Dissolved organic matter fate in coastal Mediterranean site: Toulon bay case - France

Dissolved organic matter (DOM) is a complex continuum of molecular species and plays an important role in biogeochemical processes in the aquatic ecosystem, waters and sediments (Aiken et al., 2011; Bolan et al., 2011; Burdige et al., 2004; Chen and Hur, 2015; Fu et al., 2006; Jiang et al., 2018; Stedmon et al., 2003). Once settled into marine sediments, organic matter undergoes biogeochemical transformations (Chen and Hur, 2015). These biogeochemical conditions are dependent on different parameters such as redox condition and microbial activities controlling its dynamics. Studies on pore water organic matter (PW-OM) and extracted organic matter (EOM) give understanding on the fate of sedimentary organic matter. Several studies (Burdige, 2001; Burdige et al., 2004; Chen and Hur, 2015; Dang et al., 2014; Hur et al., 2014; Murphy et al., 2008; Stedmon et al., 2003; Stedmon and Bro, 2008) have used Excitation Emission Matrices of fluorescence coupled to PARAFAC and UV-Vis spectroscopy to characterize DOM among several technics. As it is rapid and gives information on the dynamics, the aromatic structure and even the degree of humification of DOM. Toulon bay is a semi-enclosed bay located in the N-W basin of the Mediterranean Sea and in the S-E of the French coast. This bay is exposed to numerous pollution sources. The origins of organic matter in the bay is the input of two urbanized rivers (Las and Eygoutier), aquaculture, treated sewage outlets and planktonic activities (Boge et al., 2006; Nicolau et al., 2012). Sediment cores and column seawaters were collected at four points in the bay in front of Las River. Each core was sliced within a 2cm resolution under inert atmosphere (N2). Then, pore water was retrieved and filtered (0.2 µm) and solid fraction was freeze-dried, 2mm sieved, crushed. DOC and nutrients concentrations in water samples and total carbon, organic carbon (POC) and total nitrogen were measured on solid. Extraction with alkaline solution was performed and extracted organic carbon and extracted nitrogen were measured. 3D fluorescence measurements were done for all samples from 200 to 800 nm for both excitation and emission. The results showed that the degradation of OM was more intense in front of Las River than outside of the bay. Pore water and extracted OM are influenced by fresh biomass input and the latter is strongly humified. Pore water OM in the superficial layers of sediments comes from autochthonous origin and less humified with a weak aromaticity. However, in deep layers, it shows a terrestrial origin, a regain of aromaticity and an important humic character based on HIX index. Moreover, it is controlled by the production and degradation of nutrients and POC. Extracted OM derives from terrestrial origin and is strongly humified based on HIX index. The state of FDOM in superficial sediments is different from the one in deep sediments because of the reducing environment. There is a contradiction of FDOM fate/behavior between the dissolved and solid phase in deep sediments because the production of FDOM isn’t from the POC pool.

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