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 (PO 4 3−) resuspension flux (J R) based on physical representation of its identified components: diffusion stimulation by exposure of deeper sediment layer with higher PO 4 3− 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 PO 4 3− 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 PO 4 3− resuspension flux in dynamic environments. • Original model for PO 4 3− resuspension flux calculation from in situ estuarine data. • Mean resuspension flux was 272.3 ± 360.0 μmol P m−2 h−1 on a tidal scale. • Desorption pathway is critical for its magnitude and model improvement perspectives. • Standard methods underestimate PO 4 3− benthic flux in dynamic environments. [ABSTRACT FROM AUTHOR]