Abstract
Abstract. The variability of inherent optical properties is investigated in the ultra-oligotrophic waters of the Mediterranean Sea sampled during the BOUM experiment performed during early summer 2008. Bio-optical relationships found for ultra-oligotrophic waters of the three anticyclonic gyres sampled significantly depart from the mean standard relationships provided for the global ocean, confirming the peculiar character of these Mediterranean waters. These optical anomalies are diversely related to the specific biological and environmental conditions occurring in the studied ecosystem. Specifically, the surface specific phytoplankton absorption coefficient exhibits values lower than those expected from the general relationships mainly in relation with a high contribution of relatively large sized phytoplankton. Conversely, the particulate backscattering coefficient, bbp, values are much higher than the mean standard values for a given chlorophyll-a concentration, TChl-a. This feature can presumably be related to the relevant influence of highly refractive submicrometer particles of Saharan origin in the surface layer of the water column. The present measurements also show that the Mediterranean Sea is greener than TChl-a alone indicates, as already stressed in previous studies. This color anomaly is partly explained by the estimated colored dissolved organic matter and submicrometer particles absorption coefficients, and to a greater extent by the high bbp/TChl-a values assuming that these particles backscatter light similarly in the green and blue parts of the visible spectrum. The diel variation of both the particulate matter attenuation and backscattering coefficients were also investigated specifically. Despite some differences in the timing and the magnitude 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 estimating carbon community production budget has been highlighted for the first time. This result is particularly relevant for present and future geostationary spatial ocean color missions.
Highlights
In situ and remote sensing measurements of inherent optical properties (IOPs) in natural waters provide essential information to infer biogeochemical stocks and processes at Published by Copernicus Publications on behalf of the European Geosciences Union.H
General bio-optical relationships have long been established between these IOPs and some biogeochemical parameters, such as the chlorophyll-a concentration, Chl-a, and particulate organic carbon, POC, in open ocean waters (Gordon and Morel, 1983; Yentsch and Phinney, 1989; Bricaud et al, 1995; Oubelkheir et al, 2005; Gardner et al, 2006; Huot et al, 2008)
Chl-a concentration as measured from High Performance Liquid Chromatography (HPLC) does not change between 75 and 150 m (= 0.0845 ± 0.0062 mg m−3), whereas Divinyl-chlorophyll-a presents a peak at 124 m (Fig. 2c)
Summary
General bio-optical relationships have long been established between these IOPs and some biogeochemical parameters, such as the chlorophyll-a concentration, Chl-a, and particulate organic carbon, POC, in open ocean waters (Gordon and Morel, 1983; Yentsch and Phinney, 1989; Bricaud et al, 1995; Oubelkheir et al, 2005; Gardner et al, 2006; Huot et al, 2008). These relationships can be used to assess the variability of the latter biogeochemical parameters from field or remote sensing measurements. We examine the IOPs variability for ultra-oligotrophic waters of the Mediterranean Sea sampled in the frame of the BOUM (Biogeochemistry from the Oligotrophic to the Ultra-oligotrophic Mediterranean) cruise carried out in early summer, June–July 2008 (Moutin et al, 2011)
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