Estimation of picophytoplankton and nanophytoplankton abundances in the Kuroshio extension using Sentinel-3/OLCI data

Picophytoplankton and nanophytoplankton play a central role in ocean primary production, carbon cycling, and food-web dynamics. Their highly dynamic distributions, however, make in situ measurements insufficient for large-scale monitoring. Satellite remote sensing provides continuous, large-scale optical observations, enabling the estimation of phytoplankton abundances and their spatiotemporal variability across regional to basin scales. In the Kuroshio Extension region of the western North Pacific, we combined high-frequency underway flow cytometry measurements of Synechococcus (SYN), Prochlorococcus (PRO), picoeukaryotes (PEUK), and nanoeukaryotes (NEUK) with Sentinel-3/OLCI reflectance spectra. By treating reflectance as a continuous spectral function, we extracted key spectral features, including blue-to-green band ratios and red-edge characteristics, to empirically estimate group-specific abundances. The resulting OLCI-derived products capture the spatiotemporal variability of pico- and nanophytoplankton communities and their responses to mesoscale and submesoscale physical processes.