THE RESPONSE OF SALT-MARSH VEGETATION TO TIDAL REDUCTION CAUSED BY THE OOSTERSCHELDE STORM-SURGE BARRIER

In 1986 a sluice gate barrier was completed in the mouth of the Ooster schelde estuary. The barrier has been partially or completely closed d uring 1986 and the first months of 1987. Consequently the high tides w ere reduced to such a level that the salt marshes were scarcely floode d. Since April 1987 the barrier has been closed on average during two out of 706 high tides a year. Although the barrier allows tidal exchan ge the tidal flow has been restricted as a result of the reduced width of the mouth of the estuary. This restriction of the tidal flow cause d a 26 cm decrease of mean high water in the eastern part of the estua ry. As a result the inundation frequencies of the salt marshes decreas ed. The response of salt-marsh vegetation to this tidal reduction was analyzed using annual records (1982 till 1990) of species composition in 57 permanent plots in two marshes at the southern shore of the estu ary. Analysis of the response of individual species to marsh elevation in the pre and the post-barrier situation revealed that most species moved down the marsh elevation gradient. The first axis of an ordinati on (DCA-1) was significantly negatively related to inundation frequenc y. Between 1984 and 1990 all plots were displaced towards a significan tly higher ordination score, indicating a trend towards a species comp osition from higher up the marsh. The position of most plots along DCA -I remained stable until 1985 and started to increase in 1986 or 1987. The vegetation in plots dominated either by Halimione portulacoides o r by Spartina anglica, started to change in 1985. This premature chang e was attributed to frost damage in January 1985. The initially high r ate of change along DCA-I decreased in 1989 and 1990. This would sugge st that the vegetation re-equilibrated with the newly established tida l conditions. Further analysis revealed no significant difference in t he relation between inundation frequency and sample score along DCA-1 between 1984 and 1990. This corroborates the view that species composi tion had re-equilibrated with the tidal conditions. Along DCA-2 the sa mples were displaced towards a significantly higher score as well. The change was attributed to an increase of the perennial Halimione and t he annual Suaeda maritima. The annual Atriplex hastata displayed an in crease from 1986 till 1988, but strongly declined in 1990. The transie nt response of this annual was described by DCA-3. During the pre-barr ier phase several attempts had been made to predict the vegetation res ponse to tidal reduction. We developed a multivariate model (CCA) with inundation frequency as a constraining factor to describe the relatio n between inundation frequency and species composition in the pre-barr ier phase in 1984. Next we used this model to predict the score of the samples along CCA-I in the post-barrier phase from the inundation fre quencies in 1990. The actual ordination score in 1990 was calculated f rom the observed species composition. The predicted ordination scores did not differ significantly from the observed ones. In retrospect it is concluded that the response of the vegetation to the reduction of t he tides, as far as the fraction of species composition which is relat ed to inundation frequency is concerned, could have been predicted by our model.