Optical determination of phytoplankton size composition from global SeaWiFS imagery

Phytoplankton cell size is important to biogeochemical and food web processes. The goal of this study is to estimate phytoplankton cell size distribution from satellite imagery of spectral remote sensing reflectance (R_rs(λ)). Previous studies have indicated phytoplankton size classes have distinctive absorption spectra despite the physiological and taxonomic variability within an assemblage. For this study, the chlorophyll specific absorption spectra for phytoplankton size class extremes, pico- and microphytoplankton, are weighted by the percent microplankton (S_fm) and are the basis of phytoplankton size retrieval from SeaWiFS imagery. Satellite retrievals of S_fm are done through implementation of a forward optical model look-up table (LUT) that incorporates the range of absorption and scattering variability due to phytoplankton size, chlorophyll concentration ([Chl]) and dissolved and detrital matter (a_cdm(443)) in the global ocean from which R_rs(λ) is calculated by the radiative transfer software, Hydrolight. The Hydrolight modeled R_rs(λ) options for a given combination of [Chl] and a_cdm(443) within the LUT vary only due to S_fm. For a given pixel, the LUT search space was limited by satellite imagery of [Chl] and a_cdm(443). Within the narrowed search space, SeaWiFS R_rs(λ) was matched with the closest LUT R_rs(λ) option and the associated S_fm was assigned. Thresholds at which changes in R_rs(λ) due to S_fm could be discerned were established in terms of [Chl] and a_cdm(443). In situ high-precision liquid chromatography-derived estimates of cell size are used in conjunction with matched daily satellite estimates of S_fm for validation and agree well. A single month is displayed as an example of the S_fm retrieval.