Caractérisation intégrée de la matière organique dissoute : recherche d'empreintes physico-chimiques pour tracer les sources de pollutions anthropiques

Anthropogenic activities at the scale of a watershed can be very numerous (e.g. agricultural practices, industries, transport, and tourism). These activities generate pressures that result in the input of pollutants to the watercourse (e.g. major elements, organic micropollutants, trace metals). The identification of these sources of pollutants in watercourses remains a challenge to date. Organic matter in its dissolved fraction (DOM) constitutes a complex mixture of molecules whose composition and physicochemical properties depend on its origin, whether natural or anthropogenic, terrigenous or autochthonous, diffuse or point source. Because of its ubiquity, the DOM present in the rivers could be used as a tracer of the anthropic activities and thus of the sources of pollutants at the scale of a catchment. In this context, the objective of this thesis is to identify markers based on physico-chemical properties of DOM (that are characteristic of natural (e.g. terrigenous inputs, autochthonous production) and anthropogenic (e.g. wastewater treatment plant discharges, urban runoff, agriculture and livestock activities) sources at the watershed scale. For this purpose, an original database was built with more than 130 water samples representative of these different sources and analyzed with a wide range of analytical techniques (dissolved organic carbon determination, analysis by UV-Visible spectroscopy and fluorescence excitation-emission, analysis by steric exclusion chromatography coupled with UV and fluorescence detection - HPSEC/UV-fluorescence - , and molecular analysis by liquid chromatography coupled with high resolution mass spectrometry - LC-HRMS - ). The exploitation of the database allowed to improve the characterization of DOM with the identification of new optical (HPSEC/UV-fluorescence) and molecular (LC-HRMS) indicators. A protocol for processing complex data from non-targeted LC-HRMS analysis was developed and validated using quality control in order to extract the most relevant molecular compounds to differentiate DOM sources. This work also validated a sampling methodology specific to land-based diffuse sources in relation to land use to build their footprints at the watershed scale. Finally, the integrated multi-analytical and multi-source approach developed in the framework of the thesis allowed the construction of specific fingerprints of the selected sources of DOM (natural, anthropogenic, diffuse, point). The results of this work clearly show that DOM can be considered as a tracer of the origin of the water masses and the associated pollutions in the rivers.Les activités anthropiques à l'échelle d'un bassin-versant peuvent être très nombreuses (e.g. pratiques agricoles, industries, transport et tourisme). Ces activités engendrent des pressions qui se manifestent par l'apport de polluants vers les cours d'eau (e.g. éléments majeurs, micropolluants organiques, métaux traces). L'identification de ces sources de polluants dans les cours d'eau, et plus particulièrement les sources diffuses, reste un réel défi à ce jour. La matière organique dans sa fraction dissoute (MOD) constitue un mélange complexe de molécules dont la composition et les propriétés physicochimiques dépendent de son origine qu'elle soit naturelle ou anthropique, terrigène ou autochtone, diffuse ou ponctuelle. Du fait de son ubiquité, la MOD présente dans les cours d'eau pourrait ainsi être utilisée comme traceur des activités anthropiques et donc des sources de polluants à l'échelle d'un bassin versant. Dans ce contexte, l'objectif de cette thèse est de mettre en évidence des marqueurs basées sur des propriétés physico-chimiques de la MOD (e.g. distribution en poids moléculaire, aromaticité, famille de composés…) qui soient caractéristiques de sources naturelles (e.g. apports terrigènes, production autochtone) et anthropiques (e.g. rejets de stations de traitement des eaux usées STEU, ruissellement urbain, activités d'agriculture et d'élevage) à l'échelle de bassins versants. Pour cela, une base de données originale a été construite avec plus de 130 échantillons d'eau représentatifs de ces différentes sources et analysés avec un large panel de techniques analytiques (dosage du carbone organique dissous, analyse par spectroscopie UV-Visible et excitation-émission de fluorescence, analyse par chromatographie d'exclusion stérique couplée à la détection UV et fluorescence - HPSEC/UV-fluorescence - , et analyse moléculaire par chromatographie liquide couplée à la spectrométrie de masse haute résolution - LC-HRMS - ). L'exploitation de la base de données a tout d'abord permis d'améliorer la caractérisation de la MOD avec l'identification de nouveaux indicateurs optiques (HPSEC/UV-fluorescence) et moléculaires (LC-HRMS). Un protocole de traitement des données complexes issues de l'analyse LC-HRMS non-ciblée a été développé et validé à l'aide de contrôle qualité afin d'extraire les composés moléculaires les plus pertinents pour différencier les sources de MOD. Ces travaux ont également servi à valider une méthodologie d'échantillonnage spécifique aux sources diffuses terrigènes en lien avec l'occupation des sols. Finalement, l'approche intégrée multi-analytique et multi-sources développée dans le cadre de la thèse a contribué à la construction d'empreintes spécifiques des sources de MOD sélectionnées apriori (naturelles, anthropiques, diffuses, ponctuelles). Les résultats de ces travaux permettent clairement d'envisager la MOD comme traceur de l'origine des masses d'eaux et des pollutions associées dans les cours d'eau

Sampling terrigenous diffuse sources in watercourse: Influence of land use and hydrological conditions on dissolved organic matter characteristics

International audienceDiffuse and point sources of dissolved organic matter (DOM) in streams influence its composition, interactions and fate in the aquatic ecosystem. These inputs can be very numerous at the scale of a watershed, and their identification remains a challenge, especially for diffuse sources related to land use. The complexity of the transfer mechanisms and the reactivity of DOM throughout the soil-water column continuum raise questions about the sampling of diffuse sources in watercourses. To answer this issue, we compared the characteristics of soil-extracted DOM influenced by a particular land use (homogenous sub-catchment of forest and vineyard) and DOM collected from the watercourse adjacent to the soil samples. A 28-day incubation experiment of soil extracts was designed to remove the labile fraction of DOM. During the first 3 days, between 40 and 70 %of the DOC mass was lost for both types of soils. A set of optical indicators (UV–Visible, EEM fluorescence and HPSEC/UV-fluorescence) showed that the labile fraction was mostly composed by low (10 kDa) protein-like molecules. At the end of the incubation, soil-extracted DOMwas mainly composed of medium molecules (1–10 kDa) associated to terrigenous humic-like compounds. Its optical and size molecular signature tended towards that in the adjacent watercourses and was specific to land use.However, the characteristics of DOMin watercourses was also influenced by the hydrological conditions, probably due to a transfer of top soil DOM during high water periods and both deep soil and autochthonous DOM during low water periods. These results were obtained by a set of indicators, including novel ones derived from HPSEC/UV-fluorescence. Finally, this study demonstrated that it is possible to sample the DOM representative of a land use directly in the river downstream of the homogeneous sub-basin by multiplying the samples during contrasting hydrological conditions

Stream network variation in dissolved oxygen: Metabolism proxies and biogeochemical controls

International audienceAn explosion in high frequency dissolved oxygen (DO) observations at river network scales is creating new opportunities to understand dynamic signals in streams and rivers. Among the most informative metrics obtained from DO time series is stream metabolism-comprising gross primary production (GPP) and ecosystem respiration (ER)-but its estimation is non-trivial. There is thus interest in simpler metrics that can capture spatiotemporal patterns in stream metabolism and their consequences for critical ecosystem processes. Using hourly DO time series from 43 agricultural headwater streams reaches (Strahler order 1-5) across five watersheds and two years, we tested the hypothesis that simple DO metrics are useful proxies of stream metabolism, capturing key features of its spatiotemporal variation, and predicting attendant patterns in dissolved organic matter quality and catchment nitrogen processing via denitrification. Our results suggest the diel DO range scaled by stream depth is an excellent proxy for GPP throughout the network, accurately describing its spatial and temporal patterns. In contrast, we found that DO metrics were less successful as proxies for ER, with the maximum daily DO deficit scaled by depth being a good proxy for ER only in higher order streams. We also observed that DO metrics were strongly related to variation in dissolved organic matter quality and denitrification far better than GPP or ER. Finally, we found that DO metrics, GPP, and to a lesser extent ER, had power-law relationships with watershed area (scaling exponents, β = 0.2-0.5), implying increasing downstream metabolic activity. However, because lower order streams occupy ~75% of network benthic area, total network GPP and ER (g O 2 d − 1) were disproportionately provided by lower order streams, consistent with recent theoretical modeling. These findings reveal the rich inference space that simple DO metrics can provide, and support their use as proxies for stream metabolism and for inferring network patterns of biogeochemical function

Caractéristiques optiques de la matière organique dissoute dans des eaux usées domestiques

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Approche multi-analytique pour étudier les sources de matière organique dissoute dans un bassin versant périurbain

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Approche multi-analytique pour étudier les sources de matière organique dissoute dans un bassin versant périurbain

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Seawater copper content controls biofilm bioaccumulation and microbial community on microplastics

International audienceThe adsorption of trace metals on microplastics (MPs) is affected by the presence of surficial biofilms but their interactions are poorly understood. Here, we present the influence of Cu levels in real seawater (Toulon Bay, NW Mediterranean Sea) on microbial communities and Cu content of the resulting biofilms grown during incubation experiments on high density polyethylene. Two sets of incubation experiments were run with seawater supplied with MPs, sampled in two sites with contrasting Cu levels: Pt12 (most contaminated site) and Pt41P (less contaminated site). For each incubation experiment, 5 treatments were considered differing in Cu concentrations, ranging between 30 and 400 nM and between 6 and 60 nM, for Pt12 and Pt41p, respectively. A control experiment (filtered at 0.2 μm) was run in parallel for each incubation experiment. We observed that, at the time scale of the incubation period, both prokaryotic and eukaryotic richness and diversity were higher in the biofilms formed from the most contaminated site. In addition, we showed that Cu levels are shaping biofilm communities, evidencing co-occurrence patterns between prokaryotes and eukaryotes with diatoms playing a central role. These differences in biofilm formation were reflected in the amount of bioaccumulated Cu per dry weight of MPs, exhibiting higher values in the most contaminated site. Within this site, the increase of Cu seawater content enhanced its bioaccumulation onto MPs until reaching saturation. This study strongly suggests a striking link between seawater copper content, biofilm community shaping and the resulting Cu bioaccumulation onto MPs