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[1] The peculiar bio-optical properties of the northernmost waters of the Baltic Sea (Bothnian Sea and Bothnian Bay) are illustrated by exploiting unique in situ measurements of apparent and inherent optical properties collected in 2007. Specifically, this study shows that the high values of absorption by colored dissolved organic matter (ays(412) around 1.4 m−1) resulting from the presence of humic matter brought by the numerous Swedish and Finnish rivers alimenting the basin are not correlated with high concentrations of particles as generally observed in the southern Baltic Sea. As a consequence, extremely low values (i.e., less than 0.5) of the ratio of scattering to absorption are observed in the blue spectral region. The corresponding spectra of normalized water leaving radiance are close to satisfy the “black pixel” assumption in the blue and even in the green part of the visible spectrum. Consequently, the Bothnian Sea and most of all the Bothnian Bay, appear ideal candidates for studies on atmospheric corrections and vicarious calibration of ocean color sensors in coastal waters.

[1] SeaWiFS, MODIS and MERIS remote sensing radiometric products were assessed using in situ data collected at a coastal site in the northern Adriatic Sea from May 2002 to September 2005. The analysis was restricted to satellite and in situ data taken within one hour interval to minimize the effects of temporal variability of atmosphere and seawater around the measurement site. The comparison of SeaWiFS and MODIS normalized water-leaving radiances with in situ values showed averages of relative percent difference varying from 0% to −9% in the 443–555 nm spectral range. Higher values ranging from +15 to +42% were observed for the MERIS data in the equivalent spectral range (i.e., 443–560 nm).

The Aerosol Robotic Network (AERONET) was developed to support atmospheric studies at various scales with measurements from worldwide distributed autonomous sunphotometers [Holben et al. 1998]. AERONET has now extended its support to marine applications through the additional capability of measuring the radiance emerging from the sea with modified sun-photometers installed on offshore platforms like lighthouses, navigation aids, oceanographic and oil towers. The functionality of this added network component called AERONET - Ocean Color (AERONET-OC), has been verified at different sites and deployment structures over a four year testing phase. Continuous or occasional deployment platforms (see Fig. 1) included: the Acqua Alta Oceanographic Tower (AAOT) of the Italian National Research Council in the northern Adriatic Sea since spring 2002; the Martha s Vineyard Coastal Observatory (MVCO) tower of the Woods Hole Oceanographic Institution in the Atlantic off the Massachusetts coast for different periods since spring 2004; the TOTAL Abu-Al-Bukhoosh oil Platform (AABP, shown through an artistic rendition in Fig. 1) in the Persian (Arabian) Gulf in fall 2004; the Gustaf Dal n Lighthouse Tower (GDLT) of the Swedish Maritime Administration in the Baltic Sea in summer 2005; and the platform at the Clouds and the Earth's Radiant Energy System (CERES) Ocean Validation Experiment (COVE) site located in the Atlantic Ocean off the Virginia coast since fall 2005. Data collected during the network testing phase, confirm the capability of AERONET-OC to support the validation of marine optical remote sensing products through standardized measurements of normalized water-leaving radiance, LWN, and aerosol optical thickness, a, at multiple coastal sites.