Retrieval of chlorophyll concentration from MERIS measurements in the spectral range of the sun-induced chlorophyll fluorescence

The remote determination of the oceanic constituents in the upper ocean layer relies on the spectral information contained in the water leaving radiance. For waters of which the optical properties are mostly determined by phytoplankton and correlated degradation products ("case-I" waters), empirical or semi-empirical algorithms are successfully used to relate the water leaving radiance at two or more wavelengths to the chlorophyll concentration(1). These algorithms often fail in "case-II" waters where the water color is additionally influenced by sediments and non-autochtonous Colored Dissolved Organic Matter (CDOM). A promising method to derive chlorophyll concentration in case-II waters makes use of the sun-induced chlorophyll fluorescence (SICF) which is a feature specific to phytoplankton. The potential of the SICF for the space-borne retrieval of phytoplankton is reflected by the fact that several new satellite instruments (such as MERIS on ENVISAT) dispose of the required spectral channels to retrieve the SICF signal. In this article, we present an algorithm relating SICF to chlorophyll concentration in the topmost water layer. The algorithm has been derived from measurements gathered at more than 400 stations in different European waters and been applied to airborne imaging spectrometer data of European river plumes. Comparison with concurrent in situ measurements indicates that chlorophyll can be derived with a factor two error margin for the observed case-II waters. Recently, the algorithm was applied to one of the first available MERIS images and the retrieved chlorophyll concentration compared to results obtained from SeaWiFS; data using the standard OC4v4 retrieval scheme.