Browsing by Author "Van Horsten, Natasha"

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    Insights into early winter Southern Indian Ocean dissolved iron distributions and remineralisation using excess barium (GEOTRACES GIPr07 cruise)
    (Stellenbosch : Stellenbosch University, 2022-04) Van Horsten, Natasha; Roychoudhury, Alakendra N.; Sarthou, Géraldine; Mtshali, Thato; Bucciarelli, Eva; Planquette, Hélène; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The Southern Ocean is of central importance to the global ocean, connecting the Atlantic, Pacific and Indian basins via the Antarctic Circumpolar Current, which carries large amounts of heat, carbon and nutrients. Thus, this oceanic province partly regulates the Earth's climate, present and future. One of the factors influencing the biological carbon pump of the Southern Ocean is the availability of iron, which is an essential nutrient for phytoplankton. However, observations in this region have mainly been conducted in spring and summer, leaving significant gaps in our knowledge of the biogeochemical cycle of iron, in particular on the seasonality of internal processes such as mesopelagic remineralisation. During this study, the distributions of dissolved iron and excess barium (Baxs), a mesopelagic remineralisation proxy, were measured throughout the water column, at seven stations along 30 ° E in the Southern Indian Ocean during early austral winter of 2017. To our knowledge, this is the first winter study on these two parameters and in this sector of the Southern Ocean. A more in-depth evaluation of the Baxs proxy was conducted by comparing integrated remote sensing primary production data and the Baxs signal, for all observations available in the Southern Ocean, on an annual timescale. Our results suggest that remineralization is maintained at comparable levels as observed during summer, well after the bloom decline, resulting in the mesopelagic Baxs signal, at the onset of winter. Regarding dissolved iron, the winter concentrations were not significantly different to summer observations in the western Southern Indian Ocean, remaining at sub-nanomolar concentrations, from 0.02 to 0.73 nmol L-1. Concentrations in surface water were depleted, resembling late summer observations, indicating that in July there has not yet been a replenishment of the dissolved iron stock in surface waters. The distributions of dissolved iron and Baxs exhibited peaks in intermediate waters on the water column profiles, indicating iron remineralization. Using the apparent oxygen utilization proxy, we estimated that a large fraction (58 ± 1 to 160 ± 2%) of the measured mesopelagic dissolved iron stock was provided by remineralisation. Iron remineralisation fluxes were largest in the subantarctic zone and decreased to a minimum in the oligotrophic Subtropical zone. This study contributes to the emerging understanding of winter trace metal distributions and internal cycling, giving insight into the replenishment of the winter dFe reservoir, as well as obtaining better estimates of export and remineralisation on annual and basin scales, within the Southern Ocean.
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    Photosynthetic response of Southern Ocean phytoplankton under iron and light limitations : bioassay experiments
    (Stellenbosch : Stellenbosch University, 2015-12) Van Horsten, Natasha; Fietz, Susanne; Mtshali, Thato; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The Southern Ocean (SO) is of significant interest in the understanding of the global carbon cycle and therefore many studies have been conducted to determine the limiting factors controlling the biological pump within the region. During photosynthesis phytoplankton require various nutrients such as NO3, PO4, inorganic carbon and the micronutrient Fe. The SO is a High-Nutrient Low-Chlorophyll region, therefore no macronutrient limitation is experienced by resident phytoplankton but instead the micronutrient Fe is a significant limiting factor within these waters due to limited inputs. Due to deep mixed layer depths, ice cover, low sun angles and cloud cover throughout parts of the year, light is also considered a limiting factor in the SO. Fe and light limitation cause a decrease in photosynthetic efficiency and therefore a decrease in carbon fixation capabilities. During this study we conducted five bioassay shipboard incubation experiments during two cruises along the Greenwich meridian between South Africa and the ice edge, SOSCEx during March and SAFePool during January to February, in which we varied Fe concentrations and light levels to determine the effects of Fe and light limitation or co-limitation within resident phytoplankton. Spatial and temporal variations in phytoplankton response were studied to determine varying effects of limitation across water masses and different stages of bloom decline within the study area. The combined addition of Fe and light gave the largest increase in biomass, photosynthetic capacity and nutrient uptake. In support of the hypotheses tested changes in the photosynthetic apparatus led to changes in the photosynthetic efficiency and growth of the SO phytoplankton, as a result of variations in Fe and light availability. Variability was also observed in the response of phytoplankton to Fe and light amendments due to spatial and temporal variation in resident phytoplankton communities. It was therefore concluded that both Fe and light are significant controls in the resident phytoplankton photosynthetic apparatus, photosynthetic capabilities, organic carbon fixation and therefore the biogeochemical cycles within the Atlantic sector of the SO.