The Southern Bight of the North Sea is characterised by a large influence of river inputs, which results in significant eutrophication of the coastal area. High values of chlorophyll (CHL) and total suspended matter (TSM) have been reported for these waters, associated with blooms of different species and resuspension of bottom sediments. Optical properties vary considerably both seasonally and regionally. Considerable effort has been taken in the past years to retrieve CHL and TSM maps from satellite imagery. An optical characterisation of the area is essential for the understanding of the satellite imagery. In particular, algorithms for retrieval of CHL and TSM are based on assumptions regarding IOPs which may not be valid or may need to be modified for this region and/or for certain seasons. For this purpose, extensive sampling of these water constituents was carried out in the Belgian and adjacent coastal areas, in different seasons during 2004. Inherent optical properties (IOPs): particulate, non-algal particle (NAP), phytoplankton and coloured dissolved organic matter (CDOM) absorption were measured spectrophotometrically, and an absorption meter for total absorption and particle scattering was used in situ. Simultaneously, the apparent optical property (AOP) water-leaving radiance reflectance has been measured with a system of TriOS spectroradiometers. In this paper, we show some examples of the high variability of IOPs in the area and how it impacts the water-leaving reflectance, and hence retrieval of products such as CHL and TSM. The contribution of phytoplankton, CDOM and NAP has been shown to vary between coast and offshore and also between estuary and Atlantic-dominated waters. The impact of this variation is assessed for a red/NIR algorithm for CHL retrieval. The results show that neglecting CDOM and NAP absorption causes CHL overestimation at low CHL. Variation of CHL-specific phytoplankton absorption is also shown to be a key source of retrieval error. This is the first step to provide a regional ...