Tidal flushing of ammonium, iron and manganese from inter-tidal sediment pore waters

Concentrations of ammonium, manganese and iron were examined in sediment pore water and near-bottom water of inter-tidal sediments of Ria Formosa, a coastal lagoon in the South of Portugal, during the short period of time that water flooded the area. Short-sediment cores and overlying water retained during the ebb tide were sampled at air-exposed area. Immediately after the inundation sediment cores and flooding water were repeatedly collected over short periods of time, 20 min for ammonium and 60 min for iron and manganese. The pore water composition of the upper 2-cm sediment changed dramatically in NH4+, Mn and Fe during the first minutes of the flood. This decrease was recorded together with an increase of ammonium and manganese in the flooding water. The inundation causes a mixing of freshly tidal water in the interstices of sediment surface, pore water being mixed with sea water and oxygen supplied to anoxic layers of the sediment. Consequently NH4+ in pore water and weakly bound to sediment is exported to the water column, Mn(II) also escapes from the sediment, but Fe(II) is oxidised rapidly and iron is recycled inside the sediment. These processes occur on a semi-diurnal time scale that implies a continuously re-adjustment of the chemical of inter-tidal sediment surface of Ria Formosa.