Lake Biwa Branch Office, National Institute for Environmental Studies, Otsu, Shiga, Japan.
Center for Regional Environmental Research, National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan.
PLoS One. 2021 Feb 2;16(2):e0238013. doi: 10.1371/journal.pone.0238013. eCollection 2021.
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, the derivation of phytoplankton GPP in carbon units from ETRPSII remains challenging because the electron requirement for carbon fixation (Фe,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 Фe,C model for freshwater ecosystems using simultaneous measurements of ETRPSII by FRRf with multi-excitation wavelengths coupled with a traditional carbon fixation rate by the 13C 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 ETRPSII of cyanobacteria. The apparent range of Фe,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 Фe,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 Фe,C, and assesses the applicability of the method for lake GPP prediction.
直接测量水柱中的总初级生产力（GPP）是必不可少的，但可能受到空间和时间的限制。快速重复率荧光法（FRRf）是一种基于叶绿素 a（Chl-a）荧光的生物光学技术，可实时估算浮游植物光合作用系统 II（PSII）的电子传递速率（ETRPSII）。然而，从 ETRPSII 推导出浮游植物 GPP 以碳单位表示仍然具有挑战性，因为碳固定所需的电子（Фe，C）在机制上为 4 摩尔电子摩尔 C-1 或更高，这可能因多种因素而异。此外，FRRf 研究受到限制，因为在富营养化湖泊中，磷限制和蓝藻水华很常见。本研究的目的是使用 FRRf 同时测量多激发波长的 ETRPSII 并结合传统的 13C 方法测定的碳固定率，为淡水生态系统构建一个稳健的 Фe，C 模型。该研究于 2018 年 7 月至 2019 年 5 月在琵琶湖的贫营养和中营养区进行。444、512 和 633nm 的激发光组合可正确估算蓝藻的 ETRPSII。在研究期间，浮游植物群落中 Фe，C 的表观范围为 1.1-31.0 摩尔电子摩尔 C-1。广义线性模型表明，包括 12 个物理化学和生物因素的最佳拟合解释了 Фe，C 方差的 67%。在所有因素中，水温是最重要的，而光合有效辐射强度则不是。本研究量化了淡水生态系统中现场 FRRf 方法，讨论了计算 Фe，C 方法中的核心问题，并评估了该方法对湖泊 GPP 预测的适用性。
