Dynamic variability of the phytoplankton electron requirement for carbon fixation in eastern Australian waters

Fast Repetition Rate fluorometry (FRRf) generates high-resolution measures of phytoplankton primary productivity as electron transport rates (ETRs). How ETRs scale to corresponding inorganic carbon (C) uptake rates (the so-called electron requirement for carbon fixation, Phi(e,c)), inherently describes the extent and effectiveness with which absorbed light energy drives C-fixation. However, it remains unclear whether and how Phi(e,c) follows predictable patterns for oceanographic datasets spanning physically dynamic, and complex, environmental gradients. We utilise a unique high-throughput approach, coupling ETRs and C-14-incubations to produce a semi continuous dataset of Phi(e,c) (n = 80), predominantly from surface waters, along the Australian coast (Brisbane to the Tasman Sea), including the East Australian Current (EAC). Environmental conditions along this transect could be generally grouped into cooler, more nutrient-rich waters dominated by larger size-fractionated Chl-a (> 10 mu m) versus warmer nutrient-poorer waters dominated by smaller size-fractionated Chl-a (< 2 mu m). Whilst Phi(e,c) was higher for warmer water samples, environmental conditions alone explained < 20% variance of Phi(e,c), and changes in predominant size-fraction(s) distributions of Chl-a (biomass) failed to explain variance of (I),,c. Instead, normalised Stern-Volmer non-photochemical quenching (NFQ(NSV) = F-o'/F-v') was a better predictor of Phi(e,C), explaining similar to 55% of observed variability. NBQ(NSV) is a physiological descriptor that accounts for changes in both long-term driven acclimation in non-radiative decay, and quasi-instantaneous PSII downregulation, and thus may prove a useful predictor of Phi(e,C) across physically-dynamic regimes, provided the slope describing their relationship is predictable. We also consider recent advances in fluorescence-based corrections to evaluate the potential role of baseline fluorescence (F-b) in contributing to overestimation of Phi(e,C) and the correlation between Phi(e,C) and NPQ(NSV) - in doing so, we highlight the need for Fb corrections for future field-based assessments of Phi(e,C)