Keywords:ETR, Primary productivity, Quantum requirement for carbon fixation, Phytoplankton composition, Fast repetitive rate fluorometer
In the last 20 years, introduction of the active fluorescence technique, fast repetition rate fluorometer (FRRf), has opened up capability to measure instantaneous electron flow rate through phytoplankton photosystem II (electron transfer rate, ETR). However, in most of the early studies, the conversion factor (KC) from ETR to primary production was assumed to be constant. Recent studies have shown that Kc can vary over a large range in nature and that understanding of the variability of KC in relation to other physical and biogeochemical parameters is the crucial for accurately estimating primary production with FRRf. In this study, KC was determined from parallel measurements of ETR and daily net primary production (NPP) and modeled as a function of key environments and phytoplankton community structure for the first time in Ariake bay and the East China Sea. In Ariake Bay study, we firstly confirmed that KC varied considerably in nature and then demonstrated the strong correlation (R2 = 0.94) between daily photosynthetically active radiation (PAR) and KC; the novel and simple PAR-dependent relationship used for deriving KC opens the possibility for directly FRRf based NPP estimating. In the East China Sea study, we confirmed that PAR was still the main controlling factor of KC (R2 = 0.72); moreover, this factor appeared secondarily influenced by dominant phytoplankton taxa present. Results showed that the correlation between KC and PAR was improved (R2 = 0.78-0.86) by considering two clusters of taxonomical groups, large phytoplankton (>20 µm) and small phytoplankton (<20 µm). Overall, we have developed a novel algorithm for estimating KC from PAR and phytoplankton community composition and this algorithm can be applied to future FRRf-based high resolution studies of primary production.