Fluxes of water, sediments, and biogeochemical compounds in salt marshes

Tidal oscillations systematically flood salt marshes, transporting water, sediments, organic matter, and biogeochemical elements such as silica. Here we present a review of recent studies on these fluxes and their effects on both ecosystem functioning and morphological evolution of salt marshes. We reexamine a simplified model for the computation of water fluxes in salt marshes that captures the asymmetry in discharge between flood and ebb. We discuss the role of storm conditions on sediment fluxes both in tidal channels and on the marsh platform. We present recent methods and field instruments for the measurement of fluxes of organic matter. These methods will provide long-term data sets with fine temporal resolution that will help scientists to close the carbon budget in salt marshes. Finally, the main processes controlling fluxes of biogenic and dissolved silica in salt marshes are explained, with particular emphasis on the uptake by marsh macrophytes and diatoms

Composites nanotubes de ane alignés/polymère conducteur électronique pour des supercondensateurs à haute densité d'énergie

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Supercondensateurs à haute énergie et pseudo-supercondensateurs à base de matériaux dopables p et n

Development of supercapacitor electrodes based on vertically aligned carbonnanotubes is reported. This material exhibits high charge storage properties as shown inprevious studies but performances can be improved, especially their energy density. Inthis work, three original methods are studied to enhance the electrochemicalperformances of these electrodes. Long and dense VACNT were synthesis by CVD process was transferred on a thinner aluminum substrate more suitable for industrial application.Intrinsic properties of VACNT were maintained on this new Al. Influence of thickness andvolumetric mass of VACNT carpet on charge storage performances was studied. Then,interest of heteroatoms insertion was discussed. Insertion of heteroatoms such asnitrogen and oxygen was accomplished by doping during the growth and post-treatment functionalization of VACNT/Al. Heteroatoms incorporation have been validate.Improvement of capacitance have been measured, demonstrating heteroatomsincorporation benefits for supercapacitor application. Finally, conductive polymers suchas p3MT and a donor acceptor unit were synthesized and characterized especially their ndoping properties to be used as negative electrodes. They have then be associate with long and dense VACNT/Al. Both approaches, nanostructuration of ECP and acceptorinsertion, are very profitable to realize negative electrodes with high performances andstability. Type III supercapacitors based on these ECP/VACNT composite have shown exceptional performances.Ces travaux ont porté sur l’élaboration d’électrodes de supercondensateur à base de nanotubes de carbone verticalement alignés. Ce matériau, présente en effet des propriétés de stockage très intéressantes mais des progrès peuvent encore être réalisés, notamment pour augmenter leur énergie. Lors de cette thèse, trois voies innovantes et originales ont été explorées afin d’augmenter les performances de ces électrodes. Dans un premier temps, la croissance des VACNT/Al longs et denses par CVD a été transposée avec succès vers une nouvelle nuance d’aluminium plus intéressante pour les développements industriels futurs tout en conservant les propriétés intrinsèques des VACNT en terme de longueur et densité. L’influence de l’épaisseur et de la masse volumique sur les performances électrochimiques a été discutée. Dans un second temps,l’intérêt de l’insertion d’hétéroatomes a été étudiée. Celle-ci a été réalisée par deux voies: le dopage par l’azote durant la croissance et la fonctionnalisation par post-traitement d’amination et d’oxydation des VACNT/Al. L’insertion d’hétéroatomes a été validée et desgains de capacitances ont été observés, ce qui confirme l’intérêt du dopage et de la fonctionnalisation des nanotubes. Enfin, des polymères conducteurs, le p3MT et un dérivé donneur/accepteur du p3MT ont été synthétisés et caractérisés pour réaliser des électrodes négatives basées sur le dopage n du polymère. Ils ont ensuite été associés aux VACNT/Al longs et denses. Ces travaux ont montré le double avantage de la nanostructuration et de l’insertion de l’accepteur dans l’optique de réaliser des électrodes négatives performantes et stables. Des supercondensateurs de type III ont alors pu être réalisés à partir de ces électrodes composites et démontrent des performances remarquables

In situ biogenic silica variations in the invasive salt marsh plant, Spartina alterniflora : A possible link with environmental stress

International audienceAims Higher plants are an understudied component of the global silicon cycle; they absorb silicic acid (dSi) which is stored as biogenic silica (bSiO2). Si is believed to alleviate physical, chemical, and biological stresses such as storms, high salinity, heavy metal toxicity, grazing, and disease. We investigated a Si-accumulating invasive species growing in the tidal marshes of the Bay of Brest (France), viz., Spartina alterniflora. Our objectives were to determine (1) where and when bSiO2 accumulates in the plant during its life cycle, (2) whether this accumulation varies with abiotic factors: wave action, estuarine salinity, and duration of immersion, and (3) if the accumulation was limited by dSi availability in marsh porewater. Methods A 2 years field survey permitted to sample plants which were analyzed for there bSiO2 concentrations. Sediment cores were sampled seasonally and the dSi concentrations in the porewater were measured from 0 to 10 cm. Results bSiO2 accumulated more in mature leaves than in other organs. There was a strong linear relationship between bSiO2 concentration and plant length. bSiO2 concentrations did not increase, but rather decreased as a function of exposure to the three abiotic factors tested. dSi availability was not significantly different for each of the tested sites and dSi profiles did not exhibit huge losses in the root zone. Conclusions Our evidence suggests that dSi availability did not seem to be a limiting factor. bSiO2 did not increase with increasing abiotic stresses but was strongly correlated with growth. Hence, S. alterniflora is likely to have other adaptive strategies for dealing with environmental stressors but it did not exclude the possible role of Si in alleviating these stresses. If this is the case, there remain intriguing questions about Si uptake, its availability, and its role in silicification and growth

PORGEP : a decision aid for short-term management forecasting in pig production

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Resistance to Brown Ring Disease in the Manila clam, Ruditapes philippinarum: A study of selected stocks showing a recovery process by shell repair

International audienceEuropean stocks of the Manila clam Ruditapes philippinarum are affected by the Brown Ring Disease (BRD), which is caused by Vibrio tapetis. BRD is characterized by an accumulation of a brown organic matrix on the inner face of the shell. Clams that recover from BRD develop a white mineralized layer covering the brown matrix. Stocks of clams that showed resistance to BRD development, as enhanced recovery, have been monitored since 2000. We have examined two selected stocks: a Low Susceptibility (LS) stock and a High Susceptibility stock (HS), over three generations. The LS stock showed less evidence of the BRD symptoms, and more evidence of total shell repair, both in the field and following experimental challenge with V. tapetis, indicating that some clams may be less vulnerable to a V. tapetis attack than others. The inner face of the valves of the LS and HS clams of the two last generations were analysed with scanning electron microscopy. Examination of shells from BRD-affected clams showed that during the repair process, calcium crystals were progressively laid down until the affected zone was entirely covered. By the end of the shell repair process, a final organic layer covered the calcium crystal mounds. This layer seemed essential in the recovery process. The results indicate that the shell repair capability of the clams is the principal mechanism implicated in the development of BRD resistance in the Manila clam stocks. However, this resistance did not increase with generation because the broodstock was maintained at a site where selection pressure was low, due to a low prevalence of V. tapetis

Pseudo-capacitors using stable n-doped conducting polymers combined to aligned carbon nanotubes

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Pseudo-capacitors using stable n-doped conducting polymers combined to vertically aligned carbon nanotubes

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High energy ultracapacitor electrodes based on conventional and nitrogen doped vertically aligned carbon nanotubes

International audienceThe Aerosol Assisted Catalytic Chemical Vapor Deposition method allows the controlled growth of Vertically Aligned Carbon Nanotubes carpets (VACNT) on conductive substrates[1][2]. This process has been optimized for the growth of dense VACNT at low temperature (580 to 615°C) on aluminium foils, leading to a single-step process that meets industrial requirements to the development of low cost, highly conducting and light ultracapacitor electrodes[3]. VACNT can be used as a support for deposition of pseudocapacitive materials such as electronic conducting polymer (ECP). An electrodeposition of poly(3-methylthiophene) on VACNT was developed to improve the capacitance[4]. Homogenous deposition of ECP was achieved all along the carpet by pulsed chronoamperometry. To keep improving performances, our approach consists in modifying the properties of the VACNT by doping the carbon nanotubes with heteroatoms. Nitrogen doping should provide opportunities to increase the capacitance owing to the oxidation of the nitrogen sites and also the specific surface, in similar conditions of densities and diameters, because of their bamboo shape[5]. The wettability and electric conductivity should be improved by the doping[6][7]. The growth of nitrogen doped VACNT was achieved at 850°C on Si substrates by using ethylenediamine as N and C source. Such carpets exhibit a density of 4.10$^{10}$ NTC/cm$^2$ of few walled nanotubes and a N content of 4.5% at (XPS). Their storage capacity will be presente