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

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Abstract
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 × 105 cells ml−1, about half (0.6) of the total Chl 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 Chl a concentration. The euphotic depth was diminished (by ~10 m on average) and the spectral attenuation coefficients for downwelling irradiance were increased (by 40– 60% in the green and blue parts 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 Chl‐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 Chl a for these organisms impedes a quantitative assessement of their contribution to the total Chl biomass.

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