SALT MARSHES ALONG THE COAST OF THE NETHERLANDS

The area of salt marshes does no longer increase. The recent erosion c oincides with a rise on MHT-level in the last 25 years. Despite the de crease in area, sedimentation continues, especially in the lower salt marsh, which acts as a sink of nitrogen. Assimilation and mineralizati on of nitrogen are in balance in most plant communities along the grad ient from lower to higher salt marshes. Mineralization of nitrogen inc reases towards the higher salt marsh, whereas the above-ground product ion and the mean nitrogen content of plants decrease. There is a posit ive correlation between quality of food plants in salt marshes and bre eding success of Brent geese in the arctic tundra.Sedimentation on mai nland salt marshes can compensate for the expected sea level rise. Thi s is not the case for island salt marshes, if the relative sea level r ise is more than 0.5-1.0 cm yr-1. The natural succession on salt marsh es results in an accumulation of organic material, which is related to the dominance of single plant species. It is not clear to which exten t this process is enhanced by eutrophication from acid deposition and seawater. Human exploitation of unprotected salt marshes is old and he avy in the system of mound settlements. Reclamation rates by dikes in the last centuries were higher than the rate of area increase.Grazing by cattle as a management practice results in both a higher plant spec ies-richness and community diversity than abandoning; hay-making is in termediate, but shows less structural diversity than grazing with low stocking density. The invertebrate fauna is favoured by a short period of abandoning, but eventually characteristic salt marsh invertebrates are replaced by inland species. Many bird species prefer grazed salt marshes.The final section gives some perspectives. Provided that no fu rther embankments take place the optimal nature management option for plants and animals is a vegetation pattern, which includes areas with a low canopy (grazed) and areas with a tall canopy.