Phytoplankton primary productivity: A dual‐incubation approach for direct comparison of photosystem II photosynthetic flux (JVPII) and 14C‐fixation experiments

Singe-turnover active chlorophyll a fluorometry (STAF) can be used to assess phytoplankton photo syntheticrates in terms of the photosystem II photochemical flux (JVPII, μmol e� m�3 s�1) instantaneously, autonomously, and at high resolution. While JVPII provides an upper limit to rates of phytoplankton primary productivity in units of carbon fixation, the conversion between these two rates is variable, limiting our ability to utilize high-resolution JVPII data to monitor phytoplankton primary productivity. Simultaneous measurements of JVPII and 14C-fixationhelp in understanding the factors controlling the variable ratio between the two rates. However, to date, methodo-logical inconsistencies, including differences in incubation lengths and light quality, have greatly inhibited practical assessment of such electron to carbon ratios (Φe,C, mol e� mol C�1). We here present data from a range of dual-incubation experiments in northeast Atlantic waters during which JVPII and 14C-fixation were measured simultaneously on the same sample. Time-course experiments show how Φe,C increases with incubation length, likely reflecting the transition from gross to net 14C-fixation. Dual-incubation experiments conducted under different light levels show a tendency for increased Φe,C under (super-)saturating light. Finally, data from a diurnal experiment demonstrate how Φe,C increases over the course of a day, due to downregulation of 14C-fixation. We provide a detailed description of our methodological approach, including a critical discussion of improvements to the calculation of JVPII implemented in the Lab STAF instrument used for active fluorescence measurements and the limitations of the well-established 14C-fixation approach.