Using next-generation multi-spectral FRRf to improve current estimates of marine primary production (MPP) within Australian waters

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Bio-optical tools remain key technologies to address a long-standing goal in oceanography: to improve understanding of how marine primary productivity (MPP) varies over space and time. A major goal for one particular technique, Fast Repetition Rate fluorometry (FRRf), is to retrieve highly resolute patterns of carbon (C) uptake in situ to improve satellite retrieved predictions of MPP. However, this goal hinges upon the application of a highly-variable, yet poorly-understood conversion factor to scale FRRf-derived electron transport rates (ETRs) to rates of C-uptake. Understanding of the conversion factor, termed the “electron requirement for carbon fixation” (KC) is limited, in particularly for Australian waters where KC has rarely been measured. This thesis focuses on coupled ETR – C-uptake measurements, to examine how key factors drive variability in KC, utilising both laboratory and field studies to isolate the respective influences of growth environment and phytoplankton taxonomy. I performed nutrient addition bioassays upon natural phytoplankton assemblages to demonstrate for the first time how macronutrient availability (N, P and Si) regulates KC at an Australian coastal reference station when nutrient concentrations are low during summer. To examine taxonomic variability of KC together with metrics influencing phytoplankton growth and physiology (cell size and non-photochemical quenching, NPQ), I grew phytoplankton covering a broad range of taxonomic and size classes within a controlled laboratory setting where environmental variability could be excluded. Finally, to examine how well KC could be predicted in a highly-dynamic system with multiple environmental stressors and phytoplankton assemblages, I performed a novel high-throughput assessment of KC (n = 80) along the eastern Australian coast spanning multiple water masses including the Tasman Sea and the East Australian Current (EAC). Prevailing environmental variables, physiological (non-photochemical heat dissipation, NPQɴsᴠ) and phytoplankton community structure (size-fractionated Chl-a) were also measured for each sample to allow evaluation of their respective performance in empirically modelling KC variance. This thesis highlights the importance in characterising both environmental and taxonomic factors to most robustly retrieve KC, but also demonstrates that a single FRRf parameter (NPQɴsᴠ) may reliably explain ~50% of variability in eastern Australian waters. These new findings potentially provide new and unprecedented capacity to retrieve C-fixation rate from FRRf-based productivity assessments, but ultimately require further validation that may be possible through re-visiting past FRRf data sets. These findings are then considered to propose a roadmap to enable broader implementation and uptake of FRRf for widespread assessments of marine (and freshwater) primary productivity into the future.
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