-latitude tropical and subtropical (50° N to 50° S) Atlantic waters, where picophytoplankton dominate. Finally, we combined satellite-derived estimates of total chlorophyll-a during the cruise period with the tuned model parameters to map the spatial distribution of phytoplankton size classes across the broader northwestern Weddell Sea region, which confirmed the widespread dominance of nanophytoplankton. These datasets provide a valuable baseline for quantifying the rate and fate of organic carbon production in this rapidly changing region.

Seamounts are hotspots of ocean mixing and current–topography interactions. This study investigates the circulation, tidal influence, and potential for instabilities and mixing over a seamount on the South Scotia Ridge near the tip of the Antarctic Peninsula. The seamount is 20–50 km across and rises from 1200 m to within 300 m of the surface. It is known to host a stationary anticyclonic eddy associated with the bifurcation of the Antarctic Slope Front, yet the underlying dynamics remain unclear. We analyse in situ hydrographic and velocity data from two Seaglider surveys, complemented by a barotropic tide model and satellite measurements. Horizontal velocities derived from glider drift reveal a complex interplay between topography and energetic tides of up to 0.8 m s⁻¹. De-tided residuals reveal an anticyclonic circulation of 0.1–0.3 m s⁻¹ around the seamount, with a transport of about 1.5 Sv. The observations support the presence of a stratified Taylor column and topographic steering. Diagnostics suggest that tidal rectification accounts for up to 66 % of the residual current. An upscale energy transfer from small-scale turbulence over the slope may supply the additional energy required to maintain the eddy. Strong tides over the steep topography favour internal tide generation and hydraulic control, while weak stratification and strong horizontal shear leave the flow susceptible to submesoscale instability. Stratification is reduced over the summit, where vigorous turbulent mixing counteracts frictional downwelling and supports net upwelling, creating a diapycnal chimney over the seamount. Overall, the seamount emerges as a dynamic site for localised energy dissipation and upwelling, with implications for sea ice retreat, iceberg drift, and nutrient supply to the euphotic zone. Similar conditions are expected at other tidally energetic seamounts in high latitudes, where the dominant diurnal tidal constituents are subinertial.

The size of phytoplankton is a key metric that influences organic carbon production in the ocean and its transfer efficiency through marine food webs. There is evidence that the size structure of phytoplankton in Antarctic waters differs markedly from that in tropical oceans. However, logistical challenges have left many Antarctic regions poorly sampled. Here, we explore a dataset collected during an austral summer research cruise to the rarely sampled northwestern Weddell Sea continental shelf and open waters near the eastern Antarctic Peninsula. The dataset includes discrete measurements of size-fractionated chlorophyll-a concentration (pico- 20 μm) obtained using in vitro fluorescence and sequential size-fractionated filtration (SFF), phytoplankton pigments measured by high-performance liquid chromatography (HPLC), and in situ radiometric (ocean-colour) observations. We find broad agreement between HPLC-based (using diagnostic pigment analysis) and SFF-based estimates of total and size-fractionated chlorophyll-a, but with systematic differences for some size classes depending on the technique used. Ocean-colour chlorophyll-a algorithms developed in other regions of the Southern Ocean perform well in the northwestern Weddell Sea and outperform standard global algorithms, which systematically underestimate chlorophyll-a. SFF measurements indicate that medium-sized phytoplankton (nanophytoplankton) dominate both shelf and open waters of the northwestern Weddell Sea. We fitted a simple three-component model to the SFF data describing the partitioning of chlorophyll-a among the three size classes as a function of total chlorophyll-a. Model parameters were similar to those derived from an independent dataset from the western Antarctic Peninsula, supporting the broader dominance of nanophytoplankton around the entire Antarctic Peninsula, but differed markedly from parameters derived from lower-latitude tropical and subtropical (50° N to 50° S) Atlantic waters, where picophytoplankton dominate. Finally, we combined satellite-derived estimates of total chlorophyll-a during the cruise period with the tuned model parameters to map the spatial distribution of phytoplankton size classes across the broader northwestern Weddell Sea region, which confirmed the widespread dominance of nanophytoplankton. These datasets provide a valuable baseline for quantifying the rate and fate of organic carbon production in this rapidly changing region.

Typically, diatoms, haptophytes, and cryptophytes dominate phytoplankton biomass and abundance in the Southern Ocean, determining bloom dynamics and carbon transfer and respond to physical factors such as sea ice concentration and mixing. However, phytoplankton community composition remains understudied in many regions of the Southern Ocean. Here, we present pigment-based phytoplankton community-composition data from a summer cruise to the northwest Weddell Sea continental shelf. To our knowledge, these are the only pigment-based community composition data from this key region for deep-water formation and carbon transport. Taken together, the relative biomass of the three dominant phytoplankton groups (39.7% diatoms, 30.2% haptophytes, and 14.9% cryptophytes) explain the majority (R² = 0.37 – 0.82) of carbon export to 150 m. These findings highlight the need to consider frequently overlooked phytoplankton types, alongside diatoms, in Southern Ocean models of carbon export due to the key, but differing, roles these groups play in production-export dynamics.