Remote sensing reflectance and inherent optical properties in the mid Chesapeake Bay

Abstract

We used an extensive set of bio-optical data to examine the relationships between inherent optical properties and remotely sensed quantities in an optically complex and biologically productive region of the Chesapeake Bay. Field observations showed that the chlorophyll algorithms used by the MODIS (MODerate resolution Imaging Spectroradiometer) ocean color sensor (i.e. Chlor_a, chlor_MODIS, chlor_a_3 products) do not perform accurately in these Case 2 waters. This is because, at high concentrations of chlorophyll, all MODIS algorithms are based on empirical relationships between chlorophyll concentration and blue:green wavelength remote sensing reflectance (Rᵣₛ) ratios that do not account for the typically strong blue-wavelength absorption by non-covarying, dissolved and non-algal particulate components. We found stronger correlation between chlorophyll concentration and red:green Rᵣₛ ratios (i.e. Rᵣₛ(677)/Rᵣₛ(554)). Regionally-specific algorithms that are based on the phytoplankton optical properties in the red wavelength region provide a better basis for satellite monitoring of phytoplankton blooms in these Case 2 waters. Our estimates of f/Q (proportionality factor in the relationship between Rᵣₛ and the ratio of water backscattering, bb, and absorption, a, coefficients, bb/(a+bb)) were not considerably different from f/Q previously estimated for same measurement geometry but Case 1 waters. Variation in surface bb significantly affected Rᵣₛ magnitude across the visible spectrum and was most strongly correlated (R² = 0.88) with observed variability in Rᵣₛ at 670 nm. Surface values of particulate backscattering were strongly correlated with non-algal particulate absorption, aₙₐₚ (R² = 0.83). These results, along with the measured backscattering fraction and non-algal particulate absorption spectral slope, indicate that suspended non-algal particles with high inor- ganic content are the major water constituents regulating bb variability in the studied region of the Chesapeake Bay. Remote retrieval of surface aₙₐₚ from Rᵣₛ(670) could be used in conjunction with inversion of UV-blue wavelengths to separate contribution by non-algal particles and colored dissolved organic matter to total light absorption, and monitor non-algal suspended particle concentration and distribution in these Case 2 waters.