A field study of optical and physical properties in Muuga Bay, Gulf of Finland, was performed on 30 May 2007. In one of the studied stations traces of ballast water were visible on the water surface. An Advanced Synthetic Aperture Radar (ASAR) image was received and analysed from the same area. The vertical profiles of the absorption and attenuation coefficients together with the temperature and salinity profiles were measured in 13 stations. The concentrations of oil products, chlorophyll a,

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... loured dissolved organic matter, and suspended particulate matter were determined from water samples. The spatial distribution of temperature, salinity, and optically active substances indicated four distinct areas: the southern coastal area with saline, cold, and chlorophyll a poor upwelling water; the western coast of the bay with warmer and chlorophyll a richer surface water; open water dominated by higher concentrations of coloured dissolved organic matter; and Muuga harbour where the water had a high concentration of suspended particulate matter and chlorophyll a, which caused stronger light attenuation and a thinner euphotic layer compared to the other parts of the bay. Regression analysis showed that the absorption coefficient at 676 nm correlated well with the chlorophyll a concentration and the scattering coefficient at 555 nm correlated with the suspended matter concentration in Muuga Bay. The correlation coefficient between ASAR data and oil products was 0.71 although the concentration of oil products was relatively low (0.01-1.72 ppm).