Uncertainties in variable fluorescence and 14C methods to estimate primary production: a case study in the coastal waters off the Korean peninsula

To understand what controls the decoupling of photosynthetic electron transfer and carbon fixation in natural phytoplankton communities, we compared the primary production rates estimated by C-14 uptake and fast repetition rate fluorometry (FRRF) in Korean waters under diverse environmental conditions. Our comparison showed that these 2 methods produce consistent results, with the FRRF method systematically yielding 1.9 times higher values than the C-14 method. To quantify the potential factors that contribute to this discrepancy, we analyzed the variability in the electron requirement for carbon fixation, which showed a wide range of 5.5-71.3 mol electrons (mol CO2)(-1), in relation to environmental variables. The analysis revealed that nutrient availability and the state of stratification may be the major factors that control variability in the electron requirement for carbon fixation. The strong dependence on nitrogen suggests that photosynthetic processes and phytoplankton growth are not in balance in natural communities. Based on the relationship with environmental variables, we propose a regional algorithm for the electron requirement for carbon fixation, which markedly improves FRRF-based measurements of primary production in this geographical area.