As a result of the construction of a storm-surge barrier across the mouth of the Oosterschelde (SW Netherlands) in 1987, the tidal range and mean high water level in the estuary have been reduced permanently to about 88% of their original values. During the final stage of construction (1985-1987) the tidal range and mean high water level were reduced even further for more than 18 months, by up to about 65% of their original values. This paper describes the consequences of these reductions for some abiotic aspects of the salt marshes.
Strong ripening of the soil, especially in basins of the middle high salt marshes, resulted in the soils in these basins having more or less the dry nature of levees. This may cause moisture deficits for the vegetation during dry periods locally, and may lead locally to acidification of the soil as a result of oxidation of pyrite.
Erosion of the edges of the salt marshes has increased in many places since 1986, both due to lowering of the surface level of the foreland, causing wave action to affect the marsh cliff more strongly than before, and weakening of cliff strength as a consequence of desiccation of the salt marsh soil and subsequent withering of plants and plant roots. In addition, the gradual salt marsh gradients have decreased on a large scale, as a consequence of increased wave attack and frost damage to Spartina. Finally, also due to desiccation and plant withering, levees have degraded and eroded, forming shoulders in the creeks.
Settling, especially in the basins, has steepened and narrowed the height gradients between basin and levee.