Retrieving chlorophyll and non chlorophyllous matter from ocean colour satellite data in Baltic ''Case 2Y'' waters.

Based on in situ data collected in the Southern Baltic Proper at three different periods of the year this study investigates the possibility to empirically retrieve Chlorophyllous and Non Chlorophyllous (dissolved and particulate) matter absorption coefficients in the Baltic Sea using ''SeaWiFS'type'' spectral information. It relies on the classical relationship between '' remote-sensing '' reflectance and the back-scattering and absorption coefficients of the in-water optically active components (R(rs)= f/Q b(b)/a). The use of reflectance ratios minimizes the effect of the ''f/Q'' variability if one assume its low wavelength dependency. A reflectance ratio can therefore be expressed as the product of the total absorption ratio and of the total back-scattering wavelength dependency, classically defined as a power law of the wavelength ratio. Empirical relationships, valid for the considered data set, are used to estimate the exponent N of this power law from specific reflectance ratios, the 443/510 and 510/550 ratios being the best candidates. It is thus shown that, in these yellow substances dominated waters (''Case 2Y''), total absorption ratios could allow the estimation of non chlorophyllous matter absorption at 412 nm (using in particular the 665/412 ratio) and of pigment absorption at 443 nm (and consecutively of chlorophyll concentration).