ALBERT

Description: Physical, chemical and biogeochemical measurements derived from CTD-rosette deployments during three visits to site P3 (November to December, 2017) in the South Atlantic. Measurements were made during COMICS cruise DY086 on the RRS Discovery using a trace metal free Titanium Rosette (events 4, 7, 15, 19, 24, 26, 29) and a Stainless Steel Rosette (all other events). Physical parameters include temperature, salinity, density, photosynthetically active radiation and turbulence; chemical parameters include dissolved oxygen, dissolved oxygen saturation, nitrate, phosphate and silicate; biogeochemical parameters include turbidity, beam transmittance, beam attenuation, fluorescence, particulate organic carbon (POC), dissolved organic carbon (DOC), chlorophyll-a, net primary productivity (NPP), ambient leucine assimilation and bacterial cell count. To determine turbulence, a downward facing lowered acoustic doppler current profiler (LADCP, Teledyne Workhorse Monitor 300 kHz ADCP) was attached to the CTD frame. Shear and strain, which are obtained from velocity and density measurements, were used to estimate the dissipation rate of turbulent kinetic energy and the diapycnal eddy diffusivity from a fine-scale parameterisation. Estimates are calculated by parameterising internal wave-wave interactions and assuming that wave breaking modulates turbulent mixing. A detailed description of the method for calculating diffusivity from LADCP and CTD can be found in Kunze et al. (2006). Two datasets with different vertical resolutions were produced: one in which the shear is integrated from 150 to 300 m and the strain over 20-150 m, and one in which the shear is integrated from 70 to 200 m and the strain over 30-200 m. Nutrients (nitrate, phosphate, silicate) were determined via colourimetric analysis (see cruise report, Giering and Sanders, 2019), POC was determined as described in Giering et al. (2023), DOC and DOC flux were determined as described in Lovecchio et al. (2023), NPP was determined as described in Poulton et al. (2019), and ambient leucine assimilation and bacterial cell count were determined as described in Rayne et al. (2024). Bacterial abundance and leucine assimilation were made from bottle samples of six CTD casts of the stainless-steel rosette. Water was collected at six depths (6 m, deep-chlorophyll maximum, mixed layer depth + 10, 100, 250 and 500 m). Acid-cleaned HDPE carboys and tubing were used for sampling. Samples were then stored in the dark and at in-situ temperature prior to on-board laboratory sample preparation or analysis. Flow cytometry was used to measure bacterial abundance. Room temperature paraformaldehyde was used to fix 1.6 ml samples for 30 minutes. Then, using liquid nitrogen, the samples were flash frozen and stored at -80°C. Samples were then defrosted before being stained using SYBR Green I and run through the flow cytometer (BD FACSort™). The method of Hill et al. (2013) was applied to determine prokaryotic leucine assimilation using L-[4,5-³H] leucine which has a specific activity of 89.3 Ci/mmol­. In the mixed and upper layers of the water column, the protocol in Zubkov et al. (2007) was followed. Below the mixed layer, adaptions to the method included reducing the concentration of ³H-Leucine to 0.005, 0.01, 0.025, 0.04 and 0.05 nM; increasing experimental volumes to 30 ml; enhancing incubation times to 30, 60, 90 and 120 min. These adaptions were made to improve accuracy where lower rates of leucine assimilation were expected. Data were provided by the British Oceanographic Data Centre and funded by the National Environment Research Council.

Description: Dataset used in Henson et al. (2023). Binned, calibrated, merged glider data collected with 2 Slocum G2s and 1 Seaglider from 0 to 1000 m depth. The data were collected NW of South Georgia, Scotia Sea between 19 October 2017 - 13 February 2018. Gliders sampled continuously following a triangular path of ~12 km per side centred on 52.75°S, 40.16°W. The glider sampled with a vertical resolution of ~20 cm, emerging 5 to 6 times per day. Temperature, Conductivity and Depth were measured with a Seabird CTD (pumped) on Slocum gliders and a pumped Seabird CTD sail on the Seaglider. Dissolved oxygen was measured with Aanderaa optodes. Chlorophyll fluorescence and optical backscatter were measured using Seabird Triplet ECOPucks. Oxygen and chlorophyll data were calibrated against shipboard bottle samples collected using a CTD rosette. Chlorophyll data have been corrected for non-photochemical quenching. Optical backscatter was additionally converted into POC concentration using shipboard CTD bottle samples, and estimates of POC flux were then made following the methods detailed in Henson et al. (2023). Data from the 3 gliders were inter-calibrated prior to ship calibration. Estimates of primary production are made from the glider data and photosynthetic properties observed during a coincident cruise, following the methods of Mignot et al. (2018). The 2D glider profile data are interpolated into 1 m depth bins for all variables; data are provided for every profile.

Description: Discrete measurements of particulate organic carbon (POC) concentration and flux were made on the RRS Discovery during COMICS cruise DY086 at site P3 in the South Atlantic from November to December, 2017 (Giering et al. 2023). Data is from a variety of equipment including marine snow catchers, neutrally-buoyant sediment traps (PELAGRA) and a stand-alone pump system. Marine snow catchers settled on-deck for 2 hours. Slow sinking particles were collected from the base and fast sinking particles were collected from the tray. These data were used along with bottle POC data to calibrate glider backscatter data from the GOCART project.

Description: The Atlantic sector of the Southern Ocean is characterized by markedly different frontal zones with specific seasonal and sub-seasonal dynamics. Demonstrated here is the effect of iron on the potential maximum productivity rates of the phytoplankton community. A series of iron addition productivity versus irradiance (PE) experiments utilizing a unique experimental design that allowed for 24 h incubations were performed within the austral summer of 2015/16 to determine the photosynthetic parameters αB, PBmax and Ek. Mean values for each photosynthetic parameter under iron-replete conditions were 1.46 ± 0.55 (µg (µg Chl a)−1 h−1 (µM photons m−2 s−1)−1) for αB, 72.55 ± 27.97 (µg (µg Chl a)−1 h−1) for PBmax and 50.84 ± 11.89 (µM photons m−2 s−1) for Ek, whereas mean values under the control conditions were 1.25 ± 0.92 (µg (µg Chl a)−1 h−1 (µM photons m−2 s−1)−1) for αB, 62.44 ± 36.96 (µg (µg Chl a)−1 h−1) for PBmax and 55.81 ± 19.60 (µM photons m−2 s−1) for Ek. There were no clear spatial patterns in either the absolute values or the absolute differences between the treatments at the experimental locations. When these parameters are integrated into a standard depth-integrated primary production model across a latitudinal transect, the effect of iron addition shows higher levels of primary production south of 50° S, with very little difference observed in the subantarctic and polar frontal zone. These results emphasize the need for better parameterization of photosynthetic parameters in biogeochemical models around sensitivities in their response to iron supply. Future biogeochemical models will need to consider the combined and individual effects of iron and light to better resolve the natural background in primary production and predict its response under a changing climate.

Description: The seasonal and sub-seasonal dynamics of iron availability within the sub-Antarctic zone (SAZ; ∼40–45∘ S) play an important role in the distribution, biomass and productivity of the phytoplankton community. The variability in iron availability is due to an interplay between winter entrainment, diapycnal diffusion, storm-driven entrainment, atmospheric deposition, iron scavenging and iron recycling processes. Biological observations utilizing grow-out iron addition incubation experiments were performed at different stages of the seasonal cycle within the SAZ to determine whether iron availability at the time of sampling was sufficient to meet biological demands at different times of the growing season. Here we demonstrate that at the beginning of the growing season, there is sufficient iron to meet the demands of the phytoplankton community, but that as the growing season develops the mean iron concentrations in the mixed layer decrease and are insufficient to meet biological demand. Phytoplankton increase their photosynthetic efficiency and net growth rates following iron addition from midsummer to late summer, with no differences determined during early summer, suggestive of seasonal iron depletion and an insufficient resupply of iron to meet biological demand. The result of this is residual macronutrients at the end of the growing season and the prevalence of the high-nutrient low-chlorophyll (HNLC) condition. We conclude that despite the prolonged growing season characteristic of the SAZ, which can extend into late summer/early autumn, results nonetheless suggest that iron supply mechanisms are insufficient to maintain potential maximal growth and productivity throughout the season.

Description: The Atlantic sector of the Southern Ocean is characterized by markedly different frontal zones with specific seasonal and sub-seasonal dynamics. Demonstrated here is the effect of iron on the potential maximum productivity rates of the phytoplankton community. A series of iron addition productivity versus irradiance (PE) experiments utilising a unique experimental design that allowed for 24 hour incubations were performed within the austral summer of 2015/16. The addition of iron can result in the doubling of the photosynthetic parameters αB and PBmax, with subsequent changes in Ek. Mean values for each parameter under iron replete conditions were 1.46 ± 0.55 (μg (μg Chl a)−1 h−1 (μM photons m−2 s−1)−1), 72.55 ± 27.97 (μg (μg Chl a)−1 h−1) and 50.84 ± 11.89 (μM photons m−2 s−1); whereas mean values under the control conditions were 1.25 ± 0.92 (μg (μg Chl a)−1 h−1 (μM photons m−2 s−1)−1), 62.44 ± 36.96 (μg (μg Chl a)−1 h−1) and 55.81 ± 19.60 (μM photons m−2 s−1). There were no clear spatial patterns in either the absolute values or the absolute differences between the treatments at the experimental locations. When these parameters are integrated into a standard depth-integrated primary production model across a latitudinal transect, the effect of iron addition shows higher levels of primary production south of 50° S, with very little difference observed in the sub-Antarctic and Polar Frontal zone. These results emphasize the need for better parameterisation of photosynthetic parameters in biogeochemical models around sensitivities in their response to iron supply. Future biogeochemical models will need to consider the combined and individual effects of iron and light to better resolve the natural background in primary production and predict its response under a changing climate.