Evaluation of Satellite-Derived Size-Fractionated Phytoplankton Primary Production in the South China Sea

Space-borne observations of primary production (PP) are essential for understanding the role of phytoplankton in the biogeochemical cycles on a global or regional scale. In this study, the vertically generalized production model (VGPM) and two size-fractionated PP algorithms (namely U10 and B17) were evaluated in the SCS. Comparison between estimated and in situ measured total PP showed the best performance of U10 with anR2of 0.65 and mean absolute percentage deviation (MAPD) of 31.11%. B17 produced better accuracy than U10 for PP from pico-phytoplankton (PPpico) and micro-phytoplankton (PPmicro), whereas U10-derived PP from nano-phytoplankton (PPnano) agreed better with in situ measurements than B17-derived PPnano. Sensitivity analysis showed that PP estimation was most sensitive to chlorophyll-a (Chla), followed by euphotic zone depth (Zeu), and photosynthetically available radiation (PAR), and physiological parameters. With regional optimization of Zeu, phytoplankton size class (PSC), photosynthesis-irradiance curve parameters, and Chla profile modelling, B17 was tuned for the SCS. Its accuracy was greatly improved, as indicated by slopes close to 1,R2s > 0.55, and MAPDpico, and PPnanowhile no apparent trend for PPmicro. This study represents the feasibility of improving PP estimation from satellite observation for regional applications through retuning to local data. Our findings are useful for understanding the ecological response to climate change in the region and unveil the role of size-fractionated phytoplankton in biogeochemical and carbon cycles on a basin scale.