Detection of algal blooms in European waters based on satellite chlorophyll data from MERIS and MODIS

Abstract

A technique for algal-bloom detection in European waters is described, based on standard chlorophyll a concentration (Chl) data from two ocean-colour sensors, the Moderate Resolution Imaging Spectroradiometer (MODIS) and Medium Resolution Imaging Spectrometer (MERIS). Comparison of the two data sources shows good agreement in case 1 waters, whereas the difference is significant in coastal waters including turbid areas. A relationship between the water-leaving reflectance at 667 nm and Chl for case 1 waters was used to eliminate pixels where Chl retrieval is contaminated by backscatter from inorganic suspended matter. Daily Chl data are compared to a predefined threshold map to determine whether an algal bloom has occurred. In this study, a threshold map was defined as the 90th percentile of previous years' data to take account of regional differences in typical Chl levels, with separate maps for each sensor to take account of sensor-specific bias. The algal-bloom detection processing chain is described, and example results are presented.

Acknowledgements

This study was funded by the Belgian Science Policy Office's STEREO programme in the framework of the project SR/00/104 (BELCOLOUR-2) and by the European Space Agency (ESA) in the framework of the GMES-MARCOAST project. MERIS and MODIS data were received from the MARCOAST OceanColour data service provided by ACRI-ST (Cat-1 AO.1521). ESA and NASA are also acknowledged for the MERIS and MODIS data, respectively. Philippe Garnesson of ACRI-ST is acknowledged for preparation and delivery of the daily composite data. The anonymous reviewers are thanked for their careful reading of the text.