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Ship-borne measurements of ocean colour: Development of a CCD-based reflectance radiometer and trials on a longitudinal transect of the Atlantic Ocean

A ship-borne reflectance radiometer was constructed capable of making sequential downward irradiance (Ed) and upward radiance (Lu) measurements from a fixed position over a wavelength range from 350 to 800 nm with 4 nm spectral resolution. The radiometer used a cooled CCD array as the sensing element and a bifurcated fibre optic bundle for the optical inputs. It was mounted on a ship's superstructure so that the radiance collector viewed the sea surface at the Brewster angle (53° to the vertical) through a linear polarizing filter. Comparative measurements were made with the polarizer oriented horizontally and vertically in order to estimate the spectral effect of skylight reflected directly from the sea surface on reflectance spectra (Lu/Ed) measured in air. Regression algorithms based on waveband ratios were devised to relate the reflectance spectra to surface chlorophyll concentrations and to the diffuse attenuation coefficient for downwards irradiance at 490nm. The results obtained were well correlated with in situ optical measurements and with chemical analyses of water samples. Ship-borne reflectance data obtained during an Atlantic Meridianal Transect cruise from the UK to the Falklands in September 1997 showed high particle concentrations in the English Channel and high chlorophyll concentrations in the Mauritanian upwelling and the south Atlantic.