Pore-water response on seasonal environmental changes in intertidal sediments of the Weser Estuary, Germany

Abstract

In order to determine time-dependent changes in estuarine pore-water chemistry and flux variations across the sediment-water interface, sediment cores of an intertidal mud flat in the Weser Estuary were taken monthly over a one-year period. Sediment temperature, pH, Eh, Cl⁻, O₂, NO ⁻₃ , and SO ²⁻₄ pore-water concentrations were measured and showed variations that relate to the changes of surface temperature and estuarine water composition. Fick's first law was applied to quantify diffusive fluxes from concentration gradients in the diffusive boundary layer and in the pore water. Total nitrate fluxes were calculated from flux chamber experiments. Diffusive oxygen fluxes increased from 5 mmol m⁻² d⁻¹ in winter to 18 mmol m⁻² d⁻¹ in early summer, while nitrate fluxes into the sediment increased from 3 mmol m⁻² d⁻¹ in winter to 60 mmol m⁻² d⁻¹ in early summer. Oxygen and nitrate fluxes into the sediment correlated linearly to sediment temperature. Sulfate fluxes increased from 0.5 mmol m⁻² d⁻¹ in winter to 10 mmol m⁻² d⁻¹ in August and September. Converted into carbon fluxes, the sum of these oxidants ranged from 10 mmol m⁻² d⁻¹ in winter to 80 mmol m⁻² d⁻¹ in summer. An estimation of the upper limit of the annual nitrate flux into the sediment showed that about 10% of the 250,000 t of nitrate discharged annually by the river may be decomposed within the inner Weser Estuary.