| Summary: | Thesis (PhD)--Stellenbosch University, 2022. 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 ...
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