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International audience; Rising greenhouse gas emissions are causing increasing worldwide impacts and changes on climate patterns, sea level, food production, human lives and livelihoods. Maintaining or improving the ability of coastal aquatic ecosystems and oceans to remove CO2 from the atmosphere is a crucial aspect for climate mitigation. The vegetated coastal ecosystems are able to catch and to sequestrate carbon, the so-called Blue Carbon. These ecosystems are key exchange zones that mediate the biogeochemical cycles across the continent, the ocean and the atmosphere. Given the importance of these ecosystems in biogeochemical cycles and their sensitivity to natural and anthropogenic pressures, the carbon cycle within and between compartments (e.g. pelagos, benthos…) and across the interfaces (e.g. atmosphere, ocean…) need to be addressed. On a regional scale, in the extended urban area of La Rochelle located on the French Atlantic coast (La Rochelle metropolitan area), lack in situ measurements within the wetlands and littoral zone make very uncertain their role as a sink or a source of CO2 to the atmosphere. We will first present a vast research project “La Rochelle Territoire Zéro Carbone” project (https://www.agglo-larochelle.fr/projet-de-territoire/territoire-zero-carbone), that target the ambition of carbon neutrality in La Rochelle metropolitan area by 2040 through a holistic approach (from measuring CO2 to raising people's awareness and assessing the impact of exogenous natural factors). Second, we will present some first results on the Blue Carbon dynamics within the freshwater and salt marshes, and seagrasses.

International audience

International audience

International audience; The widespread use of organic compounds in daily products such as cosmetics, shampoos, pharmaceuticals or laundry detergents contributes to the continuous contamination of urban effluents. Many of these pollutants pass through classic wastewater treatment plants without substantial removal. Understanding the fate of pollutants during tertiary treatment processes initially dedicated to disinfection purpose and their transport in receiving water is essential to maintain water quality in downstream ecosystems. This work aims at studying the photodegradation rate constants and the degradation byproducts of musks (AHTN, HHCB), sunscreens (EHMC, OC), hormones (E1, E2, EE2) and pharmaceuticals (Carbamazepine, Ketoprofen, Ofloxacin, Oxazepam) when exposed to solar irradiation in simulated summertime (24 h;44 mW/cm²) and wintertime (21 mW/cm²), UVc irradiation (2h;0.75 mW/cm²) and UVc irradiation in the presence of H2O2 (45 mg/L). Irradiation experiments were carried out in ultrapure water, acidified water (3 mg/L performic acid), filtered (< 0.45 μm) and unfiltered wastewater. Aliquots of the irradiated samples were collected at specific times and internal standards were added. Half the aliquots were extracted in Ethyl Acetate and analysed by Gas Chromatography Mass Spectrometry (GC-MS) for musks and sunscreens. The other half was directly analysed by Liquid Chromatography tandem Mass Spectrometry (LC-MS-MS) for pharmaceuticals and hormones. First results in ultrapure water showed solar irradiations were efficient to remove Ofloxacin and Ketoprofen (>95% in 24 hours) while UVc irradiations highly degraded AHTN, E1 and Ketoprofen (>90% in 2 hours). In the presence of H2O2, UVc irradiations allowed the degradation (>90%) of all studied compounds. Obtained results gave important information on the efficiency of tertiary treatments in reducing domestic pollution. Calculated half-lives of studied micropollutants allowed to estimate the capacity of an aquatic system in recovering from disturbance and helped in understanding their potential dispersion pattern from a point source

The speciation of Phosphorus (P) in geologic materials can inform diverse investigations in such as aquatic biogeochemistry, soil sciences, and paleo-oceanography. Here, we describe in detail a set of sequential extraction schemes to measure P concentrations from several operationally defined phases: pore water, exchangeable or loosely sorbed P, Fe-bound P, P associated with biogenic apatite, P associated with authigenic apatite and carbonate forms, P associated with detrital apatite and inorganic forms, and organic P. The two benefits of the procedures we describe compared to prior methodological studies are: (1) fast, simple and inexpensive colorimetric methods have been developed for P determination in all the different matrices; and (2) the Fe-bound phosphorus pool is separated into two fractions, allowing the P associated with most reactive Fe-fraction to be distinguished from P bound to the remainder of the Fe-oxides. Adaptations of these procedures to optimize the characterization of P pools in a range of material types are also outlined.

In New Caledonia, a world-renowned mangrove, the "Coeur de Voh", grows in a semi-arid climate and at a high elevation in the intertidal zone. Net CO2 exchange (NEE) between the Avicennia marina shrub mangrove and the atmosphere was studied using an eddy-covariance system. The NEE and its biophysical controls were investigated, and net ecosystem production (NEP) was determined for one year. The daytime NEE ranged from -10.7 mu mol m(-2) s(-1) to +6.9 mu mol m(-2) s(-1) and the nighttime NEE ranged from +0.38 mu mol m(-2) s(-1) to +8.80 mu mol m(-2) s(-1). Both minimum values of daytime NEE and maximum values of nighttime NEE were measured during the wet, and warm season. Solar radiation, temperature and vapor pressure deficit were the controlling factors of NEE variations on both diel and seasonal scales. Water availability from tides and rainfall appeared to be key factors driving mangrove productivity. The tidal cycle also had pronounced effects on the mangrove NEE. Our results suggest that submerged conditions during high tide decreased the ecosystem respiration contribution and may increase the rate of tree photosynthesis. The absence of flooding spanning a few days rapidly decreased the NEE, suggesting that the trees experience water stress. The NEP was significantly and negatively correlated with rainfall and showed high seasonal variation. During the wet season, the NEP was very low and even negative, whereas the ecosystem photosynthesis was the greatest of the year. Indeed, the low NEP during the wet season was related to a significant increase in ecosystem respiration, whereas the low NEP during the dry season was linked to a decrease in ecosystem photosynthesis due primarily to an absence of freshwater from rainfall and seawater from tides. We estimate that the annual NEP was 73.8 gC m(-2) and may be lower if carbon tidal exports are significant. Consequently, the NEP of this dwarf mangrove in a semi-arid climate was much lower than that of mangroves in humid climates.

Lagoon environments are interfaces between continent and ocean. They receive important continental inputs providing high biological productivity. The ecological richness of these environments makes them one of the most productive worldwide ecosystems having a direct impact on economic activity. The conservation and the management of these systems are difficult because biogeochemical cycles are not yet well understood. The present work, which is included in the Programme National d'Environnement Côtier "Chantier Littoral Atlantique" and the ANR PROTIDAL, focuses on the role of the intertidal zones in the net metabolism mass budget of the Arcachon lagoon, on the biogeochemical processes at the different interfaces and their interactions with the tidal cycles, the pelagic mechanisms and the degree of the substrate biological colonization at different time scales (tide, season). Tidal creek waters, which drain tidal mudflats during ebb tide, represent the sum of seeping, diffusion and advection of anoxic interstitial waters from muddy and permeable sediments. These waters are enriched with early diagenesis products, such as ammonia, phosphate, ΣCO2, methane and dissolved metals. The rough estimations extrapolated to the entire lagoon demonstrate the importance of this tidal pumping process in the nutrient mass budget of this lagoon. The substrate colonization by the macrophyte meadows seems to be the main forcing factor on controlling biological and geochemical distribution of the biogenic species. Interstitial water stratification is detected in some profiles reflecting their production, their consumption by organic matter mineralization processes, their uptake by the macrophyte root-rhizome web, and physical and biological transport mechanisms. Contrarily to subtidal and stable sediments wherein the molecular diffusion is the primary solute transport, the geochemical vertical distribution in intertidal sediments is dominated by bioirrigation and interactions with macrophytes, inducing a high spatial and seasonal heterogeneity. A strong interaction was found between iron and phosphorus cycles with annual meadow dynamics, specifying that the ratio between meadows surface and unvegetated surface is a major control parameter for the mass balance of phosphorus in the Arcachon lagoon. The present work purposes for the first time to estimate the sources, sinks and fluxes of methane at the different interfaces in the Arcachon Bay and in a typical temperate tidal lagoon environment. The freshwater inputs and lagoon sediments constitute the two methane sources for the lagoon water column. The main quantity of produced methane is oxidised into the sediments by sulphates inducing low benthic fluxes. The atmospheric fluxes from waters and sediments of the Arcachon lagoon are low in comparison with others coastal systems. This mass budget estimation shows that the totality of supplied methane by rivers and diffused methane from sediments to the lagoon water column is transferred to the atmosphere during residence time of waters in the lagoon. This ecosystem is a negligible methane source for coastal open ocean., Les environnements lagunaires, interfaces entre le continent et l'océan, sont sujets à d'importants apports continentaux qui supportent une forte production biologique. Cette richesse écologique unique en fait des écosystèmes parmi les plus productifs de la planète à l'origine d'une activité économique importante. La gestion et la préservation de ces milieux sont difficiles car l'étude des cycles biogéochimiques est encore incomplète. Dans le cadre de cette thèse, qui s'inscrit dans le Programme National d'Environnement Côtier " Chantier Littoral Atlantique " et l'ANR PROTIDAL, un effort particulier est mis sur le rôle des zones intertidales dans le bilan du métabolisme net du système lagunaire du Bassin d'Arcachon, les processus biogéochimiques aux différentes interfaces et leurs interactions avec les cycles tidaux, les mécanismes pélagiques et le degré de colonisation biologique du substrat à différentes échelles de temps (marée, saison). Il a été démontré que les eaux des chenaux tidaux drainant les platiers intertidaux à marée descendante étaient le résultat de la percolation, diffusion et advection d'eaux interstitielles anoxiques depuis les sédiments vaseux et sableux. Ces eaux sont enrichies en produits de dégradation de la matière organique, tels que l'ammonium, les phosphates, le SCO2, le CH4 et les métaux dissous. Un premier essai de bilan à l'échelle de la lagune démontre l'importance de ce processus de pompage tidal dans le budget global en sels nutritifs de la lagune. La colonisation du substrat par les herbiers de macrophytes apparaît comme un facteur déterminant sur les associations biologiques et les distributions géochimiques des espèces biogènes. Une stratification de l'eau interstitielle est observée sur certains profils reflétant leur production, leur consommation par les processus de minéralisation de la matière organique, leur prélèvement par le système racinaire des macrophytes, et des mécanismes de transports biologiques ou physiques. A la différence des sédiments subtidaux et stationnaires, dans lesquels la diffusion moléculaire est le premier mécanisme de transport des solutés, la distribution verticale géochimique dans les sédiments intertidaux est dominée par la bioirrigation et les interactions avec les macrophytes, conférant une très forte hétérogénéité spatiale et saisonnière. Une interaction très forte entre les cycles du fer et du phosphore avec la dynamique annuelle de ces herbiers a été découverte, précisant que le rapport entre la surface couverte par les herbiers et celle non-végétalisée est un paramètre majeur du bilan de masse en phosphore dans le Bassin d'Arcachon. Ce travail propose également la première estimation d'un bilan des sources, puits et flux de méthane aux différentes interfaces dans le Bassin d'Arcachon et dans un environnement lagunaire tempéré. Les rivières et les sédiments lagunaires représentent les deux sources pour la colonne d'eau. L'essentiel du méthane produit est oxydé dans les sédiments par les sulfates entrainant de faibles flux benthiques. Les flux atmosphériques depuis les eaux et les sédiments du Bassin d'Arcachon sont faibles en comparaison à ceux estimés dans d'autres systèmes côtiers. Ce bilan montre que la totalité du CH4 apporté par les rivières et diffusé depuis les sédiments vers la colonne d'eau de la lagune, est transférée vers l'atmosphère au cours de la résidence des eaux dans la lagune, et que cet écosystème est une source nulle de méthane pour l'océan côtier ouvert.