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, Φ_e,C), inherently describes the extent and effectiveness with which absorbed light energy drives C-fixation. However, it remains unclear whether and how Φ_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 ¹⁴C-incubations to produce a semi-continuous dataset of Φ_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 μm) versus warmer nutrient-poorer waters dominated by smaller size-fractionated Chl-a (<2 μm). Whilst Φ_e,C was higher for warmer water samples, environmental conditions alone explained <20% variance of Φ_e,C, and changes in predominant size-fraction(s) distributions of Chl-a (biomass) failed to explain variance of Φ_e,C. Instead, normalised Stern-Volmer non-photochemical quenching (NPQ_NSV = F₀′/F_v′) was a better predictor of Φ_e,C, explaining ~55% of observed variability. NPQ_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 Φ_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 Φ_e,C and the correlation between Φ_e,C and NPQ_NSV – in doing so, we highlight the need for F_b corrections for future field-based assessments of Φ_e,C.