Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by browse.metadata.advisor "Fietz, Susanne"

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    Phytoplankton and trace metal dynamics in the Southern Ocean
    (Stellenbosch : Stellenbosch University, 2023-03) Viljoen, Johannes Jacobus; Fietz, Susanne; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Marine phytoplankton in the Southern Ocean are essential for the Antarctic food web and help regulate the global ocean biogeochemistry, thereby mediating the warming effect of carbon dioxide (CO2). Yet, spatially and temporally, phytoplankton distribution and their controlling factors are still poorly studied in the Southern Ocean. Owing to low sampling resolution, features such as frontal and island regions have been less sampled. Moreover, the dynamics of trace metals which serve as essential micronutrients has been the least studied in the Southern Ocean. Most studies focussed on iron (Fe) and total chlorophyll-a, few on extended suites of metals and phytoplankton groups. There are still uncertainties how the trace metals Fe, copper (Cu), zinc (Zn), nickel (Ni), cobalt (Co), manganese (Mn), and cadmium (Cd), will affect phytoplankton abundance and community composition, especially in the understudied Southern Ocean during winter. In this thesis Southern Ocean phytoplankton dynamics and the complex interplay with various parameters were elucidated. Through three research papers, phytoplankton distribution was studied utilizing multi-parameter datasets, collected during two research cruises to the Southern Ocean one during summer in the Atlantic sector (0 - 8°E) and another during winter in the Indian sector (30°E). Summer phytoplankton distribution were characterised in the Atlantic Southern Ocean at high resolution across the major zones, within fronts, in sea ice influenced zones (e.g., polynya) and close to Subantarctic islands using a suite of HPLC phytoplankton pigments. Cyanobacteria dominated the Subtropical zone; haptophytes the Subantarctic and Polar Frontal zones while diatoms and haptophytes dominated the Antarctic zone south of the Polar Front. Additionally, distinct communities were observed in frontal, sea ice and near-island regions. For example, phytoplankton abundance in fronts were up to 10-fold higher than in the major zones while haptophytes dominated communities close to the Subtropical Front. Diatoms dominated in the areas affected by recent sea ice melt while Phaeocystis and coccolithophores dominated post-bloom communities. Close to Subantarctic islands, cryptophyte blooms were observed whereas diatoms were dominant further downstream. Results suggest that a combination of nutrient, including trace element supply and mixing regime are essential in controlling the magnitude and composition of blooms close to fronts, sea ice and islands, and in turn, affect Southern Ocean food web activity and potential carbon export. Having noticed the importance of zonal changes and small-scale features, as well as the likely importance of trace element supply, the second and third study included these dynamics. The relationship between phytoplankton dynamics (abundance and community structure) and micronutrients (i.e., trace metals) were investigated for the first time during winter in the Indian sector of the Southern Ocean. This was achieved through two studies using a unique dataset of HPLC chlorophyll-a and accessory pigment concentrations with parallel sampled macronutrients and a suite of dissolved and particulate trace metals and phosphorous concentrations. Results suggest phytoplankton were still active and a dominant contributor to the uptake and remineralisation of trace metals even though their abundance were lower than during summer. Through a suite of metal* calculations, based on the macro- and micronutrient concentrations and the estimated requirements of Southern Ocean phytoplankton, varying degrees of deficiency and potential for co-limiting conditions were proposed. Accordingly, micronutrients are suggested as a major driver of winter phytoplankton abundance and community structure across the Southern Ocean. Some trace metals had stronger relationships with specific phytoplankton groups compared to total phytoplankton abundance. In turn, specific groups, such as diatoms, were confirmed to be major drivers of trace metal dynamics across the transect through preferential uptake. For example, cyanobacteria, a group not considered by previous trace metal studies in this region, was suggested to be dependent and responsible for major uptake of Co and Mn while diatoms were strongly associated with Zn. Ultimately, the preferential uptake by specific phytoplankton groups in deficient conditions, such as diatoms, can aggravate limiting conditions and lead to a change in composition. Therefore, the strong association of specific phytoplankton groups, with different remineralisation lengths, to specific micronutrients during winter would affect the release of these micronutrients for the utilisation by phytoplankton during the following spring and summer seasons. Hence, these two studies yielded new knowledge on phytoplankton-micronutrient dynamics that contributes critical seasonal information for biogeochemical models. Collectively, the research in this thesis demonstrates the importance of understanding not only phytoplankton abundance, but also its community composition and how small- and large-scale changes in the chemical environment, including the availability of trace metals, can influence phytoplankton dynamics.
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    Southern African dust characteristics and potential impacts on the surrounding oceans
    (Stellenbosch : Stellenbosch University, 2021-12) Kangueehi, Kaukurauee Ismael; Fietz, Susanne; Eckardt, Frank; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: This study supports understanding the potential impact of dust aerosols from southern Africa have on the proximal ocean ecosystems. Dust can release essential nutrients and thus fertilize the ocean, which affects the food-web and the carbon dioxide concentrations in ocean waters, i.e. climate. Dust that fertilizes the phytoplankton communities in open oceans stimulates the drawdown of carbon dioxide from the atmosphere through the process of photosynthesis. Key here is that the nutrients must be released from the aerosol particles, as they can only be uptaken by biological organisms in soluble form. However, mineral dust emitted from arid environments differ from dust emitted through industrial activities; as a result the solubility and, by extent, potential impact on the ocean may also differ. In this study, I investigated mineral characteristics and solubility of dust from three distinct regions, one with a strong human footprint (Saldanha Bay), one with mostly natural mineral dust (Namib Desert) and one off shore over the Southern Ocean. The first study targeted Saldanha Bay, a town that hosts the largest port in South Africa, with exports of up to 60 million tons of iron and manganese ore annually and is home of a steel plant and a smelter. Satellite images and photos from the area have shown extensive dispersion of dust from the area. Solubility leaching experiments revealed that dust collected in this town is highly soluble (bioaccessible) for trace metals such as Fe (up to 28%), Cu (up to 33%), Pb (up to 45%) and Zn (up to 38%). Phytoplankton communities in open oceans are sometimes depleted in these trace metals, and thus, such high solubility of dust from Saldanha Bay can prove to be an important nutrient supplier to surrounding oceans. In addition, air mass trajectories revealed that this readily available dust most likely affects the southeast Atlantic and Indian Ocean. The major implication was that harbour towns can be essential sources of trace metals to proximal oceans. Secondly, I present results from the largest non-playa environment in the Namib Desert. Some of the prominent dust emitters located in the Namib Desert provide, feed and source of hundreds of tons of mineral dust to proximal surface ocean waters. The solubility of the mineral dust, in contrast to the mixed (anthropogenic and natural sourced) dust mentioned above, is much lower (e.g. up to 2 % for Fe, up to 7.8 % for Cu, up to 16% for Zn, up to 9% for Mn, 8% for Pb). These solubilities are in similar range to other semi-arid to arid major global dust sources such as the Sahara Desert in the Northern Hemisphere. Differences in mineralogy and particle size distribution at the sampling stations are the main drivers of the solubility. For example, the solubility of trace metals such as Fe, Zn and Pb is higher at the inland station than at the coastal station. The aggregated particles along the coastal stations are less soluble probably because of persistent foggy conditions and less sunlight. The inland stations had less foggy conditions and more fine-grained particles which are rich in FeO oxides. Air mass trajectory modelling indicated that this mineral dust, with its low solubility, typically travels towards the southeast Atlantic Ocean, but can also reach the nutrient poor areas in the Southern Ocean in some cases. This study highlighted the role of non-playa environments as important dust sources and that mineralogy coupled with particle size are closely related to trace metal solubility. In addition, to assessing the solubility and potential impact of dust with natural and anthropogenic imprint, we investigated the solubility of aerosols collected over the Southern Ocean south of South Africa on board on the research vessel SA Agulhas II. The solubility of the trace metals over the Southern Ocean ranges from 0.5 to 41% Fe, 9 to 48% Al, 0.1 to 88 % Mn, 25 to 72 % Zn and 3.3 to 39 % Co. This study found large variability in trace metal composition and fractional solubility in dust aerosols collected from three distinct environments in southern Africa and south of southern Africa. Our study highlighted the importance of southern African dust of both anthropogenic and natural sources as a nutrient supplier to surrounding coastal and open oceans.
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    Understanding Southern Ocean phytoplankton ecophysiological response to iron availability
    (Stellenbosch : Stellenbosch University, 2022-04) Singh, Asmita; Ryan-Keogh, Thomas; Fietz, Susanne; Thomalla, Sandy; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Over the past century, climate change has been of growing concern, due to its ecological and economic global impact. The Southern Ocean buffers the impacts of climate change by accounting for a significant proportion of the total oceanic uptake of CO2. Phytoplankton primary production and carbon export (the biological carbon pump) plays an important role in the Southern Ocean carbon cycle and despite their ability to remove a significant amount of the global organic carbon flux each year, it is often constrained by the availability of light and nutrients. The micronutrient iron is particularly important in the production of key metabolic and photosynthetic proteins in phytoplankton and its scarcity in the Southern Ocean dictates its role as a key driver of variability in Southern Ocean productivity. A greater understanding of the response of Southern Ocean phytoplankton to seasonal and regional variability to their environmental drivers (with a focus on iron in particular) is thus required in order to increase the accuracy in assessing and predicting the impact of climate change. Active chlorophyll-a fluorescence is a non-invasive, powerful instantaneous tool, which can assess the phytoplankton photosynthetic efficiency in response to potential environmental drivers and in particular under stressful growth conditions, i.e. under iron limitation. However, our understanding of the seasonal cycle of photophysiological responses of phytoplankton to iron and other biogeochemical drivers remains limited, primarily due to the prevalence of experiments and measurements only being conducted in summer. In this research, three individual studies of phytoplankton photophysiology across all seasons of the Atlantic Southern Ocean were used to investigate physical and biogeochemical drivers of inter-zonal, inter-annual and intra-seasonal variability in phytoplankton photophysiology (Fv/Fm) in summer. Results from this study point to a combination of drivers (notably sea surface temperature, macronutrients and community structure) that elicit simultaneous and oftentimes antagonist responses in Fv/Fm, making it difficult to ascertain one dominant driver over another. In addition, the degree of iron stress in the Sea-Ice Zone in autumn, and the full zonal extent of the open Atlantic Southern Ocean in spring and winter, was determined using a series of short-term (24 hrs) in situ iron addition experiments. Key results suggest that phytoplankton in the Sea-Ice Zone of Dronning Maud Land are not iron-limited in autumn, and presumably have a sufficient year-round supply of iron potentially from shallow topography. However, both winter and spring showed some positive responses to iron addition with zonal variability being linked to the timing of the incubation experiments relative to the seasonal cycle of the mixed layer depth, highlighting the importance of convective overturning as a dominant seasonal iron supply mechanism. The research presented in this thesis contributes to a greater understanding of the complex interplay of multiple drivers of phytoplankton photosynthesis across the seasonal cycle. It is recommended that future research continues to address seasonal and regional variability in phytoplankton photophysiology but with a focus on resolving the relationship between multiple drivers in line with anticipated climate-mediated adjustments in environmental conditions.