Modeling of phosphate flux induced by flood resuspension on a macrotidal estuarine mudflat (Seine, France)

Coastal marine sediments can be either major scrubbers or eutrophication contributors to surface waters. Standard methods for direct measurement of nutrient fluxes at the sediment-water interface do not consider hydrodynamic forcing although several ex-situ studies suggest that sediment resuspension can dramatically increase dissolved fluxes. We provide a new model to quantify dissolved phosphate (PO₄³⁻) resuspension flux (J_R) based on physical representation of its identified components: diffusion stimulation by exposure of deeper sediment layer with higher PO₄³⁻ concentration in the porewater (J_D), pore water mixing with overlying water (J_M) and net adsorption/desorption from suspended sediments (J_K). This approach was applied to field data from a Seine intertidal mudflat periodically submitted to millimetric erosion. On a tidal scale, the model output reveals a J_R of 272.3 ± 360.0 μmol m⁻² h⁻¹ (± 52% from parameter uncertainty), well above flux calculated by application of Fick's first law (0.15 ± 0.85 μmol m⁻² h⁻¹) or by ex situ core incubation (40.8 μmol m⁻² h⁻¹). Iron bound phosphorus within suboxic layers buffers PO₄³⁻ concentrations in superficial sediments leading to negligible contributions of J_D and J_M to total fluxes. Conversely, J_K appears to be the main exchange pathway, even though improvements in turbidity measurement would allow this term to be defined more precisely. Correction required to enhance and control model robustness are described. These results show the importance of considering the dissolved PO₄³⁻ resuspension flux in dynamic environments.