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-thic k 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 dif fered from those of case 1 waters with the same Chin concentration. Th e euphotic depth was diminished (by similar to 10 m on average) and th e spectral attenuation coefficients for downwelling irradiance were in creased (by 40-60% in the green and blue pans of the spectrum); the ab sorption band characteristic of phycoerythrin was easily detected on f iltered material, as well as within the in-water light field and in th e water-leaving upward radiant flux. The amplitude of the departures o f the actual optical properties from those predicted for normal case 1 waters are described and then analyzed through radiative transfer com putations. The peculiar Chi-specific absorption capabilities of Synech ococcus (originating from the small size and the presence of phycobili ns and zeaxanthin), rather than a significant increase in the bulk sca ttering 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 ra diometric sensitivity and dedicated spectral channels. If detection is feasible, quantifying the cyanobacterial biomass from satellite infor mation remains a difficult task. The variability in the relative propo rtions of phycobilins and Chi a for these organisms impedes a quantita tive assessement of their contribution to the total Chl biomass.