Retrieval of phytoplankton size structure based on the spectral slope of phytoplankton absorption in the northern South China Sea
基于南海北部海区浮游植物吸收光谱斜率变化的粒级结构反演

Phytoplankton size structure is an important biological parameter that governs functioning of pelagic food-webs and consequently affects the rate of carbon cycle. Relationships between spectral phytoplankton absorption (aph(λ)) and phyto-plankton size structure were investigated based on in situ data collected in the northern South China Sea. The spectral slope of phytoplankton absorption in the range of 443 and 510 nm was used as an index (S) for describing phytoplankton size classes, which tended to increase with increasing dominant roles of large-sized phytoplankton. There existed positive correlations be-tween S and phytoplankton absorption coefficient and chlorophyll a concentration, which were then discussed by considering size-fractioned chlorophyll a contribution. Taking 40% as the cutoff, relatively clear division between pico- and mi-cro-phytoplankton dominated waters was found, with chlorophyll a concentration of ~0.70 mg m-3 and aph(443) of ~0.05 m-1, and accordingly the value of S being around 0.0004 (m nm)-1. A bio-optical model was developed between S and blue-to-green band ratio of remote sensing reflectance (Rrs(443)/Rrs(555 )), which showed good correlations with R2 being about 0.91. This model pro-vided much insight into the retrieval of phytoplankton size structure from ocean color remote sensing.