Retrieval of phytoplankton size parameter from phytoplankton absorption spectra

Data from three cruises conducted in the northern South China Sea (SCS),the Daya Bay and the Pearl River Estuary,respectively,were examined to access the variations of phytoplankton size structure. A spectral mixing model was established to retrieve phytoplankton size parameter from phytoplankton absorption spectra,with Sf as a proxy of the size parameter. There were significant similarities in phytoplankton size structure in the estuarine and coastal waters; however,it is very dif-ferent in the open ocean. In the estuarine and coastal waters,the micro-phytoplankton plays a dominant role,while in the open ocean the pico-phytoplankton was the dominant species. Phytoplankton size parameter tended to decrease with increasing micro-phytoplankton,and to increase with increasing pico-phytoplankton. There was an increasing trend for chlorophyll a concentration from the open ocean to the coastal sea. Phytoplankton size parameter showed a power function relation with chlorophyll a concentration and decreased with increasing chlorophyll a concentration. The results showed that there was a relationship between phytoplankton size parameter retrieved from the spectral mixing model and the bio-optical characteristics in different circumstances of the SCS. This spectral mixing model provided a simple tool for extracting ecological information of phytoplankton species from optical measurements,and can be used to analyze the influence of dominant phytoplankton size structure on optical characteristics.