On the challenges of retrieving phytoplankton properties from remote-sensing observations

Abstract

Remote-sensing satellites provide the only means to observe the entire ocean at high-temporal resolution. Optical sensors assess ocean color through estimates of remote-sensing reflectance (Rrs(λ)). We emphasize a physical degeneracy in the radiative transfer equation that relates Rrs(λ) to the absorption and backscattering coefficients (a(λ), bb(λ)) known as inherent optical properties (IOPs). This degeneracy stems from Rrs(λ) depending on the ratio bb(λ)/a(λ), preventing the independent retrieval of non-water IOPs without prior knowledge. We demonstrate that multi-spectral satellite observations lack the statistical power to recover more than three parameters describing non-water absorption and backscattering. Due to exponential-like absorption by colored dissolved organic matter and detritus at shorter wavelengths, multi-spectral Rrs(λ) data cannot detect phytoplankton absorption without strict priors, leading to biased and uncertain estimates. These results challenge decades of IOP retrieval literature, including assessments of phytoplankton growth and biomass. While hyperspectral observations hold promise to recover additional parameters, significant hurdles remain in accurately quantifying IOPs and phytoplankton biomass at a global scale.