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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 ( Κ   C ) and oxygen (O 2 ) production ( Κ   O2 ), 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  Κ   C  and  Κ   O2  through measures of Pulse Amplitude Modulated (PAM) fluorometry, C fixation and O 2  production in an Arctic fjord (Godthåbsfjorden, W Greenland). Through short- (2h) and long-term (24h) experiments, rates of electron transfer (ETR PSII ), C fixation and/or O 2  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 ETR PSII  and gross  14 C fixation (R 2  = 0.81) during light-limited photosynthesis, giving a  Κ   C  of 7.6 ± 0.6 (mean ± S.E.) mol é (mol C) −1 . Diel net rates also demonstrated a linear relationship between ETR PSII  and C fixation giving a  Κ   C  of 11.2 ± 1.3 mol é (mol C) −1  (R 2  = 0.86). For net O 2  production the electron requirement was lower than for net C fixation giving 6.5 ± 0.9 mol é (mol O 2 ) −1  (R 2  = 0.94). This, however, still is an electron requirement 1.6 times higher than the theoretical minimum for O 2  production [i.e. 4 mol é (mol O 2 ) −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 O 2 , paving the road for improved spatial and temporal resolution of primary production estimates.

Phytoplankton Productivity in an Arctic Fjord (West Greenland): Estimating Electron Requirements for Carbon Fixation and Oxygen Production