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

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

Low temporal variation in the intact polar lipid composition of North Sea coastal marine water reveals limited chemotaxonomic value

Temporal variations in the abundance and composition of intact polar lipids (IPLs) in North Sea coastal marine water were assessed over a one-year seasonal cycle, and compared with environmental parameters and the microbial community composition. Sulfoquinovosyldiacylglycerol (SQDG) was the most abundant IPL class, followed by phosphatidylcholine (PC), phosphatidylglycerol (PG) and diacylglyceryl-(N,N,N)-trimethylhomoserine (DGTS) in roughly equal concentrations, and smaller amounts of phosphatidylethanolamine (PE). Although the total concentrations of these IPL classes varied substantially throughout the year, the composition of the IPL pool remained remarkably constant. Statistical analysis yielded negative correlations between IPL concentrations and dissolved inorganic nutrient concentrations, but no changes in the overall planktonic IPL composition due to nutrient limitation were observed. Significant correlations between SQDG, PC, PG and DGTS concentrations and chlorophyll a concentrations and algal abundances indicated that eukaryotic primary producers, in particular Phaeocystis globosa, were the predominant source of IPLs at this site. However, while IPL concentrations in the water were closely tied to total algal abundances, the rapid succession of different algal groups blooming throughout the year resulted in only minor shifts in the IPL composition. Principal component analysis showed that the IPLs were derived from multiple sources, and that no IPL species could be exclusively assigned to a particular algal taxa or (cyano)bacteria. Thus, the most commonly occurring IPLs appear to have limited chemotaxonomic potential, highlighting the need to use targeted assays of more specific biomarker IPLs

Large-Scale Spatial Dynamics of Intertidal Mussel (<i>Mytilus edulis L.</i>) Bed Coverage in the German and Dutch Wadden Sea

Intertidal blue mussel beds are important for the functioning and community composition of coastal ecosystems. Modeling spatial dynamics of intertidal mussel beds is complicated because suitable habitat is spatially heterogeneously distributed and recruitment and loss are hard to predict. To get insight into the main determinants of dispersion, growth and loss of intertidal mussel beds, we analyzed spatial distributions and growth patterns in the German and Dutch Wadden Sea. We considered yearly distributions of adult intertidal mussel beds from 36 connected tidal basins between 1999 and 2010 and for the period 1968-1976. We found that in both periods the highest coverage of tidal flats by mussel beds occurs in the sheltered basins in the southern Wadden Sea. We used a stochastic growth model to investigate the effects of density dependence, winter temperature and storminess on changes in mussel bed coverage between 1999 and 2010. In contrast to expectation, we found no evidence that cold winters consistently induced events of synchronous population growth, nor did we find strong evidence for increased removal of adult mussel beds after stormy winter seasons. However, we did find synchronic growth within groups of proximate tidal basins and that synchrony between distant groups is mainly low or negative. Because the boundaries between synchronic groups are located near river mouths and in areas lacking suitable mussel bed habitat, we suggest that the metapopulation is under the control of larval dispersal conditions. Our study demonstrates the importance of moving from simple habitat suitability models to models that incorporate metapopulation processes to understand spatial dynamics of mussel beds. The spatio-dynamic structure revealed in this paper will be instrumental for that purpose