Possibilities for restoration of Zostera marina beds in the Dutch Wadden Sea

The Wadden Sea lost its economically valuable seagrass (Zostera marina) beds during the 1930s. The beds never recovered and only sparse beds remain around mean sea level. As the conditions in the Dutch Wadden Sea have improved since the late 1980s (improved water quality, local prohibition of fisheries activities), the University of Nijmegen and Rijkswaterstaat have initiated investigations on the possibilities for restoration of the beds. In this study, it was concluded that potential Zostera marina habitats are located above approx. -0.80 m mean sea level (when turbidity remains at the same level as in the early 1990s), at sheltered, undisturbed locations, and preferably where some freshwater influence is present. A decrease of the nitrogen loads will increase the area of potential Z. marina habitats. A distinction has to be made between a higher and a lower zone of potential habitats, each suitable for different morphotypes of Z. marina. Suitable donor populations are available. With regard to Zostera marina ecology in general, it is concluded that (1) a low salinity (23 or 26 PSU) is more favourable to Z. marina development than a high salinity (30 PSU), (2) ammonium was shown to be toxic at loads of 175 kg N ha-1 yr-1 (25 æM NH4 in the supply medium), (3) at a low salinity, high nitrogen loads stimulate Z. marina development, whereas this has a negative effect at a high salinity, and (4) in some cases, the depth limit of intertidal Z. marina beds is determined by increasing water dynamics. Resulting new hypotheses: 1. Restoration of intertidal Zostera marina requires restoration of shelter that was formerly provided by the beds themselves and/or, for instance, by stable mussel beds. Therefore, in an active restoration program, seagrass restoration could be elegantly integrated with restoration of stable mussel beds (that have largely disappeared from the Dutch Wadden Sea in 1990).2. It is postulated that, in general, a high growth rate will allow plants to use increased nitrogen loads for their growth, whereas plants having a low growth rate (caused for example by a high salinity, phosphorus limitation, inherent plant properties, etc.), will be burdened by extra nitrogen loads. This may allow the explanation and prediction of many indirect and interactive effects of increased nitrogen loads, even in nitrogen limited ecosystem