| Summary: | Marine phytoplankton primary productivity, the photosynthetic conversion of CO₂ into organic carbon by microscopic photosynthetic algae in the surface ocean, plays a fundamental role in ecosystem dynamics and global biogeochemical cycles. Consequently, the ability to accurately measure, monitor and predict environmental influences on this process over a range of spatial and temporal scales is crucial. The work presented in this thesis evaluates the application of fast repetition rate fluorometry (FRRF) for instantaneous, high resolution estimates of phytoplankton primary productivity. Results from both laboratory experiments and field work in Arctic and Subarctic marine waters show that the conversion factor required to derive carbon-based primary productivity estimates from FRRF-derived rates of electron transport in photosystem II (ETR) varies significantly in response to the interacting effects of iron and light availability (Chapter 2), over diurnal cycles (Chapter 3), and in response to nitrogen and light availability under low temperatures (Chapter 4). At a photo-physiological level, a high conversion factor is observed under conditions of excess excitation energy, where the amount of light energy absorbed in the pigment antenna exceeds the capacity for downstream metabolic processes, i.e. carbon fixation. Phytoplankton employ numerous mechanisms to alleviate excess excitation energy after charge separation, and these processes are postulated to be responsible for the increased de-coupling of ETR and carbon fixation. Consistent with this hypothesis, a strong correlation was observed between the derived conversion factor and the dissipation of excess excitation energy before charge separation, which can be estimated as non-photochemical quenching (NPQ). Because NPQ can be estimated from FRRF measurements, it can be used as a proxy for the magnitude and variability of the conversion factor between carbon fixation and ETR, and this approach holds potential to significantly improve carbon-based primary productivity estimates from FRRF measurements. The work presented in this thesis advances our understanding of the coupling between light absorption, photo-chemistry, and carbon fixation in response to various environmental gradients. The experimental approach taken demonstrates how an appreciation of photo-physiological processes of photosynthesis is critical for improved estimates of phytoplankton primary productivity at regional scales. Science, Faculty of Earth, Ocean and Atmospheric Sciences, Department of Graduate
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