How the origin of sedimentary organic matter impacts the benthic nutrient fluxes in shallow coastal mudflats

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 NH4+ and PO43- fluxes, respectively. The NH4+ 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 PO43- fluxes were driven by the sedimentation of particulate P exported through agricultural soil erosion.