Inherent optical property inversion of ocean color spectra and its biogeochemical interpretation: 1. Time series from the Sargasso Sea

A nonlinear statistical method for the inversion of ocean color spectra is used to determine three inherent optical properties (IOPs), the absorption coefficients for phytoplankton and dissolved and detrital materials, and the backscattering coefficient due to particulates. The inherent optical property inversion model assumes that (1) the relationship between remote-sensing reflectance and backscattering and absorption is well known, (2) the optical coefficients for pure water are known, and (3) the spectral shapes of the specific absorption coefficients for phytoplankton and dissolved and detrital materials and the specific backscattering coefficient for particulates are known. This leaves the magnitudes for the three unknown coefficients to be determined. A sensitivity analysis is conducted to determine the best IOP model configuration for the Sargasso Sea using existing bio-optical models. The optical and biogeochemical measurements used were collected as part of the Bermuda Bio-Optics Project and the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time Series (BATS). The results demonstrate that the IOP model is most sensitive to changes in the exponential decay constant used to model absorption by dissolved and detrital materials. The retrieved chlorophyll a estimates show excellent correspondence to chlorophyll a determinations (r2 = 81%), similar to estimates from standard band ratio pigment algorithms, while providing two additional retrievals simultaneously. The temporal signal of retrieved estimates of absorption by colored dissolved and detrital materials is mirrored in ratios of K d (410) to K d (488), a qualitative indicator for nonalgal light attenuation coefficients. The backscatter coefficient for particles is nearly constant in time and shows no correspondence with the temporal signal observed for chlorophyll a concentrations. Last, the TOP model is evaluated using only those wavelengths which closely match the Sea Viewing Wide Field of View Sensor wave bands. This results in only a to 6% decrease in hindcast skill with the BATS biogeochemical data set. This is encouraging for the long-range goal of applying the IOP model to data from upcoming ocean color satellite missions.

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