Accurate quantification of pelagic primary production is essential for quantifying the marine carbon turnover and the energy supply to the food web. Knowing the electron requirement (Κ) for carbon (C) fixation (ΚC) and oxygen (O2) production (ΚO2), variable fluorescence has the potential to quantify primary production in microalgae, and hereby increasing spatial and temporal resolution of measurements compared to traditional methods. Here we quantify ΚC and ΚO2 through measures of Pulse Amplitude Modulated (PAM) fluorometry, C fixation and O2 production in an Arctic fjord (Godthåbsfjorden, W Greenland). Through short- (2h) and long-term (24h) experiments, rates of electron transfer (ETRPSII), C fixation and/or O2 production were quantified and compared. Absolute rates of ETR were derived by accounting for Photosystem II light absorption and spectral light composition. Two-hour incubations revealed a linear relationship between ETRPSII and gross 14C fixation (R2 = 0.81) during light-limited photosynthesis, giving a ΚC of 7.6 ± 0.6 (mean ± S.E.) mol é (mol C)-1. Diel net rates also demonstrated a linear relationship between ETRPSII and C fixation giving a ΚC of 11.2 ± 1.3 mol é (mol C)-1 (R2 = 0.86). For net O2 production the electron requirement was lower than for net C fixation giving 6.5 ± 0.9 mol é (mol O2)-1 (R2 = 0.94). This, however, still is an electron requirement 1.6 times higher than the theoretical minimum for O2 production [i.e. 4 mol é (mol O2)-1]. The discrepancy is explained by respiratory activity and non-photochemical electron requirements and the variability is discussed. In conclusion, the bio-optical method and derived electron requirement support conversion of ETR to units of C or O2, paving the road for improved spatial and temporal resolution of primary production estimates.