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

The time-course evolution of ammonium concentration has been examined in th e flood water during the first 25 min of tidal inundation. The way this tra nsport fluctuates with the tidal ranges and wind conditions was investigate d. 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 tra nsect 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 ne ap-spring tidal cycle and wind conditions. Ammonium was determined in the f lood 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 beginni ng of the inundation, ranging from 0.2 to 4.8 mmol m(-2) d(-1). The highest transport was recorded at the lower intertidal site under spring tide cond itions, which corresponds to the higher energetic situation and shorter eme rsion period. The lowest transport was observed at the upper intertidal sit e during the first inundation that followed three days of neap tide and con tinuous exposure of the sediment to the air. The value rates (0.2-4.8 mmol m(-2) d(-1)) were one order of magnitude higher than those calculated from molecular diffusion (0.07-0.16 mmol m(-2) d(-1)). This study points to the importance of the tidal flushing of ammonium from the intertidal sediments, and its spatial and tidal fluctuation. (C) Elsevier, Paris / Ifremer / Cnr s / Ird.