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

Biogeosciences, 8, 3295–3317, 2011 www.biogeosciences.net/8/3295/2011/ doi:10.5194/bg-8-3295-2011 © Author(s) 2011. CC Attribution 3.0 License. Biogeosciences 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 H. Loisel1 , V. Vantrepotte1 , K. Norkvist1 , X. Mériaux1 , M. Kheireddine2 , J. Ras2 , M. Pujo-Pay3,4 , Y. Combet5 , K. Leblanc5 , G. Dall’Olmo6 , R. Mauriac7 , D. Dessailly1 , and T. Moutin6 1 INSU-CNRS, UMR 8187, LOG, Laboratoire d’Océanologie et des Géosciences, Université Lille Nord de France, ULCO, 32 avenue Foch, 62930 Wimereux, France 2 INSU-CNRS, LOV, Lab. d’Océanographie de Villefranche, UMR 7093, Université Paris VI, Villefranche sur mer, France 3 INSU-CNRS, UMR 7621, LOMIC, Laboratoire d’Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls/mer, France 4 UPMC Univ Paris 06, UMR 7621, LOMIC, Laboratoire d’Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls/mer, France 5 INSU-CNRS, UMR 6535, LOPB, Laboratoire d’Océanographie Physique et Biogéochimique, Université de la Méditerranée, Centre d’Océanologie de Marseille, Campus de Luminy Case 901, 13 288 Marseille cedex 9, France 6 Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, PL1 3DH, UK 7 INSU-CNRS, UMR 6535, LOPB, Laboratoire d’Océanographie physique et biogéochimique Université de la Méditerranée, Centre d’Océanologie de Marseille, Campus de Luminy Case 901, 13 288 Marseille cedex 9, France Received: 31 July 2011 – Published in Biogeosciences Discuss.: 8 August 2011 Revised: 27 October 2011 – Accepted: 28 October 2011 – Published: 14 November 2011 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 Correspondence to: H. Loisel () also show that the Mediterranean Sea is greener than TChla 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. 1 Introduction 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. 3296 H. Loisel et al.: Bio-optical anomaly and diurnal variability different temporal and spatial scales (Smith and Baker, 1978; Nelson et al., 1998; Stramski et al., 1999; Oubelkheir et al., 2005; Boss et al., 2007; Vantrepotte et al., 2011). 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 then be used to assess the variability of the latter biogeochemical parameters from field or remote sensing measurements. The universal (i.e. global) status of these relationships can only be stated by characterizing and understanding the variability around these averaged laws which describe the mean trends observed between the IOPs and the biogeochemical parameters concentration. At first order, the IOPs variability is driven by the concentration of the optically significant material present in the water masses. For instance, robust statistical relationships (Loisel and Morel, 1998; Bricaud et al., 1998) have been established between the particulate attenuation, cp , and absorption, ap , coefficients with Chl-a over the whole trophic range (covering about three orders of magnitude). The biooptical characteristics of the particulate and dissolved matter, as well as the respective proportion between these different optically active pools, drive the natural variability observed around the averages relationships between IOPs and biogeochemical parameters. Here 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). While numerous field measurements were acquired to established bio-optical relationships representative of the open ocean (see Morel, 2009 and references therein), in situ measurements performed in ultra oligotrophic waters (i.e. Chl-a lower than about 0.05 mg m−3 ) are still very scarce. This is particularly true for the particulate backscattering coefficient, bbp , which has been rarely measured in oligotrophic waters. Recent bio-optical relationships (Huot et al., 2008; Bricaud et al., 2010) were however established, thanks to the large trophic gradient covered in the frame of the BIOSOPE cruise which was carried out in the eastern South Pacific Ocean (Claustre et al., 2008). Compared to the South Pacific gyre system, where ultra ologotro (...truncated)