| Summary: | 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.
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