Importance of deep mixing and silicic acid in regulating phytoplankton biomass and community in the iron-limited Antarctic Polar Front region in summer

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Controls of primary production in two phytoplankton blooms in the Antarctic Circumpolar Current

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Ocean acidification and calcium carbonate saturation states in the coastal zone of the West Antarctic Peninsula

The polar oceans are particularly vulnerable to ocean acidification; the lowering of seawater pH and carbonate mineral saturation states due to uptake of atmospheric carbon dioxide (CO2). High spatial variability in surface water pH and saturation states (Ω) for two biologically-important calcium carbonate minerals calcite and aragonite was observed in Ryder Bay, in the coastal sea-ice zone of the West Antarctic Peninsula. Glacial meltwater and melting sea ice stratified the water column and facilitated the development of large phytoplankton blooms and subsequent strong uptake of atmospheric CO2 of up to 55 mmol m-2 day-1 during austral summer. Concurrent high pH (8.48) and calcium carbonate mineral supersaturation (Ωaragonite ~3.1) occurred in the meltwater-influenced surface ocean. Biologically-induced increases in calcium carbonate mineral saturation states counteracted any effects of carbonate ion dilution. Accumulation of CO2 through remineralisation of additional organic matter from productive coastal waters lowered the pH (7.84) and caused deep-water corrosivity (Ωaragonite ~0.9) in regions impacted by Circumpolar Deep Water. Episodic mixing events enabled CO2-rich subsurface water to become entrained into the surface and eroded seasonal stratification to lower surface water pH (8.21) and saturation states (Ωaragonite ~1.8) relative to all surface waters across Ryder Bay. Uptake of atmospheric CO2 of 28 mmol m-2 day-1 in regions of vertical mixing may enhance the susceptibility of the surface layer to future ocean acidification in dynamic coastal environments. Spatially-resolved studies are essential to elucidate the natural variability in carbonate chemistry in order to better understand and predict carbon cycling and the response of marine organisms to future ocean acidification in the Antarctic coastal zone

The influence of tides on the marine carbonate chemistry of a coastal polynya in the south-eastern Weddell Sea

Tides significantly affect polar coastlines by modulating ice shelf melt and modifying shelf water properties through transport and mixing. However, the effect of tides on the marine carbonate chemistry in such regions, especially around Antarctica, remains largely unexplored. We address this topic with two case studies in a coastal polynya in the south-eastern Weddell Sea, neighbouring the Ekström Ice Shelf. The case studies were conducted in January 2015 (PS89) and January 2019 (PS117), capturing semi-diurnal oscillations in the water column. These are pronounced in both physical and biogeochemical variables for PS89. During rising tide, advection of sea ice meltwater from the north-east created a fresher, warmer, and more deeply mixed water column with lower dissolved inorganic carbon (DIC) and total alkalinity (TA) content. During ebbing tide, water from underneath the ice shelf decreased the polynya's temperature, increased the DIC and TA content, and created a more stratified water column. The variability during the PS117 case study was much smaller, as it had less sea ice meltwater input during rising tide and was better mixed with sub-ice shelf water. The contrasts in the variability between the two case studies could be wind and sea ice driven, and they underline the complexity and highly dynamic nature of the system. The variability in the polynya induced by the tides results in an air–sea CO2 flux that can range between a strong sink (−24 mmol m−2 d−1) and a small source (3 mmol m−2 d−1) on a semi-diurnal timescale. If the variability induced by tides is not taken into account, there is a potential risk of overestimating the polynya's CO2 uptake by 67 % or underestimating it by 73 %, compared to the average flux determined over several days. Depending on the timing of limited sampling, the polynya may appear to be a source or a sink of CO2. Given the disproportionate influence of polynyas on heat and carbon exchange in polar oceans, we recommend future studies around the Antarctic and Arctic coastlines to consider the timing of tidal currents in their sampling strategies and analyses. This will help constrain variability in oceanographic measurements and avoid potential biases in our understanding of these highly complex systems

Dissolved inorganic carbon and total alkalinity of seawater samples from the Weddell Sea for RV POLARSTERN expedition PS117

Discrete seawater samples for the determination of dissolved inorganic carbon (DIC) and total alkalinity (TA) were collected from CTD stations during RV POLARSTERN expedition PS117, between 15 December 2018 and 7 February 2019. Seawater samples were collected from stations that used the AWI-operated CTD, as well as the Ultra-Clean-CTD, operated by NIOZ. DIC and TA were measured using coulometric titration and potentiometric titration, respectively, on a VINDTA 3C system. Nutrients were measured with UV-Vis spectrophotometry and a continuous gas-segmented flow auto-analyser. Data for station 41 have previously been published on Pangaea and are excluded from this dataset (https://doi.org/10.1594/PANGAEA.946363). Bottle data (including nutrients) from the AWI CTD and Ultra-Clean-CTD stations have already been published and are stored on Pangaea under https://doi.org/10.1594/PANGAEA.910673 and https://doi.org/10.1594/PANGAEA.940209, respectively. Data quality flags follow the WOCE quality code definitions for water sample measurements. Details on sample collection and analysis methods for DIC and TA can be found in Droste et al. (2022)

Dissolved inorganic carbon and total alkalinity of seawater samples from a Weddell Sea coastal polynya during two tidal observation case studies for RV POLARSTERN expeditions PS89 and PS117

Discrete seawater samples in a Weddell Sea coastal polynya along the Ekström Ice Shelf were collected from two sets of repeat CTD casts, capturing tidal variability in the water column. One set was collected during RV POLARSTERN expedition PS89, between 8 and 11 January 2015. The second set was collected during RV POLARSTERN expedition PS117, between 11 and 12 January 2019. Dissolved inorganic carbon (DIC) and total alkalinity (TA) were measured using coulometric titration and potentiometric titration, respectively, on a VINDTA 3C system. DIC and TA have been normalised to salinity: nDIC and nTA. Nutrients were measured with UV-Vis spectrophotometry and a continuous gas-segmented flow auto-analyser