Tidally driven N, P, Fe and Mn exchanges in salt marsh sediments of Tagus estuary (SW Europe)

Short-sediment cores and flooding water were collected at 0, 5, 15, 25 and 50 min of tidal inundation in the two sites colonised by pure stands of Spartina maritima (low marsh) and Sarcocornia fruticosa (high marsh) from the Rosario salt marsh (Tagus estuary, SW Europe). Concentrations of NH (4) (+) , NO (3) (-) + NO (2) (-) and HPO (4) (2-) , Fe and Mn were measured in tidal flooding water and pore water. Flooding water is enriched in nutrients, particularly ammonium due to local discharge of untreated urban effluents. Nevertheless, NH (4) (+) and NO (3) (-) + NO (2) (-) concentrations in flooding waters at t = 5 min (NH (4) (+) = 246 +/- 7 mu M, NO (3) (-) + NO (2) (-) = 138 +/- 1 mu M for S. fruticosa and NH (4) (+) = 256 +/- 8 mu M, NO (3) (-) + NO (2) (-) = 138 +/- 1 mu M for S. maritima) rose sharply at both vegetated sites. An increase was also registered for HPO (4) (2-) and total dissolved Fe although the subsequent decrease was smoother. Advective transport induced by the two daily pulses of inundation is several orders of magnitude higher than the diffusive fluxes during submerged periods. In addition, solutes are exported from the sediment with the inundation and imported in submerged periods. The exported amount of inorganic nitrogen during tidal inundation (export of 3,200 mu mol N m(-2) day(-1)to the water column), is not counterbalanced by the sink of -290 mu mol N m(-2) day(-1) occurred during the submerged period.