TIDAL REDUCTION AND ITS EFFECTS ON INTERTIDAL HARD-SUBSTRATE COMMUNITIES IN THE OOSTERSCHELDE ESTUARY

Developments of intertidal hard-substrate communities in the Oostersch elde estuary were examined in perpendicular transects between high-wat er line and low-water line in the period 1982-1992. Prior to the begin ning of the Oosterschelde estuary works a typology of communities was established and an overall survey of the estuary was carried out. The communities contain flora (algae) as well as fauna. Due to asphalting of dikes in 1986, much of the surface of several communities has been destroyed. The originally well developed communities with large specie s-richness have not returned. The small reduction in tidal amplitude d ue to the construction of the storm-surge barrier had a minor effect o n the zonation of communities. In the upper part of the intertidal zon e the boundaries of the communities moved 0.5-1.0 m downward in the tr ansects along the dike-slopes. At an average inclination of 18-degrees this means a vertical shift of about 15-30 cm. This reflects the redu ction of the tidal amplitude: the high-water line shifted ca 22 cm dow nward. In a number of places sedimentation has caused a reduction in t he number of smaller seaweed species in the lower eulittoral zone. At monitoring locations presence of the original communities is rather un changed. Rare species like Pelvetia canaliculata, Actinia equina and G elidium pusillum have been able to maintain quite successfully.