Roadmaps and Detours: Active Chlorophyll-a Assessments of Primary Productivity Across Marine and Freshwater Systems

Assessing phytoplankton productivity over space and time remains a core goal for oceanographers and limnologists. Fast Repetition Rate fluorometry (FRRf) provides a potential means to realize this goal with unprecedented resolution and scale yet has not become the “go-to” method despite high expectations. A major obstacle is difficulty converting electron transfer rates to equivalent rates of C-fixation most relevant for studies of biogeochemical C-fluxes. Such difficulty stems from methodological inconsistencies and our limited understanding of how the electron requirement for C-fixation (Φe,C) is influenced by the environment and by differences in the composition and physiology of phytoplankton assemblages. We outline a “roadmap” for limiting methodological bias and to develop a more mechanistic understanding of the ecophysiology underlying Φe,C. We 1) re-evaluate core physiological processes governing how microalgae invest photosynthetic electron transport-derived energy and reductant into stored carbo...