Detection of offshore plankton blooms with AVHRR and SAR imagery

Abstract

The ability of the Advanced Very High Resolution Radiometer (AVHRR) to detect plankton blooms in sediment-free oceanic waters was evaluated. After atmospheric correction, the AVHRR visible channel was found to detect plankton blooms resulting in chlorophyll-a concentrations higher than 3-4 mg m -3 in upwelling features off the US west coast. The AVHRR-derived radiance value also showed positive correlation (R2 = 0.64) with increasing chlorophyll concentrations. In the absence of higher quality ocean colour data, the AVHRR was utilised to image patterns of high plankton concentrations in offshore-flowing upwelling plumes, and to relate the plankton distributions to the occurrence of low backscatter patterns in time-coincident ERS-1 Synthetic Aperture Radar (SAR) imagery. The results indicate that the observed low backscatter areas were caused by the accumulation of biological surfactants released by the plankton blooms. The local surface flow field strongly affected the actual locations and shapes of the surfactant features. Our findings suggest the potential for using satellite ocean colour data in conjunction with SAR imagery to help separate natural slicks from those caused by oil or human activity.