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

Spaceborne 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 South China Sea (SCS). Comparison between estimated and in situ measured total PP showed the best performance of U10 with an
$R^{2}$
of 0.65 and mean absolute percentage deviation (MAPD) of 31.11%. B17 produced better accuracy than U10 for PP from picophytoplankton (PP
_pico
) and microphytoplankton (PP
_micro
), whereas U10-derived PP from nanophytoplankton (PP
_nano
) agreed better with in situ measurements than B17-derived PP
_nano
. Sensitivity analysis showed that PP estimation was most sensitive to chlorophyll-a (Chla), followed by euphotic zone depth (
$Z_{\mathrm {eu}}$
), photosynthetically available radiation (PAR), and physiological parameters. With regional optimization of
$Z_{\mathrm {eu}}$
, phytoplankton size class (PSC), photosynthesis-irradiance curve parameters, and Chla profile modeling, B17 was tuned for the SCS. Its accuracy was greatly improved, as indicated by slopes close to 1,
$R^{2}\text{s} >$
0.55, and MAPD < 145%, for all size classes and total PP. Application of the tuned algorithm to satellite produced data demonstrated a decreasing trend for the total PP, PP
_pico
, and PP
_nano
while no apparent trend for PP
_micro
. 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.