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A hand-held spectroradiometer was used for above-water determination of subsurface spectral irradiance reflectance in the Scheldt Estuary (Belgium/The Netherlands), the North Sea off the Belgian coast, and the Hudson/Raritan Estuary (New York/New Jersey). On the North Sea the measurement conditions were adverse, and elsewhere broken cloud caused considerable spectral variation. Despite this variation the retrieval of chlorophyll a (Chl-a) from three reflectance spectra at all sampling stations was stable. The algorithm calibrated for the freshwater IJssel Lagoon (The Netherlands) proved to be applicable to these estuarine and coastal waters(N=30; standard error of estimate = 7 mg m(-3) for corrected Chi-a ranging from 1 to 93 mg m(-3)).