Sources of dissolved organic matter during storm and inter-storm conditions in a lowland headwater catchment: constraints from high-frequency molecular data

International audienceThe transfer of dissolved organic matter (DOM) at soil–river interfaces controls the biogeochemistry of mi-cropollutants and the equilibrium between continental and oceanic C reservoirs. Understanding the mechanisms controlling this transfer is fundamental to ecology and geochem-istry. DOM delivery to streams during storms is assumed to come from the flushing of preexisting soil DOM reservoirs mobilized by the modification of water flow paths. We tested this hypothesis by investigating the evolution of the composition of stream DOM during inter-storm conditions and five storm events monitored with high-frequency sampling. The composition of DOM was analyzed using thermally assisted hydrolysis and methylation (THM) with tetramethylammo-nium hydroxide (TMAH) coupled to a gas chromatograph and mass spectrometer. In inter-storm conditions, stream DOM is derived from the flushing of soil DOM, while during storm events, the modification of the distribution of chemical biomarkers allows the identification of three additional mechanisms. The first one corresponds to the destabilization of microbial biofilms due to the increase in water velocity, resulting in the fleeting export of a microbial pool. The second mechanism corresponds to the erosion of soils and river banks, leading to a partition of organic matter between particulate and dissolved phases. The third mechanism is linked to the increase in water velocity in soils that could induce the erosion of macropore walls, leading to an in-soil partition between soil microparticles and dissolved phase. The contribution of this in-soil erosive process would be linked to the magnitude of the hydraulic gradient following the rise of the water table and could persist after the recession, which could explain why the return to inter-storm composition of DOM does not follow the same temporal scheme as the discharge. These results are the most important factors in understanding the transfer of nutrients and micropollutants at the soil–river interfaces during the hot moments that are storm events

Hydrologically driven seasonal changes in the sources and production mechanisms of dissolved organic carbon in a small lowland catchment

International audienceTo obtain better constraints on the control of seasonal hydrological variations on dissolved organic carbon (DOC) dynamics in headwater catchments, we combined hydrometric monitoring with high-frequency analyses of DOC concentration and DOC chemical composition (specific UV adsorption, 13C) in soil and stream waters during one complete hydrological cycle in a small lowland catchment of western France. We observed a succession of four hydrological periods, each corresponding to specific DOC signatures. In particular, the rise of the upland water table at the end of the rewetting period yielded to a strong increase of the specific UV absorbance (from 2.5 to 4.0 L mg C 1 m 1) and of the 13C values (from 29 to 27%) of the soil DOC. Another striking feature was the release of large amounts of DOC during reduction of soil Fe-oxyhydroxides at the end of the highflow period. Comparison of hydrometric data with DOC composition metrics showed that soils from the upland domains were rapidly DOC depleted after the rise of the water table in these domains, whereas wetland soils acted as quasi-infinite DOC sources. Results from this study showed that the composition and ultimate source of the DOC exported to the stream will depend on the period within the annual hydrological cycle. However, we found that the aromatic DOC component identified during the high-flow period will likely represent the dominant DOC component in stream waters on an annual basis, because most of the annual stream DOC flux is exported during such periods

Level set for face feature extraction: Application to lip tracking

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Pollution des captages d'eau brute de Bretagne par les matières organiques. Guide pratique pour localiser les zones sources dans les bassins versants et suivre l'évolution des pollutions dans le temps.

Rapport DRASS et Région Bretagn

Is trace metal release in wetland soils controlled by organic matter mobility or Fe-oxyhydroxides reduction?

Aerobic and anaerobic incubation experiments on a wetland soil samples were used to assess the respective roles of organic matter (OM) release, Fe-oxyhydroxides reduction and redox/speciation changes on trace metal mobility during soil reduction. Significant amounts of Cu, Cr, Co, Ni, Pb, U, Th and Rare Earth Elements (REE) were released during anaerobic incubation, and were accompanied by strong Fe(II) and dissolved organic matter (DOM) release. Aerobic incubation at pH 7 also resulted in significant trace metal and DOM release, suggesting that Fe-oxyhydroxide reduction is not the sole mechanism controlling trace metal mobility during soil reduction. Using these results and redox/speciation modeling, four types of trace metal behavior were identified: (i) metals bound to organic matter (OM) and released by DOM release (REE); (ii) metals bound to both OM and Fe-oxyhydroxides, and released by the combined effect of DOM release and Fe(III) reduction (Pb and Ni); (iii) metals bound solely to soil Fe-oxyhydroxides and released by its reductive dissolution (Co); and (iv) metals for which release mechanisms are unclear because their behavior upon reduction is affected by changes in redox state and/or solution speciation (Cu, Cr, U and Th). Even though the process of soil Fe-oxyhydroxide reduction is important in controlling metal mobility in wetland soils, the present study showed that the dominant mechanism for this process is OM release. Thus, OM should be systematically monitored in experimental studies dedicated to understand trace metal mobility in wetland soils. Due to the fact that the process of OM release is mainly controlled by pH variations, the pH is a more crucial parameter than Eh for metal mobility in wetland soils

Application de l'électrophorèse capillaire au dosage des anions et des cations majeurs en solution dans les eaux douces naturelles

International audienceLe laboratoire de Géochimie de l'UMR Géosciences Rennes est équipée d'uneélectrophorèse capillaire, appareil utilisé classiquement pour la séparation, l'identification etle dosage des molécules organiques. Nous souhaitons élargir cette méthode analytique à laséparation et au dosage des anions et des cations majeurs en solution dans les eaux doucesnaturelles. Les anions et les cations testés sont Cl-, SO42-, NO3-, K+, Na+, Ca2+, Mg2+, NH4+.L'ensemble des tests et résultats présentés dans ce mémoire démontre que lesélectrophorèses capillaires sont des instruments adaptés à ce type de dosage, pour peu que lesespèces visées soient présentes à des niveaux de concentrations suffisamment élevées dans leséchantillons analysés.Analyse des anions minéraux. Pour des concentrations que l’on trouve habituellement dans leseaux douces naturelles, la répétabilité obtenue est inférieure à 5%. A titre de comparaison, lesrésultats publiés dans la littérature pour la technique par éléctrophorèse capillaire font état derépétabilités comprises entre 3 et 3.5 %. Les limites de détection et de quantification que nouscalculons sont identiques voire inférieures à celles rapportées dans la littérature pour lesélectrophorèses capillaires. Elles sont du même ordre de grandeur que celles obtenues pour leschromatographies ioniques. Cependant, on doit noter que l’erreur sur la mesure augmenteconsidérablement lorsque la quantité à doser s’approche de la limite de quantification, faisantdes électrophorèses capillaires des instruments peu adaptées à la mesure des bassesconcentrations (<5 ppm). Concernant la justesse, les tests effectués sur le standard CRM 617ont donné des résultats satisfaisants, les concentrations mesurées étant identiques, aux erreursanalytiques près, à celles données par le constructeur.Analyse des cations minéraux. Il est classiquement reconnu que l’analyse des cationsminéraux par éléctrophorèse capillaire est plus délicate que celles des anions. Les tests derépétabilité présentés dans ce travail confirme ce fait, les écarts types calculés sur lesmoyennes des répétitions effectuées étant plus élevés pour les cations (3 à 6%) que pour lesanions (1 à 2%). En outre, les tests de justesse effectués sur le standard standard CRM 617sont moins bons que pour les anions. Seuls le calcium et le magnésium ont livré desconcentrations mesurées identiques, aux erreurs près, aux valeurs certifiées du standard.Pourle potassium et le sodium, les concentrations mesurées se sont avérées être plus faibles que lesvaleurs données par le constructeur - de 10 et 5.8%, respectivement – sans qu'il soit possibled'expliquer la raison de ces écarts. Deux cations (magnésium et potassium) ont livré deslimites de quantification particulièrement basses: <0.5 mg.L-1

Stanols as a tool to track the origin of microbial contamination of oysters, Crassostrea gigas, in shellfish areas.

International audienceRunoff of cattle manures (cows, pigs, sheeps) or discharge of effluent from wastewater treatment plants (WWTP) into aquatic ecosystems can lead to microbiological contamination of waters and living organisms. In coastal ecosystems and particularly in shellfish harvesting areas, the presence of pathogen microorganisms in waters induces fecal contamination of filter feeding bivalves (oysters, mussels, scallops...), therefore leading to human health risks associated to the consumption of these contaminated organisms. Watershed management plans that aim at limiting these risks require the development of tools able to identify fecal contamination sources. The fecal indicator bacteria used in the regulations to determine fecal contamination are not source specific since they are found in the feces of most warm-blooded animals. Thus, microbiological biomarkers have been developed in association with chemical biomarkers as Microbial Source Tracking (MST) methods. Fecal stanols, by-products of sterols obtained by human and animal microbial gut flora, are found in considerable amounts in feces with different relative proportions depending on their animal or human source. Recently, in association with microbiological biomarkers, the stanol fingerprint of contaminated waters has been successfully used to determine the main source of fecal contamination (cow, pig or human sources) in rural watersheds (Brittany, France). Up to now, the use of the stanol fingerprint to track the fecal contamination in shellfish tissues, especially bivalves, has been limited to the analysis of coprostanol, a stanol commonly associated to human contamination. Therefore, whether the stanol fingerprint can be used as a MST method in bivalves or not is still unknown. The first aim of this study was to compare several organic extraction procedures of stanols in the oyster Crassostrea gigas to determine a reliable method for stanol fingerprint analysis in bivalves. Solvent extraction and purification steps have been carried out with attention as they are critical for stanol quantification. Secondly, the evolution of the stanol fingerprint of oysters with time was evaluated during 6 days by artificially contaminating microcosms with two concentrations of a WWTP effluent. In the microcosms, the fingerprint of stanols as a chemical biomarkers of fecal (human) contamination was compared to counts of Escherichia coli, a commonly used microbial indicator. In association with microbial markers, the method developed from the two previous steps will be applied at the watershed scale in order to identify sources of fecal contamination in Brittany and Normandy (France)

Are fecal stanols suitable to record and identify a pulse of human fecal contamination in short-term exposed shellfish? A microcosm study

In this study, the capacity of oysters to bioaccumulate fecal stanols and to record a source-specific fingerprint was investigated by the short-term contamination of seawater microcosms containing oysters with a human effluent. Contaminated oysters bioaccumulated the typical fecal stanols coprostanol and 24-ethylcoprostanol and their bioaccumulation kinetics were similar to that of the Fecal Indicator Bacteria Escherichia coli used in European legislation. Although stanol fingerprints of contaminated water allowed the identification of the human specific fingerprint, this was not the case for oysters. This discrepancy is attributed to (i) high concentrations of endogenous cholestanol and sitostanol, responsible for “unbalanced” stanol fingerprints, (ii) different accumulation/depuration kinetics of fecal coprostanol and 24-ethylcoprostanol and (iii) the limits of the analytical pathway used. These results show that fecal stanols bioaccumulated by oysters are useful to record fecal contamination but the usefulness of stanol fingerprints to identify specific sources of contamination in shellfish currently seems limited

DOC sources and DOC transport pathways in a small headwater catchment as revealed by carbon isotope fluctuation during storm events

International audienceMonitoring the isotopic composition ( 13CDOC) of dissolved organic carbon (DOC) during flood events can be helpful for locating DOC sources in catchments and quantifying their relative contribution to stream DOC flux. Highresolution ( 2 ‰), and others yielding a very restricted range of values ( 80% of the stream DOC flux flows through the most superficial soil horizons of the riparian domain and (ii) the riparian soil DOC flux is comprised of DOC coming ultimately from both riparian and upland domains. Based on its 13C fingerprint, we find that the upland DOC contribution decreases from ca. 30% of the stream DOC flux at the beginning of the high-flow period to < 10% later in this period. Overall, upland domains contribute significantly to stream DOC export, but act as a sizelimited reservoir, whereas soils in the wetland domains act as a near-infinite reservoir. Through this study, we show that 13CDOC provides a powerful tool for tracing DOC sources and DOC transport mechanisms in headwater catchments, having a high-resolution assessment of temporal and spatial variability

Increasing PH drives organic matter solubilization from wetland soils under reducing conditions

International audienceIn wetlands, large quantities of dissolved organic matter (DOM) are solubilized under reducing conditions. Controlled incubations of a wetland soil were performed under oxic and anoxic conditions to investigate the extent to which the following processes account for this phenomenon: i) production of organic metabolites by microbes during soil reduction; ii) release of organic matter (OM) from Mn- and Fe-oxyhydroxides that undergo reductive dissolution; and iii) desorption of OM from soil minerals due to pH changes. Anaerobic incubation releases 2.5% of the total soil organic carbon (OC) as dissolved organic carbon (DOC), and is accompanied by a pH rise from 5.5 to 7.4 and by the soil Mn- and Fe-reduction. The three processes above all take place. However, anaerobic incubation at a constant pH of 5.5 (preventing OM desorption) releases only 0.5% of the total soil OC, while aerobic incubation at pH 7.4 (preventing Mn- and Fe-reduction) releases 1.7% of the total soil OC. By contrast, aerobic incubation at pH 5.5 (preventing both Mn- and Fe-reduction and pH rise) does not solubilize any DOC. The DOC released is markedly aromatic, indicating little contribution from microbial metabolites, but, rather, the presence of microbes leading to OM mineralization. The pH rise is the key factor controlling OM solubilization under reducing conditions. This rise of pH accounts for >60% of the total released DOC, which is not due to reductive dissolution as such