Phytoplankton photosynthetic efficiency and primary production rates estimated from fast repetition rate fluorometry at coastal embayments affected by upwelling (Rias Baixas, NW of Spain)

Previous studies have not always found a significant relationshipbetween fast repetition rate fluorometry (FRRF)-derived and¹⁴C-based primary production rates. This apparent discrepancymight be related to environmental control of the coupling betweenphotosynthetic electron transport through photosystem II (PSII)and carbon uptake in phytoplankton. In this study, we lookedat this relationship under upwelling conditions favourable forphytoplankton growth. The combination of both techniques allowedthe calculation of the quantum efficiency of carbon fixation,which averaged 0.12 mol C (mol quanta)^–1 indicating thatphytoplankton populations were very efficient in convertingabsorbed photons into photosynthetically produced organic carbon.The tight coupling observed between phytoplankton photosyntheticelectron transport through PSII and carbon uptake resulted ina statistically significant linear relationship between FRRF-derivedand ¹⁴C-based carbon incorporation rates, with a slope of 1.43.In conclusion, the results of the this study indicate that FRRFcan be a useful tool to derive high spatial and temporal resolutionprimary production estimates under environmental conditionsfavouring the close coupling between PSII electron transportand carbon uptake, such as those characteristic of this coastalupwelling system.