Characterization of the bio-optical anomaly and diurnal variability of particulate matter, as seen from scattering and backscattering coefficients, in ultra-oligotrophic eddies of the Mediterranean Sea

摘要

The variability of inherent optical properties is investigated in theultra-oligotrophic waters of the Mediterranean Sea sampled during the BOUMexperiment performed during early summer 2008. Bio-optical relationshipsfound for ultra-oligotrophic waters of the three anticyclonic gyres sampledsignificantly depart from the mean standard relationships provided for theglobal ocean, confirming the peculiar character of these Mediterraneanwaters. These optical anomalies are diversely related to the specificbiological and environmental conditions occurring in the studied ecosystem.Specifically, the surface specific phytoplankton absorption coefficientexhibits values lower than those expected from the general relationshipsmainly in relation with a high contribution of relatively large sizedphytoplankton. Conversely, the particulate backscattering coefficient,b_bp, values are much higher than the mean standard values for a givenchlorophyll-a concentration, TChl-a. This feature can presumably be related to therelevant influence of highly refractive submicrometer particles of Saharanorigin in the surface layer of the water column. The present measurementsalso show that the Mediterranean Sea is greener than TChl-a alone indicates, asalready stressed in previous studies. This color anomaly is partly explainedby the estimated colored dissolved organic matter and submicrometerparticles absorption coefficients, and to a greater extent by the highb_bp/TChl-a values assuming that these particles backscatter light similarly inthe green and blue parts of the visible spectrum. The diel variation of boththe particulate matter attenuation and backscattering coefficients were alsoinvestigated specifically. Despite some differences in the timing and themagnitude of the daily oscillations found for these optical parameters,potential for the backscattering coefficient daily oscillation to be used,similarly to that for the attenuation coefficient, as a proxy for estimatingcarbon community production budget has been highlighted for the first time.This result is particularly relevant for present and future geostationaryspatial ocean color missions.