Fast repetition rate fluorometry (FRRf) provides a potential means to examine marine primary productivity; however, FRRf-based productivity estimations require knowledge of the electron requirement (K) for carbon (C) uptake (KC) to scale an electron transfer rate (ETR) to the CO2 uptake rate. Most previous studies have derived KC from parallel measurements of ETR and CO2 uptake over relatively short incubations, with few from longer-term daily-integrated periods. Here we determined KC by comparing depth-specific, daily ETRs and CO2-uptake rates obtained from 24-h on-deck incubation experiments undertaken on seven cruises in Ariake Bay, Japan, from 2008 to 2010. The purpose of this study was to determine the extent of variability of KC and to what extent this variability could be reconciled with the prevailing environmental conditions and ultimately to develop a method for determining net primary productivity (NPP) based on FRRf measurements. Both daily ETR and KC of the upper layer varied considerably, from 0.5 to 115.7 mmol e− mg Chl-a−1 day−1 and 4.1–26.6 mol e− (mol C)−1, respectively, throughout the entire data set. Multivariate analysis revealed a strong correlation between daily photosynthetically active radiation (PAR) and KC (r2 = 0.94). A simple PAR-dependent relationship derived from the data set was used for generating KC, and this relationship was validated by comparing the FRRf-predicted NPP with the 13C uptake measured in 2007. These new observations demonstrate the potential application of FRRf for estimating regional NPP from ETR.
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