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The Wadden Sea is a coastal sea which is an ecologically important part of the shallow and sedimentary south-eastern North Sea. It is also a highly dynamic ecosystem. The main objective of this study is the analysis and quantification of the variability and development of the Wadden Sea ecosystem. Therefore, both biodynamics and morphodynamics in the northern Wadden Sea at the islands Sylt and Rømø have been taken into consideration on different time scales. Shorelines, surface sediments and megaripples were utilised as morphodynamic study targets while seagrass beds and mussel beds were taken as biodynamic parameters. Furthermore, the interactions that exist between these study objects were identified and the factors driving their development analysed. Data were collected by remote sensing of high-resolution aerial photographs with a Geographic Information System (GIS) as well as in ground-truth surveys and sampling campaigns. The five study parameters are affected by global climate change, which is among other things indicated in the Wadden Sea by increased hydrodynamics. However, there are also close interactions between them which can serve to amplify the effects of climate change. It is assumed that in the future the Wadden Sea faces substantial change brought about by erosion and the inundation of tidal flats together with the degradation of important habitats, as a consequence of global climate change. It can be expected that morphodynamic and biodynamic processes will accelerate and variability increase.

Within the microbenthic community, diatoms play an important role in terms of food source for grazers and suspension feeders. Furthermore, they contribute significantly to the primary production on intertidal flats and in coastal waters world wide. Numerous studies have now documented microalgal biomass, primary production and other aspects of their ecology in many habitats. However, knowledge on the resuspension of diatoms or the effect of hydrodynamic forces at the species level is sparse but a better understanding of processes at the sediment/water interface and of potential consequences for the ecosystem regarding climate change is urgently required. In addition, knowledge of the effect of hydrodynamics in the intertidal is important because this force links and alters various processes from small scale areas at the sediment/water interface to ecosystem as a whole.The microscopic algae inhabit the sediment and water column and a terminology exists for referring to the habitats occupied by the diatoms, including pennate and centric forms, concerning their life mode: epipsammic (attached to a substratum), epipelic (cells are associated with the sediment, mainly motile) and pelagic diatoms that will occur mainly in the water column. A variety of diatoms are frequently found in both habitats, and often described as tychopelagic: a life mode with pelagic forms that are likely to have settled from the plankton and conversely, benthic diatoms that are readily resuspended. One aim of this thesis was to investigate the diatom community in the water and on the sediment surface and to establish the life mode of selected species in order to gather background information for further studies concerning bentho-pelagic coupling. The increased resolution of the identification and cell counts revealed a discrimination at the species level. Several species, notably of the genera Diploneis and Achnanthes, were found to react to altered flow conditions by presenting enhanced cell numbers on the sediment under increased flow or reduced flow, respectively. Furthermore, Diploneis, previously assumed to be a benthic species, is now shown to adopt a tychopelagic life style. Settling processes during slack water phases and resuspension caused by wind-induced waves and tidal currents lead to a blurring of the boundaries between the two communities. The present study investigated for the first time the influence of altered flow conditions and turbulence on diatoms in situ. This enabled us to take into consideration the effects of wind direction, wind force and tidal currents on the coupling. For this purpose a three-current-flume construction was deployed modifying the flow velocity. The study was carried out on a Wadden Sea intertidal flat at Sylt island in the North Sea, Germany, on 6 dates between July and September 2003. Analyses of chlorophyll a (chl a), suspended matter (SPM), cell counts, particulate organic carbon (POC), particulate organic nitrogen (PON) and species analysis on the sediment surface and in the water column served to characterise the relationship between the two communities. Flow characteristics were measured using the Acoustic Doppler Velocimeter (ADV) technique to calculate the physical parameters describing bottom shear stress and turbulence in order to assess the impact of hydrodynamics on diatom species and cell-size groups.Results showed that wind direction, à ?à ¯Ã ?à ¿Ã ?à ½velocity and fetch had the greatest influence on the variability of the samples, superimposed as they were on tidal forces. An increase in flow velocity reduces turbulence, whereas under reduced flow conditions turbulence exerts stochastic resuspension events by lowering the critical bed shear stress. This affects small diatoms (< 20 à ?�à ?à µm) of the benthic community that are entrained into the water column.

In aquatic ecosystems, phytoplankton primary production, zooplankton grazing, and pelagic respiration are important processes of carbon dynamics. The aim of this study is to quantify the annual dynamics of pelagic primary production, zooplankton grazing, and respiration in a shallow coastal system and to investigate possible benthic impacts. Pelagic primary production, zooplankton grazing, and respiration were investigated as weekly/monthly time series over a one year period in the northern Wadden Sea. Studies were related to the Sylt long term time series, providing data on temperature, salinity, inorganic and organic nutrients, chlorophyll a and suspended matter concentrations (e.g. MARTENS&ELBRÄCHTER 1998). This study was conducted in the framework of the European Research Project COSA (Coastal Sands as Biocatalytical Filters) to relate the temporal dynamics of pelagic carbon dynamics with benthic processes.

Pacific oysters (Crassostrea gigas Thunberg 1793) have been introduced into the Wadden Sea (North Sea) by aquaculture in the 1980s. Subsequently, natural spatfalls occurred and wild oyster populations became established. For settlement, oyster larvae need hard substrates to which they attach themselves permanently. By settling on top of each other, they may create massive biogenic reefs. On the sedimentary tidal flats of the Wadden Sea, epibenthic mussel beds (Mytilus edulis L.) represent the main insular hard substrates, wherefore the oysters attached themselves mainly to the shells of living and dead blue mussels. Resident mussel beds became more and more overgrown by C. gigas and the question arose, whether they all might soon be replaced by oyster reefs. In this context, the bjective was to assess the impact of C. gigas on the native ecosystem by investigating the population development in the northern Wadden Sea, and by evaluating the scope for coexistence with resident mussels. In general, this may be a test case whether an introduced species is capable of displacing a native analogue in a sedimentary shore environment.The invasion of C. gigas in the northern Wadden Sea started in 1991 when the first wild oysters had settled on an intertidal mussel bed in the vicinity of an oyster farm that has started its business in 1986 in the List tidal basin (island of Sylt, Germany). At first, abundances on intertidal mussel beds remained low and patchy (1995: 3.56 ± 3.21 individuals m-2; 1999: 3.71 ± 3.79 individuals m-2). The population slowly expanded its range from intertidal to subtidal locations as well as from Sylt north- and southwards along the coastline. However, a succession of three summers (2001 2003) with anomalous high water temperatures led to a massive increase in oyster abundances (2003: 125.80 ± 119.47 individuals m-2, 2004: 244.44 ± 172.84 individuals m-2). It is assumed that the further invasion of C. gigas in the northern Wadden Sea will benefit from high late-summer water temperatures when these oysters reproduce. However, length frequency distributions revealed that successful cohorts survived for at least 5 years, allowing for population persistence even when warm summers are rare.Studies on recruitment showed differential settlement of oysters and mussels that may lead to niche separation and coexistence of both species. As oysters settle preferentially on conspecifics, a positive feedback of adults on recruitment may facilitate rapid reef formation. Mussels may find a refuge underneath a cover of the brown macroalga Fucus vesiculosus. Potentially, mussels may overgrow oyster reefs in high recruitment years especially if a facilitating barnacle cover is high. However, biotic interactions with C. gigas that reaches about three to four times the size of mussels may prevent M. edulis to become abundant on oyster reefs.Growth experiments revealed a faster growth of C. gigas compared to M. edulis in intertidal and subtidal habitats. Whereas oyster growth is not hampered by the presence of oysters, mussels, and barnacle epigrowth, the growth of mussels is reduced in the presence of these species, thus suggesting competitive inferiority.In field experiments, a high survival rate of juvenile oysters was found and presumably caused by very low predation pressure. About 70% of juvenile C. gigas survived the first three months on an intertidal mussel bed and about 40% reached their first reproductiveperiod one year after settlement. Only early recruitment in the subtidal zone was reduced due to predation. Laboratory feeding preference experiments confirmed that the main benthic predators, shore crabs (Carcinus maenas L.) and starfish (Asterias rubens L.), strongly prefer mussels to oysters. Size selective feeding by the main mussel predators together with an early size refuge from predation due to faster growth and larger size may facilitate a competitive advantage of C. gigas over M. edulis.As C. gigas is well adapted to the Wadden Sea ecosystem and competitive superior to their native congeners, a further increase of the oyster population in the Wadden Sea is expected. The development of massive intertidal and possibly also subtidal oyster reefs that may contain a variable amount of mussel epigrowth depending on recruitment success in different years is considered as a likely future scenario. As oyster recruitment depends on high summer water temperatures whereas high mussel recruitment usually follows severe winters, a possible climate change leading to warmer summers and milder winters will further support the displacement of M. edulis by C. gigas. This regime shift is expected to have profound impacts on the Wadden Sea ecosystem, mainly because oysters are less integrated in the basic food web. A massive increase of the oyster population may lead to food limitation of other suspension feeders, especially in the wake of decreasing eutrophication, and to a decline of benthic predators. However, in which way the resident community will adapt to this new invader will be a future task to tackle. I conclude that the invasion of C. gigas in the Wadden Sea is facilitated by a high efficiency of using space and food resources and by low predation pressure by resident predators.

It has been assumed that the distribution of marine benthic populations depends mainly on pre-settlement processes. Juvenile dispersal after settlement is here investigated whether it attains a key role in determining spatial patterns of adult populations. Distribution patterns of juvenile and adult polychaetes in the Wadden Sea near the Island Sylt are described. A total of 43 polychaete species is recorded. Juvenile abundance was significantly higher in structured habitats than in non-structured ones. Spatial separation of juveniles from adults across habitats was found in some species. This may indicate juvenile migration as a mandatory process in such populations. A second indication of dispersal by juvenile benthic stages was found, when small-scale distribution patterns (4 m2) were analyzed almost daily over a two months period. At high spatio-temporal resolution, distribution patterns for Spio martinensis, Typosyllis hyalina, Ophelia rathkei and Capitella minima exhibited a high variability explained mostly by ongoing immigration and emigration. In contrast to extensive drifting in juvenile bivalves, only few juvenile polychaetes were found in water samples or in nets above the bottom. With the hypothesis that juveniles crawl at or below the sediment-water interface, an in situ experiment was set up. A combination of drift nets and covered grooves placed level with the sediment surface were used in order to trace crawling performance in juveniles. No juvenile worms were found in the drift nets, while in the grooves small species were able to crawl actively at least one meter per day. It is assumed that post-settlement dispersal plays an important role in the population ecology of some polychaetes. Juveniles are able to settle in habitats favorable specifically for juveniles irrespective of habitats preferred by adults. Active migrations at the bottom may bring juveniles in due time to sites suitable for the completion of their life cycle. keine dt.-spr. Zusammenfassung

Polychaete meroplanktonic larvae from the List tidal basin were identified, their horizontal and vertical distribution patterns described, and temporal abundance fluctuations analyzed. Tides cause marked changes in the depth of this basin, while seasonality and weather dependence cause high environmental and biological variability. The taxonomic composition was analyzed from samplings undertaken between 1996-2002, during flood and ebb periods. Spionid, phyllodocid, and polynoid larvae are representative for this basin, accounting 60% of the total number of taxa (46 taxa). The larval production is numerically dominated by Pygospio elegans, Spio martinensis, Polydora cornuta and Lanice conchilega. Since their benthic stages are well known inside the basin, a large proportion of the larval production can be considered local. The low correspondence between benthic and planktonic assemblages suggested a qualitatively important input of long-distance larval vagrants. Sites within the basin differed in species composition, suggesting that larvae are not randomly mixed by the tidal hydrodynamics. Polychaete larvae are retained inside the basin, in spite of the continuous exchange of water with the North Sea. Analyses of the spatial distribution undertaken at three spatial scales during flood and ebb periods indicated that benthic occurrence, coastal morphology, and larval behavior could enhance species-specific distribution patterns. Significant fluctuations in total and partial larval abundances occurred between 1996-2001 were probably related to variables with influence over gonadal maturation and spawning in adult stages, as well as larval feeding and performances.