Phytoplankton and trace metal dynamics in the Southern Ocean

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Stellenbosch : Stellenbosch University
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.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Thesis (PhD)--Stellenbosch University, 2023.