Opfics & Laser Technology, Vol. 29, No. 1, pp. 3-8, 1997 Copyright fZ 1996 Elsevier Science Ltd ELSEVIER ADVANCED TECHNOLOGY PII: SOO30-3992(96)00047-3 Printed in Great Britain. All rights reserved 0030&3992/97 $17.00 + 0.00 Observations of diffuse upwelling irradiance and chlorophyll in case I waters near the Canary Islands (Spain) K. WILD-ALLEN, P. TETT, D. BOWERS 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 SeaWlFS 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. Copyright @ 1996 Elsevier Science Ltd. zyxwvutsrqponmlkjih KEYWO RDS: upwelling irradiance, chlorophyll, drifting buoy Intro d uc tio n The principal optical characteristics of case I waters are defined by their phytoplankton content. The phytoplankton pigment chlorophyll absorbs, scatters and reflects light giving the ocean a green tint. The intensity of this ocean colour reflects the concentration of phytoplankton biomass and its proximity to the surface. Large-scale synoptic observations of ocean colour by satellite have been used to estimate pigment biomass112 and, combined with ship data, primary productivity3. Optical instruments have also been deployed on moorings to give continuous measurements of bio-optical properties for comparison with model algorithms4. More recently, optical instruments have been incorporated in drifting buoys, to observe the evolution of phytoplankton biomass in upwelling filaments off the California coast5. KW-A and DB are in the School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Gwynedd LL59 5EY, UK. PT is in the Department of Biological Sciences, Napier University, 10 Colinton Road, Edinburgh EHlO 5DT, UK. Received 20 November 1995. Accepted 11 December 1995. This paper aims to establish the calibration of a 4-channel downward-looking colour sensor with chlorophyll concentration for case I waters near the Canary Islands. The colour sensor was designed at the University of Wales, Bangor6 and incorporated into a novel instrumented drifting buoy (Fig. 1). Methods Me a sure m e nt o f o c e a n c o lo ur Ocean colour was measured as diffuse upwelling irradiance in four colour channels (corresponding to some of the SeaWiFS channels), centred on wavelengths 440, 490, 570 and 670 nm. The aim was to use colour ratios which are known to vary with chlorophyll concentration-for example, of blue (440 nm) to yellow (570 nm) light-to estimate chlorophyll concentration in the surface mixed layer. It should therefore be possible to develop an instrument specific calibration between colour ratio and chlorophyll concentration for case I waters. Observed colour ratios can also be transformed to surface reflectance ratios which are then directly comparable with the universal relationship established by Gordon and More12. To establish a calibration between colour ratio and chlorophyll, it is necessary to evaluate the depth of 3