Fluorescence and absorption properties of chromophoric dissolved organic matter (CDOM) in coastal surface waters of the northwestern Mediterranean Sea, influence of the Rhône River

J Para,B Charrière,M Tedetti,R Sempéré,C Fontana,P G Coble

doi:10.5194/bg-7-4083-2010

Abstract

Abstract. Seawater samples were collected monthly in surface waters (2 and 5 m depths) of the Bay of Marseilles (northwestern Mediterranean Sea; 5°17'30" E, 43°14'30" N) during one year from November 2007 to December 2008 and studied for total organic carbon (TOC) as well as chromophoric dissolved organic matter (CDOM) optical properties (absorbance and fluorescence). The annual mean value of surface CDOM absorption coefficient at 350 nm [aCDOM(350)] was very low (0.10 ± 0.02 m−1) in comparison to values usually found in coastal waters, and no significant seasonal trend in aCDOM(350) could be determined. By contrast, the spectral slope of CDOM absorption (SCDOM) was significantly higher (0.023 ± 0.003 nm−1) in summer than in fall and winter periods (0.017 ± 0.002 nm−1), reflecting either CDOM photobleaching or production in surface waters during stratified sunny periods. The CDOM fluorescence, assessed through excitation emission matrices (EEMs), was dominated by protein-like component (peak T; 1.30–21.94 QSU) and marine humic-like component (peak M; 0.55–5.82 QSU), while terrestrial humic-like fluorescence (peak C; 0.34–2.99 QSU) remained very low. This reflected a dominance of relatively fresh material from biological origin within the CDOM fluorescent pool. At the end of summer, surface CDOM fluorescence was very low and strongly blue shifted, reinforcing the hypothesis of CDOM photobleaching. Our results suggested that unusual Rhône River plume eastward intrusion events might reach Marseilles Bay within 2–3 days and induce local phytoplankton blooms and subsequent fluorescent CDOM production (peaks M and T) without adding terrestrial fluorescence signatures (peaks C and A). Besides Rhône River plumes, mixing events of the entire water column injected relative aged (peaks C and M) CDOM from the bottom into the surface and thus appeared also as an important source of CDOM in surface waters of the Marseilles Bay. Therefore, the assessment of CDOM optical properties, within the hydrological context, pointed out several biotic (in situ biological production, biological production within Rhône River plumes) and abiotic (photobleaching, mixing) factors controlling CDOM transport, production and removal in this highly urbanized coastal area.

Highlights

Dissolved organic matter (DOM) represents one of the largest bioreactive organic reservoirs at earth’s surface (Hedges, 1992, 2002) and constitutes the main substrate for heterotrophic bacteria growth (Azam et al, 1983)
Since no significant seasonal trend of aCDOM(350) appeared during this period, our results suggest that external influences such as Rhone River plume intrusion and mixing events control
Our results showed low and variable values of humification index (HIX) and high constant values of biological index (BIX), suggesting a predominantly autochthonous origin of DOM and the presence of organic matter freshly released in marine surface waters

Summary

Introduction

Dissolved organic matter (DOM) represents one of the largest bioreactive organic reservoirs at earth’s surface (Hedges, 1992, 2002) and constitutes the main substrate for heterotrophic bacteria growth (Azam et al, 1983). J. Para et al.: Fluorescence and absorption properties of chromophoric dissolved organic matter (CDOM). Fueling alternative labile carbon source to sustain local carbon demand in addition to autochthonous carbon source derived from phytoplankton and heterotrophic microbial food web (Sempereet al., 2000) and increasing light attenuation (Blough and Del Vecchio, 2002; Nelson and Siegel, 2002). Chromophoric (or colored) dissolved organic matter (CDOM), which is the fraction of DOM that absorbs light over a broad range of ultraviolet (UV) and visible wavelengths, is essentially controlled by in situ biological production, terrestrial inputs (sources), photochemical degradation, microbial consumption (sinks), as well as deep ocean circulation (Siegel et al, 2002; Nelson et al, 2007; Coble, 2007) and upwelling and/or vertical mixing (Coble, 1996; Parlanti et al, 2000). CDOM is the major factor controlling the attenuation of UV radiation in the ocean (Kirk, 1994) and is highly photoreactive and efficiently destroyed upon exposure to solar radiation (Mopper and Kieber, 2000, 2002)

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