Natasha René van Horsten

Distributions en fer dissous et en baryum particulaire pour l'étude de la reminéralisation en début d'hiver dans le secteur Indien de l'Océan Austral (mission GEOTRACES GIPr07): L'océan Austral est d'une importance centrale pour l'océan mondial, reliant les bassins Atlantique, Pacifique et Indien grâce notamment au courant circumpolaire antarctique, qui transporte de grandes quantités de chaleur, de carbone et de nutriments. Ainsi, cette province océanique régule en partie le climat de la Terre. Un des facteurs influençant la pompe à carbone biologique de l’Océan Austral est la disponibilité en fer, qui est un nutriment essentiel à la cellule phytoplanctonique. Or, les observations dans cette région ont été principalement menées au printemps et en été, laissant des lacunes importantes dans notre connaissance du cycle biogéochimique du fer notamment sur la saisonnalité des processus internes tels que la reminéralisation mésopélagique. Au cours de cette thèse, les distributions en fer dissous et d'excès de baryum (Baxs) qui est un proxy de la réminéralisation mésopélagique, ont été mesurés dans toute la colonne d'eau, à sept stations le long de 30°E dans le sud de l'océan Indien au début de l'hiver austral 2017. Nos résultats suggèrent que la reminéralisation est maintenue à des niveaux comparables à l'été, bien après la fin de la floraison, au début de l'hiver. Concernant le fer dissous, les concentrations hivernales ne sont pas significativement différentes des observations estivales mesurées dans l'ouest de l'océan Indien Austral, restant à des concentrations subnanomolaires, de 0.02 à 0.73 nmol L-1. Les concentrations dans les eaux de surface sont diminués, indiquant qu'en juillet, il n'y a pas encore eu de reconstitution du stock de fer dissous dans les eaux de surface. Les distributions de fer dissous et d'excès de baryum présentent des pics dans les eaux intermédiaires sur les profils de colonne d'eau, indiquant une reminéralisation du fer.En utilisant le proxy d'utilisation apparente de l'oxygène nous avons estimé qu’une fraction importante (de 58 ± 1 à 160 ± 2 %) du stock de fer dissous mésopélagique mesuré était fourni par reminéralisation.

Combined observations of phytoplankton abundance, community structure, and trace metals in winter are scarce but crucial for understanding Southern Ocean biogeochemistry. This study provides the first early winter insights of phytoplankton‐micronutrient dynamics in the Indian sector of the Southern Ocean. Depth‐resolved measurements of chlorophyll‐ a , phytoplankton composition, and micronutrients iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd) reveal low phytoplankton abundance (0.27–0.08 μg L ⁻¹ chlorophyll‐ a ) and variable phytoplankton–micronutrient dynamics between the zones. In the Subtropical Zone, cyanobacteria, and haptophytes dominated, driven by higher temperatures, limiting macronutrients, Fe and Zn, and shaped Co and Mn through biological uptake. In the Subantarctic and Polar Frontal Zones, cyanobacteria contributions declined but continued to play a key role in shaping Co dynamics. Diatoms were limited by Mn, Zn, and silicic acid, enabling the dominance of prasinophytes. Other groups such as coccolithophores may have benefitted from the ability to substitute Zn for Cd and contributed to the formation of particulate Cd. Coccolithophores along with Phaeocystis also contributed to particulate Cd in the Antarctic Zone, where Fe and Mn limited the abundance of the dominating diatoms. Diatoms, in turn, strongly contributed to particulate Zn. This perspective on phytoplankton‐micronutrient relationships shows that despite their low productivity in the harsh low light wintertime Southern Ocean, phytoplankton continue to influence trace metal distribution and that there are strong phytoplankton group‐specific associations with various essential metals, rather than Fe alone, that vary latitudinally.

Combined observations of phytoplankton abundance, community structure, and trace metals are limited but crucial for understanding Southern Ocean biogeochemistry. While summer studies exist, winter research is scarce. This study provides a fundamental first overview of phytoplankton-micronutrient (i.e. trace metals) dynamics during early winter in the Indian sector of the Southern Ocean. Depth-resolved measurements were taken for chlorophyll-a, phytoplankton composition, and micronutrients iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd). Phytoplankton abundance (0.27 - 0.08 µg L ^-1 chlorophyll-a), and presumably, metal uptake were low in winter. In the Subtropical Zone, cyanobacteria and haptophytes dominated, driven by the interplay of higher temperatures, limiting macronutrients, and trace metals such as Fe and Zn, while also influencing Co and Mn dynamics through biological uptake. In the Subantarctic and Polar Frontal Zones prasinophytes dominated, cyanobacteria were linked to Co uptake, while Mn and Zn deficiency and silicic acid limitation constrained diatoms. In contrast, flagellate groups, such as the dominant prasinophytes, may have compensated through substitution. In the Antarctic Zone, Fe and Mn were the primary limiting nutrients for the dominating diatoms, as silicic acid was no longer limiting. In turn, diatoms strongly influenced Zn particulates through biological uptake, while Phaeocystis and coccolithophores appear to have contributed more to Cd uptake. This winter perspective on phytoplankton-micronutrient relationships shows that phytoplankton continue to influence particulate trace metal distribution despite their low productivity and that there are strong phytoplankton group-specific associations with various essential metals, rather than iron alone, that vary latitudinally.

The Southern Ocean (SO) is of global importance to the carbon cycle, and processes such as mesopelagic remineralisation that impact the efficiency of the biological carbon pump in this region need to be better constrained. During this study early austral winter barium excess (Ba_xs) concentrations were measured for the first time, along 30^∘ E in the southern Indian Ocean. Winter Ba_xs concentrations of 59 to 684 pmol L⁻¹ were comparable to those observed throughout other seasons. The expected decline of the mesopelagic Ba_xs signal to background values during winter was not observed, supporting the hypothesis that this remineralisation proxy likely has a longer timescale than previously reported. A compilation of available SO mesopelagic Ba_xs data, including data from this study, shows an accumulation rate of ∼0.9 µmol m⁻² d⁻¹ from September to July that correlates with temporally integrated remotely sensed primary productivity (PP) throughout the SO from data spanning ∼20 years, advocating for a possible annual timescale of this proxy. The percentage of mesopelagic particulate organic carbon (POC) remineralisation as calculated from estimated POC remineralisation fluxes over integrated remotely sensed PP was ∼2-fold higher south of the polar front (19 ± 15 %, n=39) than north of the polar front (10 ± 10 %, n=29), revealing the higher surface carbon export efficiency further south. By linking integrated remotely sensed PP to mesopelagic Ba_xs stock, we could obtain better estimates of carbon export and remineralisation signals within the SO on annual and basin scales.

The Southern Ocean is of global importance and processes such as mesopelagic remineralisation that impact the efficiency of the biological carbon pump in this region is of substantial interest. During this study the proxy barium excess which is utilised to shed light on mesopelagic remineralisation was measured at seven stations along 30° E in the Southern Indian Ocean during early austral winter of 2017. To our knowledge this is the first reported winter study utilising this proxy in the Southern Ocean. Concentrations of 59 to 684 pmol L⁻¹ were comparable to those observed throughout other seasons, indicating that this proxy has a longer timescale than previously thought. Background barium excess values observed in deep waters were also similar to previous studies, not having declined down to an expected true Southern Ocean background value. It is apparent that processes driving the mesopelagic barium excess signal are still underway during early winter. Indicating that continuous remineralisation is sustained at levels comparable to summer, well after bloom termination. Moreover, linking integrated remote sensing primary production to the mesopelagic barium excess signal reiterates a longer timescale. The significant positive correlations obtained in the Antarctic and Subantarctic zones suggest that mesopelagic barium excess stock can be used as a remineralisation proxy on an annual timescale and possible inference of carbon remineralisation from remote sensing data on an annual and basin scale.

The Southern Ocean is of global importance and processes such as mesopelagic remineralisation that impact the efficiency of the biological carbon pump in this region is of substantial interest. During this study the proxy barium excess which is utilised to shed light on mesopelagic remineralisation was measured at seven stations along 30° E in the Southern Indian Ocean during early austral winter of 2017. To our knowledge this is the first reported winter study utilising this proxy in the Southern Ocean. Concentrations of 59 to 684 pmol L⁻¹ were comparable to those observed throughout other seasons, indicating that this proxy has a longer timescale than previously thought. Background barium excess values observed in deep waters were also similar to previous studies, not having declined down to an expected true Southern Ocean background value. It is apparent that processes driving the mesopelagic barium excess signal are still underway during early winter. Indicating that continuous remineralisation is sustained at levels comparable to summer, well after bloom termination. Moreover, linking integrated remote sensing primary production to the mesopelagic barium excess signal reiterates a longer timescale. The significant positive correlations obtained in the Antarctic and Subantarctic zones suggest that mesopelagic barium excess stock can be used as a remineralisation proxy on an annual timescale and possible inference of carbon remineralisation from remote sensing data on an annual and basin scale.

The Southern Ocean is of global importance and processes such as mesopelagic remineralisation that impact the efficiency of the biological carbon pump in this region is of substantial interest. During this study the proxy barium excess which is utilised to shed light on mesopelagic remineralisation was measured at seven stations along 30° E in the Southern Indian Ocean during early austral winter of 2017. To our knowledge this is the first reported winter study utilising this proxy in the Southern Ocean. Concentrations of 59 to 684 pmol L⁻¹ were comparable to those observed throughout other seasons, indicating that this proxy has a longer timescale than previously thought. Background barium excess values observed in deep waters were also similar to previous studies, not having declined down to an expected true Southern Ocean background value. It is apparent that processes driving the mesopelagic barium excess signal are still underway during early winter. Indicating that continuous remineralisation is sustained at levels comparable to summer, well after bloom termination. Moreover, linking integrated remote sensing primary production to the mesopelagic barium excess signal reiterates a longer timescale. The significant positive correlations obtained in the Antarctic and Subantarctic zones suggest that mesopelagic barium excess stock can be used as a remineralisation proxy on an annual timescale and possible inference of carbon remineralisation from remote sensing data on an annual and basin scale.

Seasonal progression of dissolved iron (DFe) concentrations in the upper water column was examined during four occupations in the Atlantic sector of the Southern Ocean. DFe inventories from euphotic and aphotic reservoirs decreased progressively from July to February, while dissolved inorganic nitrogen decreased from July to January with no significant change between January and February. Results suggest that between July and January, DFe loss from both euphotic and aphotic reservoirs was predominantly in support of phytoplankton growth (iron‐to‐carbon uptake ratio of 16 ± 3 μmol/mol), highlighting the importance of the “winter DFe reservoir” for biological uptake. During January to February, excess loss of DFe relative to dissolved inorganic nitrogen (iron‐to‐carbon uptake ratio of 44 ± 8 μmol/mol and aphotic DFe loss rate of 0.34 ± 0.06 μmol·m⁻²·day⁻¹) suggests that scavenging is the dominant removal mechanism of DFe from the aphotic, while continued production is likely supported by recycled nutrients.

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.