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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.

The evolution of organic carbon export to the deep ocean, under anthropogenic forcing such as ocean warming and acidification, needs to be investigated in order to evaluate potential positive or negative feedbacks on atmospheric CO2 concentrations, and therefore on climate. As such, modifications of aggregation processes driven by transparent exopolymer particles (TEP) formation have the potential to affect carbon export. The objectives of this study were to experimentally assess the dynamics of organic matter, after the simulation of a Saharan dust deposition event, through the measurement over one week of TEP abundance and size, and to evaluate the effects of ocean acidification on TEP formation and carbon export following a dust deposition event. Three experiments were performed in the laboratory using 300 L tanks filled with filtered seawater collected in the Mediterranean Sea, during two ‘no bloom’ periods (spring at the start of the stratification period and autumn at the end of this stratification period) and during the winter bloom period. For each experiment, one of the two tanks was acidified to reach pH conditions slightly below values projected for 2100 (~ 7.6–7.8). In both tanks, a dust deposition event of 10 g m-2 was simulated at the surface. Our results suggest that Saharan dust deposition triggered the abiotic formation of TEP, leading to the formation of organic-mineral aggregates. The amount of particulate organic carbon (POC) exported was proportional to the flux of lithogenic particles to the sediment traps. Depending on the season, the POC flux following artificial dust deposition ranged between 38 and 90 mg m-2 over six experimental days. Such variability is likely linked to the seasonal differences in the quality and quantity of TEP-precursors initially present in seawater. Finally, these export fluxes were not significantly different at the completion of the three experiments between the two pH conditions. [ABSTRACT FROM AUTHOR]