Diel changes of the optical backscattering coefficient of oceanic particulate matter determined from diel changes in apparent optical properties: a case study in the Mediterranean Sea (BOUSSOLE site)

Using in situ measurements of radiometric quantities and of the optical backscattering coefficient of particulate matter (bbp) at an oceanic site, we show that diel cycles of bbp are large enough to generate measurable diel variability of the ocean reflectance. This means that biogeochemical quantities such as net phytoplankton primary production, which are derivable from the diel bbp signal, can be potentially derived also from the diel variability of ocean color radiometry (OCR). This is a promising avenue for basin-scale quantification of such quantities because OCR is now performed from geostationary platforms that enable quantification of diel changes in the ocean reflectance over large ocean expanses. To assess the feasibility of this inversion, we applied three numerical inversion algorithms to derive bbp from measured reflectance data. The uncertainty in deriving bbp transfers to the retrieval of its diel cycle, making the performance of the inversion better in the green part of the spectrum (555 nm), with correlation coefficients >0.75 and a variability of 40% between the observed and derived bbp diel changes. While the results are encouraging, they also emphasize the inherent limitation of current inversion algorithms in deriving diel changes of bbp, which essentially stems from the empirical parameterizations that such algorithms include.