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The effect of tidal range on the flushing of ammonium from intertidal sediments of the Tagus estuary, Portugal

1999; EDP Sciences; Volume: 22; Issue: 3 Linguagem: Inglês

10.1016/s0399-1784(99)80053-x

1878-4143

Maria Teresa Cabrita, Fernando Catarino, Carlos Vale,

Marine and coastal ecosystems

The time-course evolution of ammonium concentration has been examined in the flood water during the first 25 min of tidal inundation.The way this transport fluctuates with the tidal ranges and wind conditions was investigated.Flood water was collected at three sites, located along a transect from the lower to the upper intertidal area of the Tagus estuary.At spring and intermediate tides, the periods of air exposure vary slightly along the transect due to the high tidal amplitude and the flatness of the area, but the upper site remains uncovered at neap tide over the entire tidal cycle.At each site, sampling was performed at different tidal ranges covering the neap-spring tidal cycle and wind conditions.Ammonium was determined in the flood water at short time intervals: 1, 2, 3, 4, 5, 10, 15, 20 and 25 min.A clear pattern was observed along the transect: considerable quantities of ammonium were exported from the sediment to the water column at the beginning of the inundation, ranging from 0.2 to 4.8 mmol m-' d-'.The highest transport was recorded at the lower intertidal site under spring tide conditions, which corresponds to the higher energetic situation and shorter emersion period.The lowest transport was observed at the upper intertidal site during the first inundation that followed three days of neap tide and continuous exposure of the sediment to the air.The value rates (0.2-4.8 mmol me2 d-') were one order of magnitude higher than those calculated from molecular diffusion (0.07-0.16 mmol mm2 dd).This study points to the importance of the tidal flushing of ammonium from the intertidal sediments, and its spatial and tidal fluctuation.