Variability of the inherent and apparent optical properties in a highly turbid coastal area: Impact for the validation of remote sensing algorithms

Rosa Astoreca, Véronique Rousseau, Jean-Yves Parent, Christiane Lancelot
Ecologie des Systèmes Aquatiques (ESA), Université Libre de Bruxelles, Campus Plaine -
CP 221, Boulevard du Triomphe, B-1050 Brussels, Belgium.
rastorec@ulb.ac.be

Kevin Ruddick, Barbara Van Mol
Management Unit of the North Sea Mathematical Models (MUMM),
100 Gulledelle, B-1200 Brussels, Belgium.

ABSTRACT

The Southern Bight of the North Sea is characterised by the large influence of river inputs, which results in heavy eutrophication of the area. High values of chlorophyll and suspended matter have been reported for this area, in relation with blooms of different species and influence of bottom sediments. These characteristics make this area interesting from an optical point of view, since the main components of the water are co-varying seasonally and regionally. A considerable effort has been put in the past years to retrieve chlorophyll and suspended matter maps from satellite imagery under the scope of the BELCOLOUR project. An optical characterisation of the area is essential for understanding the satellite imagery, since the main components of the water are largely contributing to the total absorption, this will have an impact on the retrieval of chlorophyll a and total suspended matter.

For this purpose, extensive sampling of these water constituents has been carried out in the area, at different seasons during 2003 and 2004. Inherent optical properties (IOPs): particle, detrital, phytoplankton and CDOM absorption have been measured spectrophotometrically, and in situ using an ac-9 for total absorption and particle scattering. In parallel, apparent optical properties (AOPs), i.e. water-leaving reflectance (Rrs) have been measured with a TriOS instrument.

In this paper we will show some examples of the high variability of IOPs in the area and how it impacts the AOPs. The contribution of phytoplankton, CDOM and non-algal particles has been shown to vary between coast and offshore, and also between estuary and Atlantic-dominated waters. The impact of this variation on the Rrs spectra will be assessed and the values of Rrs measured will be compared with those processed with the in-situ data. This is the first step to validate the CHL and TSM retrieval algorithms for this highly turbid water area and estimate their sensitivity.

Last Update: 2005-04-7