Seasonally-resolved nutrient export fluxes and filtering capacities in a macrotidal estuary

A fully transient, two-dimensional nested grid reactive-transport model of the Scheldt tidal river-estuary continuum is used to quantify seasonally-resolved silica and nitrogen filtering capacities and export fluxes to the coastal zone over a period of one year. Simulation results indicate that a pronounced seasonal variability in nutrient cycling results from the combined effect of biogeochemical transformations and nutrient flux imbalances, which arise from the time-lagged response of the scalar fields to hydrological perturbations. Dissolved silica reveals a larger mass storage capacity and a slower flux response to upstream perturbations than nitrogen. As a consequence, the nutrient export flux ratio to the coastal zone, which is instrumental for the marine phytoplankton dynamics, depends strongly on the transient propagation of the scalar fluxes. The estuarine nutrient retention shows a strong temporal variability, which is driven by the complex interplay between reaction and transport. Results reveal that seasonal filtering capacities, with values comprised between 0.01 and 0.81 for silica and 0.13 and 0.78 for nitrogen, cannot be estimated by existing empirical relationships between this parameter and freshwater residence time. At the seasonal scale, nutrient export fluxes to the coastal zone cannot be quantified from the river loads and the estuarine filtering capacities.