Consequences of a Synechococcus bloom upon the optical properties of oceanic (case 1) waters

A dense Synechococcus population has been observed in oceanic offshore waters (400 km from the Mauritanian coast) within the upper 30-m-thick layer. With cell counts >3 x 10(5) cells ml(-1), about half (0.6) of the total Chi a concentration (1.25 mg m(-3)) was attributable to the cyanobacteria. The optical properties of this water body markedly differed from those of case 1 waters with the same Chin concentration. The euphotic depth was diminished (by similar to 10 m on average) and the spectral attenuation coefficients for downwelling irradiance were increased (by 40-60% in the green and blue pans of the spectrum); the absorption band characteristic of phycoerythrin was easily detected on filtered material, as well as within the in-water light field and in the water-leaving upward radiant flux. The amplitude of the departures of the actual optical properties from those predicted for normal case 1 waters are described and then analyzed through radiative transfer computations. The peculiar Chi-specific absorption capabilities of Synechococcus (originating from the small size and the presence of phycobilins and zeaxanthin), rather than a significant increase in the bulk scattering properties, are at the origin of the deviations observed. The possibility of detecting such a particular algal bloom, through remote-sensing techniques based on the ocean reflectance, requires a high radiometric sensitivity and dedicated spectral channels. If detection is feasible, quantifying the cyanobacterial biomass from satellite information remains a difficult task. The variability in the relative proportions of phycobilins and Chi a for these organisms impedes a quantitative assessement of their contribution to the total Chl biomass.