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The origin of sedimentary organic matter (SOM) has often been mentioned as a driver of SOM reactivity. This was quantified by statistically relating the isotopic and lipid composition of SOM to benthic nutrient fluxes in 200 intertidal mudflats sampled along the Brittany coast (France). The origin of SOM explained 24% and 31% of the variance of $\text{NH}_4^+$ and $\text{PO}_4^{3-}$ fluxes, respectively. The $\text{NH}_4^+$ fluxes were driven by the uptake by phytoplankton of dissolved anthropogenic N exported from agricultural catchments. Their sedimentation is favoured by low hydrodynamic conditions, enriching the sediments with labile OM. The $\text{PO}_4^{3-}$ fluxes were driven by the sedimentation of particulate P exported through agricultural soil erosion.

A broad sampling program was carried out in the spring of 2019 on the Brittany coast to assess how the surface sediment characteristics drive the benthic effluxes of ammonium ($\mathrm{NH}_{4}^{+}$) and phosphate ($\mathrm{PO}_{4}$) from intertidal mudflats. A total of 200 sediment samples were characterized by their porosity, grain-size, elemental composition and pigment contents, as well as the benthic fluxes of $\mathrm{NH}_{4}^{+}$ and $\mathrm{PO}_{4}$ determined by core incubations. The results showed that (1) a high phaeopigment and iron-bound phosphorus content (Fe-P) and a low porosity were significantly related to the high $\mathrm{PO}_{4}$ flux, and (2) a high porosity and the TN:Org-P ratio in the sediment organic matter (SOM) were related to the high $\mathrm{NH}_{4}^{+}$ flux. This indicated that $\mathrm{PO}_{4}$ fluxes would be more driven by the redox status of the sediment through the desorption of Fe-P under specific anoxic conditions during the algal decomposition. $\mathrm{NH}_{4}^{+}$ fluxes would be more driven by high $\mathrm{NH}_{4}^{+}$ recycling rates from SOM mineralization and high sediment–water exchanges, enhancing the diffusion of $\mathrm{NH}_{4}^{+}$ to the overlying water. The present study allowed to highlight the large variability in the benthic nutrient fluxes at the regional scale, as a result of the connections between microbial (SOM mineralization), chemical (adsorption–desorption) and physical (diffusion) processes.

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. [ABSTRACT FROM AUTHOR]

The evolution of eutrophication (i.e., phytoplankton biomass) during recent decades was examined in the coastal waters of Vilaine Bay (VB, France) in relation to those in their main external nutrient sources, the Loire and Vilaine Rivers. Dynamic Linear Models, corroborated by the Mann-Kendall test, were used to study long-term trends and seasonality of dissolved inorganic nutrient and chlorophyll a concentrations (Chl a) in rivers and coastal waters between 1980 and 2013. The reduction in dissolved riverine inorganic phosphorus concentrations (DIP) from the early 1990s led to the decrease in their Chl a levels. However, dissolved inorganic nitrogen concentrations (DIN) decreased only slightly in the Vilaine and actually increased in the Loire, especially during summer. Simultaneously, phytoplankton in the VB has undergone profound changes with: (1) increase in biomass, (2) change in the position of the annual peak from spring to summer, and (3) increase in diatom:dinoflagellate ratios, especially in summer. The increase in phytoplankton biomass in VB, particularly in summer, was probably due to increased DIN loads from the Loire, sustained by internal regeneration of DIP and dissolved silicate from sediments. This long-term ecosystem-scale analysis reports the consequence of nutrient management scenarios focused solely on P reduction. Freshwater ecosystems upstream reveal successful recoveries through the control of P alone, while eutrophication continues to increase downstream, especially during the period of N limitation. Therefore, nutrient management strategies, paying particular attention to diffuse N-sources, are required to control eutrophication in receiving coastal waters. [ABSTRACT FROM AUTHOR]

The article investigates the occurrence and seasonality of Vibrio aestuarianus in sediment and Crassostrea gigas haemolymph at two oyster farms in France. Materials and methods used in sediment DNA and bacterial DNA extractions are described. The process of cloning and sequencing of 16S rRNA genes is detailed. A comparison between environmental parameters and vaest cultured counts in sediments and haemolymph of oysters is presented. The identification of isolated Vibrio aestuarianus strains is discussed.