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 ( K C ) to scale an electron transfer rate (ETR) to the CO 2 uptake rate. Most previous studies have derived K C from parallel measurements of ETR and CO 2 uptake over relatively short incubations, with few from longer-term daily-integrated periods. Here we determined K C by comparing depth-specific, daily ETRs and CO 2 -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 K C 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 K C 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 K C ( r 2  = 0.94). A simple PAR-dependent relationship derived from the data set was used for generating K C , and this relationship was validated by comparing the FRRf-predicted NPP with the 13 C uptake measured in 2007. These new observations demonstrate the potential application of FRRf for estimating regional NPP from ETR.