Observations of diffuse upwelling irradiance and chlorophyll in case I waters near the Canary Islands (Spain)

Two drifting buoys equipped with downward looking 4-channel colour sensors were deployed south-west of Gran Canaria island in the vicinity of a cyclonic eddy. Diffuse upwelling irradiance was measured in four channels (corresponding to some of the SeaWIFS channels) centred on wavelengths 440, 490, 570 and 670 nm. Chlorophyll concentrations were obtained at the start and end of each buoy deployment from profiles of fluorescence calibrated against extracted chlorophyll samples. A weighted chlorophyll concentration was calculated which corresponded to the depth detected by the colour sensor. The ratio of blue (440 nm) to green (570 nm) wavelength diffuse upwelling irradiance was found to vary in an 'n' shape throughout the day, and the size of the 'n' varied between days and buoy deployments. Possible causes for the observed variations and the problems encountered in calibrating the ratio of blue to green light with chlorophyll concentration are discussed. The surface reflectance ratio of blue to green irradiance was consistent with results in the literature for oceanic waters, with very low near-surface chlorophyll concentrations.