Impact of scattering and absorption of photosynthetic pigments on fluorescence retrieval algorithms for coastal waters

Abstract

Fluorescence Line Height (FLH) algorithms are effective for fluorescence retrieval in the open ocean where elastic reflectance in the fluorescence zone does not deviate much from the baseline. In coastal waters, FLH algorithms are significantly complicated by the overlap of the fluorescence and elastic reflectance peaks. To test accuracy of MODIS, MERIS and other FLH algorithms, we compared numerical simulations using an extensive synthetic database suitable for case II waters, with results of extensive field measurements of reflectance, absorption and attenuation spectra by us in the Chesapeake Bay, as well as satellite FLH data from several areas that typically show low correlation between [Chl] and FLH. Our synthetic datasets were created using the HYDROLIGHT radiative transfer code with IOP's connected to parameterized microphysical models in accordance with procedures used to generate the IOCCG dataset, but with some added improvements. These included higher (1 nm) spectral resolution, a wider range of parameters typical for coastal waters, including chlorophyll specific absorptions with significant variations in spectral shapes and magnitude. HYDROLIGHT simulations of elastic reflectance using measured attenuation/extinction spectra followed by subtraction from measured reflectance, permitted retrieval of the fluorescence contribution to the latter, for comparisons with the data set simulations. We find relatively small fluorescence contributions to surface reflectance for mineral concentrations > 5 mg/l because of strong attenuation in the excitation zone and enhanced elastic reflectance making fluorescence detection unrealistic. For lower mineral concentrations, we find that some combinations of NIR observation bands permit reasonably good FLH retrievals in conditions where specific absorption spectral variation is not very high, and that application of multi-spectral algorithms can be more efficient for the retrieval of fluorescence contributions in coastal areas.