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

Abstract

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 (ETRPSII) at photosystem II (PSII) of phytoplankton in real time. However, derivation of phytoplankton GPP in carbon units from ETRPSII remains challenging because the electron requirement for carbon fixation ([FE]e,C) can vary depending on multiple factors. Also, the FRRf is still relatively novel, especially in freshwater ecosystems where phosphorus limitation and cyanobacterial blooms are common. The goal of the present study is to construct a robust [FE]e,C model for freshwater ecosystems using simultaneous measurements of ETRPSII by FRRf with multi-excitation wavelengths coupled with traditional carbon fixation rate by the 13C method. The study was conducted in oligotrophic and mesotrophic areas in Lake Biwa from July 2018 to May 2019. The combination of excitation light at 444, 512 and 633 nm correctly estimated ETRPSII of cyanobacteria. The range of [FE]e,C in the phytoplankton community varied from 1.1 to 31.0 mol e- mol C-1 during the study period. Generalized liner model showed the best model including 12 physicochemical and biological factors explained 67% of the variance in [FE]e,C. Among all factors, water temperature was the most significant, while PAR intensity was not. The GPP values estimated by FRRf (GPPf) with the best [FE]e,C model relative to 13C (GPP13C) varied 0.5-1.5. Further, GPPf estimated with more parsimonious [FE]e,C models were also comparable to GPP13C. This study quantifies the applicability of the in situ FRRf methodology, and supports continuous monitoring of GPP by FRRf in lakes with large spatio-temporal variability of environmental conditions and phytoplankton assemblages.