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.