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In coastal areas and estuaries, such as those encountered in the western part of France (Brittany region), the recycling of carbon and nutrients from sediments can participate in the development of micro and macro-algal blooms with harmful consequences for these ecosystems. One of the main processes controlling this recycling is the microbial mineralization of sedimentary organic matter (SOM). Mineralization is controlled by the origin, quantity and accessibility of the SOM, three factors whose relative importance remain, however, poorly quantified, mainly due to the great diversity of OM sources in coastal areas. The first objective of the present work was to assess the variability of the SOM origin at the regional scale representative of the complexity of the sources likely to be involved. The second objective was to determine the link between the SOM origin and its biodegradability, and how the OM sources can drive nutrient dynamics at the sediment-water interface. To this end, a broad sediment sampling campaign was carried out on Brittany mudflats, particularly affected by the eutrophication, during the spring period. A total of 200 samples were collected at 45 sites. They were characterized by their porosity and grain-size, as well as their chemical composition through elemental, isotopic and molecular biomarker analysis. A wide range of OM sources were identified in the sediments, including both natural (bacteria, algae, macrophytes, terrestrial plants), and anthropogenic (combustion products, crude oil, petroleum products – e.g. from the processing of crude oil at refineries- and fecal matter) sources. Sediment slurry incubations were carried out to determine the spatial variability of potential mineralization rates under oxic conditions. In addition, the measurements of NH4+ and PO4 fluxes at the sediment-water interface were made from sediment core incubations under realistic redox conditions of sediment. The physical and chemical sedimentary characteristics explained 58 % of the variability of mineralization rates under oxic conditions, with a negligible independent effect of the SOM origin (3 %). Conversely, under insitu redox conditions, the prevalent role of SOM origin over quantity/accessibility on the sediment biodegradability was highlighted with a significant effect 5 and 1.5 fold higher on the PO4 and NH4+ fluxes respectively. The anthropogenic inputs from the watershed to the coastal sediment, through agricultural runoff and/or sewages discharge, seem to significantly drive the nutrient dynamics at the sediment-water interface. Higher values of NH4+ and PO4 fluxes were measured for the sediment with a chemical composition impacted by human activities.

International audience; Nitrogen (N) and Phosphorus (P) losses from rural landscapes are of major concern as they can cause eutrophication of inland and coastal waters. Spatial management of landscape features distribution is sometimes proposed as a viable solution to limit nutrient transfer to streams and rivers. However, the effectiveness of such measures is rarely quantified at the catchment scale. We propose a spatially explicit data analysis to assess whether the spatial distribution of nutrient sources in agricultural landscapes influences nitrate and total phosphorus (TP) exports.We sampled water in 19 independent headwater catchments (1 to 14 km²) every two weeks for 18 months and analyzed nitrate and TP. The headwaters studied were located within the same 375 km² catchment and they had similar soil type, climate and farming systems but their landscapes differ in their composition (i.e. percentage of agricultural land) and distribution (i.e. agricultural land distance to ditches/streams & degree of overlay with flow accumulation zones). The method consisted in comparing a model only accounting for the land use composition (lumped) with a spatially explicit index that weights agricultural land according to its spatial distribution. We derived a hydrological distance to the stream (HDS) map, and a flow accumulation map (FAcc) from a 10m resolution digital elevation model. We aggregated land-use, HDS and FAcc maps within a catchment-level metric defined as Σ(LU*FAcc^a / HDS^b), where LU takes the value of 1 for agricultural land use and 0 for other land-uses. The parameters a and b were calibrated against median concentration of TP and nitrate, with the objective of maximizing the correlation on ranks.Results showed that median nitrate concentrations depended largely on landscape composition whereas the landscape distribution had a great influence on TP exports. From the optimal values of a and b we showed that both distance to the stream and overlay with flow accumulation influence TP exports. Our research provides useful insights to design new landscape management strategies.