Abstract
AbstractDirect 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 chlorophylla(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 ETRPSIIremains challenging because the electron requirement for carbon fixation (Фe,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 Фe,Cmodel for freshwater ecosystems using simultaneous measurements of ETRPSIIby FRRf with multi-excitation wavelengths coupled with traditional carbon fixation rate by the13C 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 ETRPSIIof cyanobacteria. The range of Фe,Cin the phytoplankton community varied from 1.1 to 31.0 mol e−mol C−1during the study period. Generalized liner model showed the best model including 12 physicochemical and biological factors explained 67% of the variance in Фe,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 Фe,Cmodel relative to13C (GPP13C) varied 0.5–1.5. Further,GPPfestimated with more parsimonious Фe,Cmodels were also comparable toGPP13C. This study quantifies the applicability of thein situFRRf methodology, and supports continuous monitoring of GPP by FRRf in lakes with large spatio-temporal variability of environmental conditions and phytoplankton assemblages.
Publisher
Cold Spring Harbor Laboratory