Eutrophication as a possible cause of decline in the seagrass Zostera noltii of the Dutch Wadden Sea

Samenvatting

The present thesis describes the results of research on the distribution and possible causes for the decrease of the seagrass Z. noltii in the Wadden Sea, carried out from 1986 to 1990. Chapter 2 examines the relation between the distribution of Zostera dominated ecosystems in the international Wadden Sea and several possibly limiting environmental factors, i.e. period of emersion, sediment type, sediment stability, mean salinity and geographical region. Chapter 3 describes the former and present distribution of Z.noltii dominated ecosystems in the Dutch Wadden Sea in relation with their environment, i.e. period of emersion, sediment type, sediment stability, macroalgae and lugworms. The seasonal dynamics and structures of a Z. noltii dominated ecosystem, including periphyton and mudsnails, on the tidal flats of Terschelling in the Dutch Wadden Sea are studied in Chapter 4. Results of a shading experiment on the effects of periphyton on the growth, biomass and survival mechanisms of Z. noltii on the tidal flats of Terschelling are discussed in Chapter 5. Chapter 6 describes the effects of the length of light periods on the growth, biomass and photosynthesis-irradiance curves of Z.noltii originating from the Netherlands and Mauritania. Chapter 7 deals with results of an exclosure experiment on the role of the periphyton-grazing mudsnail H. ulvae on the growth and biomass of Z.noltii In Chapter 8, the results of a transplantation experiment on the interaction between the seagrass Z. noltii and the bioturbating lugworm A. marina on the tidal flats of Terschelling is described. Chapter 9 summarizes and discusses the main results of the previous chapters in relation to the hypothesis.