Development of photosynthetic carbon fixation model using multi-excitation wavelength fast repetition rate fluorometry in Lake Biwa

Direct measurements of gross primary productivity (GPP) in the water column are essential, but can be spatially and temporally restrictive. Fast repetition rate fluorometry (FRRf) is a bio-optical technique based on chlorophyll a (Chl-a) fluorescence that can estimate the electron transport rate (ETR(PSII)) at photosystem II (PSII) of phytoplankton in real time. However, the derivation of phytoplankton GPP in carbon units from ETR(PSII) remains challenging because the electron requirement for carbon fixation (Ф(e,C)), which is mechanistically 4 mol e(−) mol C(−1) or above, can vary depending on multiple factors. In addition, FRRf studies are limited in freshwater lakes where phosphorus limitation and cyanobacterial blooms are common. The goal of the present study is to construct a robust Ф(e,C) model for freshwater ecosystems using simultaneous measurements of ETR(PSII) by FRRf with multi-excitation wavelengths coupled with a traditional carbon fixation rate by the (13)C method. The study was conducted in oligotrophic and mesotrophic parts of Lake Biwa from July 2018 to May 2019. The combination of excitation light at 444, 512 and 633 nm correctly estimated ETR(PSII) of cyanobacteria. The apparent range of Ф(e,C) in the phytoplankton community was 1.1–31.0 mol e(−) mol C(−1) during the study period. A generalised linear model showed that the best fit including 12 physicochemical and biological factors explained 67% of the variance in Ф(e,C). Among all factors, water temperature was the most significant, while photosynthetically active radiation intensity was not. This study quantifies the in situ FRRf method in a freshwater ecosystem, discusses core issues in the methodology to calculate Ф(e,C), and assesses the applicability of the method for lake GPP prediction.