Estimation of chlorophyll-a concentration in waters over the continental shelf of the Bay of Biscay: a comparison of remote sensing algorithms

Abstract

Ocean colour imagery is used increasingly as a tool to assess water quality via chlorophyll-a concentration (chl-a) estimations in European waters. The Bay of Biscay is affected by major river discharges, which alter the constituents of the marine waters. Chlorophyll-a algorithms, designed for use at global scales, are less accurate due to the variability of optically active in-water constituents. Hence, regionally parameterized empirical algorithms are necessary. The main objective of the present study was to develop a regional algorithm to retrieve chl-a in surface water using in situ R _rs, for a subsequent application to Medium Resolution Imaging Spectrometer (MERIS) satellite images. To address this objective, a platform was developed initially and a measurement procedure adapted for the field HR4000CG Spectrometer. Subsequently, the procedure was tested during a survey over the south-eastern Bay of Biscay (North-East Atlantic Ocean), to establish a MERIS chl-a algorithm for the area, by comparing different global remote sensing chl-a algorithms, with band ratios. Results validated with the jackknife resampling procedure show a satisfactory relationship between the R _rs(510)/R _{r s}(560) and chl-a (R ² _jac = 0.681). This ratio is better correlated to chl-a than those obtained with established chl-a remote sensing algorithms. High content in coloured dissolved organic matter (CDOM > 0.4 m⁻¹) and suspended particulate matter (SPM > 2.8 mg l⁻¹) influenced this relationship, with yellow substances having a stronger effect.

Acknowledgements

This research was supported by the Ministry of Education and Science (Spanish Government, Ref.: ESP2006-10411) and the Funds for Aquitania–Euskadi cooperation (Basque Government). S. Novoa benefited from a PhD scholarship granted by the Fundación de Centros Tecnológicos Iñaki Goenaga. MERIS data were provided by the ESA (ESA Category-1 scheme, proposal nr. 5395). The authors acknowledge the contribution of Dr J. Franco for his valuable technical support on chlorophyll estimation; thanks are also due to M. Santos, coordinator of 2008 BIOMAN campaign, and to Dr A. Borja and Professor Michael Collins (School of Ocean and Earth Science, University of Southampton and AZTI-Tecnalia) for their useful comments. This is the contribution 490 from the AZTI-Technalia Marine Research Division.