TIDAL FLUSHING OF AMMONIUM, IRON AND MANGANESE FROM INTER-TIDAL SEDIMENT PORE WATERS

Concentrations of ammonium, manganese and iron were examined in sedime nt pore water and near-bottom water of inter-tidal sediments of Ria Fo rmosa, a coastal lagoon in the South of Portugal, during the short per iod of time that water flooded the area. Short-sediment cores and over lying water retained during the ebb tide were sampled at air-exposed a rea. Immediately after the inundation sediment cores and flooding wate r were repeatedly collected over short periods of time: 20 min for amm onium and 60 min for iron and manganese. The pore water composition of the upper 2-cm sediment changed dramatically in NH4+, Mn and Fe durin g 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 o f sediment surface, pore water being mixed with sea water and oxygen s upplied to anoxic layers of the sediment. Consequently NH: in pore wat er 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 i ron is recycled inside the sediment. These processes occur on a semi-d iurnal time scale that implies a continuously re-adjustment of the che mical of inter-tidal sediment surface of Ria Formosa. (C) 1997 Elsevie r Science B.V.