Remote sensing of size structure of phytoplankton communities using optical properties of the Chukchi and Bering Sea shelf region

Abstract. Recent ocean warming and subsequent sea ice decline resulting from climate change could affect the northward shift of the ecosystem structure in the Chukchi Sea and Bering Sea shelf region (Grebmeier et al., 2006b). The size structure of phytoplankton communities provides an index of trophic levels that is crucial to understanding the mechanisms underlying such ecosystem changes and their implications for the future. This study proposes a new ocean color algorithm for deriving this characteristic by using the region's optical properties. The size derivation model (SDM) estimates the phytoplankton size index F_L on the basis of size-fractionated chlorophyll-a (chl-a) using the light absorption coefficient of phytoplankton, a_ph(λ), and the backscattering coefficient of suspended particles including algae, b_bp(λ). F_L was defined as the ratio of algal biomass attributed to cells larger than 5 μm to the total. It was expressed by a multiple regression model using the a_ph(λ) ratio, a_ph(488)/a_ph(555), which varies with phytoplankton pigment composition, and the spectral slope of b_bp(λ), γ, which is an index of the mean suspended particle size. A validation study demonstrated that 69% of unknown data are correctly derived within F_L range of ±20%. The spatial distributions of F_L for the cold August of 2006 and the warm August of 2007 were compared to examine application of the SDM to satellite remote sensing. The results suggested that phytoplankton size was responsive to changes in sea surface temperature. Further analysis of satellite-derived F_L values and other environmental factors can advance our understanding of ecosystem structure changes in the shelf region of the Chukchi and Bering Seas.