Validation of chlorophyll concentration algorithms in Apulian coastal areas

Monitoring and managing small coastal ecosystems require a considerable understanding of temporal dynamics of biophysical factors describing coastal water systems. For this reason, daily observation from space could be a very efficient tool. For this purpose, high resolution, multispectral satellite data have been successfully applied in the detection of chemical, biological and physical processes happening in coastal waters. However, monitoring coastal waters using the existing satellite instruments, is a challenging task which requires very sophisticate procedures. We know that in open oceans, case I waters, phytoplankton is the principal agent responsible for the optical properties of waters. Instead, the optical properties of shallow and coastal waters, case II waters, are influenced not only by phytoplankton, but also by other substances, generally classified as suspended inorganic particles and yellow substance [1]. In the framework of IMCA project (Integrated Monitoring of Coastal Area) an important set of in situ measurements has been carried out. Beside lab-measurements of water quality parameters and their inherent optical properties, a set of radiometric measurements have been performed. This measurements consist of downwelling irradiance above water, and in water profiles of downwelling irradiance, upwelling radiance, fluorimetric chlorophyll-a concentration, temperature and other ancillary quantities. These measurements are performed using the Satlantic Profile II, a free fall profiler with OCR- 507 I and OCR-507 R radiometers connected. The areas of interest are located in the south-east of Italy in the region of Apulia. In this region, three sites have been selected: 1) Taranto seas on the Ionian Sea. This site is characterized by a strong human activity for the presence of important industrial settlements with an high traffic commercial port and a base of Italian Military Marine; 2) Margherita di Savoia and estuarine of Ofanto river on the Adriatic Sea. The waters of this area are very turbid due to the presence of the river and for some industrial settlements near the small town; 3) Tremiti islands on the Adriatic sea, an important marine protected area characterized by clear water conditions. In these areas a total of 25 stations located within 3 km from the shore have been selected, and the measurements have been performed about twice per month, according on the cloud free satellite overpasses from Spring 2006 to Spring 2008. The aim of this paper is the validation of some bio-optical algorithms for the retrieval of chlorophyll-a concentration. The remote sensing reflectances derived from in situ measurements have been used as input for the algorithms, and the resulting chlorophyll concentrations have been compared to the one measured by the fluorimeter and with the one derived by the water samples using the HPLC technique. Three algorithms have been selected. The first one is a simple empirical algorithm[2]. It has been selected to test the hypothesis that the Tremiti site could be considered as case I area. The other two algorithms are semi-analytical ones: the GSM algorithm [3] and Carder algorithm[4].

This work has been supported by MIUR (Ministero dell’Università e della Ricerca), within the framework of IMCA project (Integrated Monitoring of Coastal Areas).