A multiscale study of mercury transformations and dynamics at the chemocline of the Petit-Saut tropical reservoir (French Guiana)

International audienc

The impact of induced redox transitions on nutrient diagenesis in coastal marine sediments (Gulf of Trieste, northern Adriatic Sea)

International audienc

Response of a kleptoplastidic foraminifer to heterotrophic starvation: photosynthesis and lipid droplet biogenesis

International audienc

Contribution du compartiment sédimentaire à la demande en oxygène dissous durant un évènement hypoxique dans l'estuaire de la Loire (France)

International audienc

Modeling of phosphate flux induced by flood resuspension on a macrotidal estuarine mudflat (Seine, France)

International audienceCoastal marine sediments can be either major scrubbers or eutrophication contributors to surface waters. Standard methods for direct measurement of nutrient fluxes at the sediment-water interface do not consider hydrodynamic forcing although several ex-situ studies suggest that sediment resuspension can dramatically increase dissolved fluxes. We provide a new model to quantify dissolved phosphate (PO4$^{3−}$) resuspension flux (J$_R$) based on physical representation of its identified components: diffusion stimulation by exposure of deeper sediment layer with higher PO4$^{3−}$ concentration in the porewater (J$_D$), pore water mixing with overlying water (J$_M$) and net adsorption/desorption from suspended sediments (J$_K$). This approach was applied to field data from a Seine intertidal mudflat periodically submitted to millimetric erosion. On a tidal scale, the model output reveals a J$_R$ of 272.3 ± 360.0 μmol m$^{-2}$ h$^{-1}$ (± 52% from parameter uncertainty), well above flux calculated by application of Fick's first law (0.15 ± 0.85 μmol m$^{-2}$ h$^{-1}$) or by ex situ core incubation (40.8 μmol m$^{-2}$ h$^{-1}$). Iron bound phosphorus within suboxic layers buffers PO4$^{3−}$ concentrations in superficial sediments leading to negligible contributions of J$_D$ and J$_M$ to total fluxes. Conversely, J$_K$ appears to be the main exchange pathway, even though improvements in turbidity measurement would allow this term to be defined more precisely. Correction required to enhance and control model robustness are described. These results show the importance of considering the dissolved PO4$^{3−}$ resuspension flux in dynamic environments

Manganese, iron and phosphorus cycling in an estuarine mudflat sediment, Loire, France

International audienceThe sampling of surface sediment fromtwo sites of amudflat of the Loire Estuary during four contrasting seasonshas led to new information about geochemical cycling under transient diagenesis fuelled by flood deposition.Based on stocks of reactive iron-oxides andmanganese-oxides (ascorbate-extracted) and pore water concentrations,the progressive evolution of flood deposits is described. Three major steps are observed: at first, there is nomanganese, iron and phosphorus release into pore waterwithin the flood-deposited layer. Then, during a periodof approximately 1 month, Mn oxides are consumed while the dissolved Mn concentration increases. Simultaneously,the Fe oxide-rich layer from flood deposition prevents (or at least limits) phosphorus release intoporewater as shown by the increasing P/Fe ratio of the ascorbate extractions. During spring and summer, Fe oxidesare reductively dissolved until complete depletion results. This period also corresponds to the saturation ofFe oxides by phosphorus and probably maximum P release to the water column. The site located closer to theshore showed higher density of benthic faunas leading to more intense bioirrigation. The importance of bioturbationon the year scale for biogeochemical processes is discussed according to both bioirrigation and biomixingprocesses

Carbon and nutrient accumulation in tropical mangrove creeks, Amazon region

International audienc

Multiscale analysis of living benthic foraminiferal heterogeneity: Ecological advances from an intertidal mudflat (Loire estuary, France)

International audienceAn unprecedented sampling effort on the Loire estuary allowed a multi scale approach to identify parameters controlling density variations of benthic foraminifera. Indeed, the distances between the samples analysed for this study vary from 1 cm to hundreds of kilometres. To catch this range of distance variations, a model called Scale Variance Analysis was build describing the participation of each scale to the total observed variance. The SVA model requires, for each scale, the stability of relative variance. A comparison with the Moran's Index and experimental variogram is proposed showing coherent conclusions with the SVA analysis. The analysis shows that in order to maximize information on foraminiferal density variation, sampling campaigns should be designed with stations distant from few meters to 1 km, with a particular focus on the hectometre scale. A range of scale too rarely investigated in the community of benthic foraminifera ecology. Next, based on two intertidal mudflat stations separated of few hundred meters, the present study shows that for Ammonia tepida, the scale dependant preponderant parameters is the Chl a concentration in the top first centimetre. Contrastingly, the indicators of food quality such as the lability index and the oxygen penetration depth do not seem to affect A. tepida densities. This high quantity, low quality diet is interpreted as an opportunistic behaviour that is indirectly confirmed by a kinetic approach. This approach compares the deep infaunal microhabitat density with the shallow infaunal microhabitat density. The identical ratio indicates quick saturation of the available resources