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 sen sors were deployed south-west of Gran Canaria island in the vicinity o f a cyclonic eddy. Diffuse upwelling irradiance was measured in four c hannels (corresponding to some of the SeaWIFS channels) centred on wav elengths 440, 490, 570 and 670 nm. Chlorophyll concentrations were obt ained at the start and end of each buoy deployment from profiles of fl uorescence calibrated against extracted chlorophyll samples. A weighte d chlorophyll concentration was calculated which corresponded to the d epth detected by the colour sensor. The ratio of blue (440 nm) to gree n (570 nm) wavelength diffuse upwelling irradiance was found to vary i n an 'n' shape throughout the day, and the size of the 'n' varied betw een days and buoy deployments. Possible causes for the observed variat ions 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 resu lts in the literature for oceanic waters, with very low near-surface c hlorophyll concentrations. Copyright (C) 1996 Elsevier Science Ltd.