K.S. Dijkema

Restoration of salt marshes in the Netherlands

The conquest of land from the sea has been a long tradition in the Netherlands. When salt marshes were high enough, they were embanked when it was economically feasible, and transformed into intensively exploited agricultural land. This resulted in the transformation of halophytic communities to glycophytic communities. Often as an alternative, a low levee, a summerdike was built, which greatly reduced the flooding frequency of the landward summerpolder, hence creating a sedimentation deficit therein. Such summerpolders now cover 1200 ha in the Netherlands, 2100 ha in NW-Germany and small areas in England. Due to continuous embankments, the present salt-marsh area is relatively small with respect to the tidal basins. Discussions have been started how to increase the salt-marsh area. Two options will be discussed, firstly de-embankment of summerpolders and maintenance of the protective seawall, secondly increase of the effects of saline seepage behind the seawall by top soil removal. Both options include the restoration of salt-marsh communities (target communities) in intensively agriculturally exploited sites that have been salt marshes before. From the few examples abroad and experiments it is discussed (1) to which extent the sedimentation deficit in summerpolders could be compensated for, (2) if the soil seed bank is likely to contribute to re-establishment of salt-marsh communities, (3) if the dispersal of propagules of halophytic plants will be possible by hydrochory when the summerdike is breached, (4) to what extent is dispersal by endozoochory through waterfowl important in case re-establishment in a saline seepage area behind the seawall without open connection to the sea is envisaged. Two case studies of de-embanked summerpolders in the Netherlands revealed that the sedimentation deficit can be counteracted by rapid sedimentation, provided enough transport is possible from the foreshore. Dispersal by incoming tidal water from the nearby salt-marsh source area into the target area is possible for many salt-marsh plant species. The rate of success seems to depend on the relative position of source area and target area. A case study in a saline seepage area after top soil removal in the Netherlands, showed that the number of viable seeds dispersed by droppings from waterfowl is limited. Hence the possibilities for restoration of inland halophytic plant communities seem much lower than after de-embankment of summerpolders.

The effects of decreased management on plant-species distribution patterns in a salt marsh nature reserve in the Wadden Sea

To restore natural salt-marsh habitats, maintenance of the artificial drainage system was discontinued and cattle grazing was reduced in man-made salt marshes in the Dollard estuary, the Netherlands. We studied the vegetation development in these marshes shortly after these marshes became a nature reserve, and again 8–9 years later. Cattle distribution showed a gradient of intensive use close to the landward seawall to a low or zero use more seawards. Elymus repens was mainly found at the highest elevations and in the landward parts of the marsh, and lost much of its dominance. Conversely, Aster tripolium increased landwards, and showed a shift to higher marsh elevations. These changes are explained by the interaction between increased soil waterlogging in the marsh, due to the neglect of the drainage system, and cattle grazing. Phragmites australis, Spartina anglica and Scirpus maritimus were mainly found in the seaward parts of the marsh. Spartina anglica and Scirpus maritimus decreased strongly, partly through competitive replacement by Phragmites australis and partly due to herbivory by greylag geese (Anser anser). Bare soil increased in depressions due to increased waterlogging in the marsh combined with livestock grazing and goose grubbing. Phragmites australis probably forms the climax vegetation in the brackish Dollard marshes. Its increase in the seaward parts is irreversible in practice, but will benefit reed-bed breeding birds. On the landward side, the combination of neglect of the drainage system with moderate grazing may be effective in maintaining young successional stages suitable for a wider range of halophytic plants, and for breeding redshank and grazing waterfowl.

Measuring sedimentation in tidal marshes: a review on methods and their applicability in biogeomorphological studies

It is increasingly recognised that interactions between geomorphological and biotic processes control the functioning of many ecosystem types as described e.g. by the ecological theory of ecosystem engineering. Consequently, the need for specific bio-geomorphological research methods is growing recently. Much research on bio-geomorphological processes is done in coastal marshes. These areas provide clear examples of ecosystem engineering as well as other bio-geomorphological processes: Marsh vegetation slows down tidal currents and hence stimulates the process of sedimentation, while vice versa, the sedimentation controls ecological processes like vegetation succession. This review is meant to give insights in the various available methods to measure sedimentation, with special attention to their suitability to quantify bio-geomorphological interactions. The choice of method used to measure sedimentation is important to obtain the correct parameters to understand the biogeomorphology of tidal salt marshes. This review, therefore, aims to be a tool for decision making regarding the processes to be measured and the methods to be used. We, subdivide the methods into those measuring suspended sediment concentration (A), sediment deposition (B), accretion (C) and surface-elevation change (D). With this review, we would like to further encourage interdisciplinary studies in the fields of ecology and geomorphology.

Biological and abiotic factors influencing the settlement and survival of Salicornia dolichostachya in the intertidal pioneer zone

During the last two decades a decrease of salt marsh area of at least 6% was found along the mainland coast of the Dutch Wadden Sea. However, it was not clear what determined the seaward boundary of the pioneer vegetation in this area. In the period 1993–1995, abiotic and biological variables were monitored along a transect from the mudflat to the low salt marsh in two sites, the Negenboerenpolder and the Noordpolder. At the Negenboerenpolder site the pioneer zone extended further from the dike and had a dense cover with vegetation when compared to the Noordpolder site. The observed difference in extension of the pioneer zone could not be attributed to differences in tidal frequency, sedimentation rate or nutrient availability. During the winter a dramatic loss in seed numbers was found at both sites. This loss decreased from mudflat to low marsh. The mud/sand ratio, on the other hand, increased from mudflat to low marsh and was higher in the Negenboerenpolder. The shear strength of the top soil layer measured with an in‐situ erosion flume was correlated to the mud/sand ratio and increased with an increasing mud content of the soil. The strength of the soil seems to be the key factor for the settlement and survival of Salicornia dolichostachya in the pioneer zone.

Drainage systems in the Netherlands foreland salt marshe and natural creek systems

The salt marshes of nearly the entire mainland coast of the Wadden Sea have been developed with the support of land reclamation techniques, originally with the purpose of embankments. At present land reclamation works (foreland works) are still maintained to protect the salt marshes against erosion. The maintenance should be continued in a way as natural as possible. Examinated were the differences in drainage-pattem between land-reclamation salt marshes and natural salt marshes.