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

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 (K) for carbon (C) fixation (K-C) and oxygen (O-2) production (K-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 K-C and K-O2 through measures of Pulse Amplitude Modulated (PAM) fluorometry, C fixation and O-2 production in an Arctic fjord (Godthabsfjorden, W Greenland). Through short- (2h) and long-term (24h) experiments, rates of electron transfer (ETRPSII), 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 ETRPSII and gross C-14 fixation (R-2 = 0.81) during light-limited photosynthesis, giving a K-C of 7.6 +/- 0.6 (mean +/- S.E.) mole (mol C)(-1). Diel net rates also demonstrated a linear relationship between ETRPSII and C fixation giving a K-C of 11.2 +/- 1.3 mole (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 mole (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 mole (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.