Relationship between light, community composition and the electron requirement for carbon fixation in natural phytoplankton

Fast repetition rate fluorometry (FRRF) provides a means to examine primary productivity at high resolution across broad scales, but must be coupled with independent knowledge of the electron requirement for carbon uptake (K-C) to convert FRRF-measured electron transfer rate (ETR) to an inorganic carbon (C) uptake rate. Previous studies have demonstrated that variability of K-C can be explained by key environmental factors (e.g. light, nutrients, temperature). However, how such reconciliation of K-C reflects changes of phytoplankton physiological status versus that of community composition has not been well resolved. Therefore, using a dataset of coupled FRRF and C uptake measurements, we examined how the environmental dependency of K-C potentially varied with parallel changes in phytoplankton community structure. Data were combined from 14 campaigns conducted during the summer season throughout 2007 to 2014 in the East China Sea (ECS) and Tsushima Strait (TS). K-C varied considerably, but this variability was best explained by a linear relationship with light availability (R-2 = 0.66). Co-variability between K-C and light availability was slightly improved by considering data as 2 clusters of physico-chemical conditions (R-2 = 0.74), but was best improved as 2 taxonomic clusters: samples dominated by micro-phytoplankton (> 20 mu m) versus small phytoplankton (nano + pico, < 20 mu m; R-2 = 0.70-0.81). Interaction of phytoplankton community structure with light availability therefore explains the majority of variance of K-C. The algorithms generated through our analysis therefore provide a means to examine C uptake with high resolution from future FRRF observations from these waters.