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The European flat oyster (Ostrea edulis) provided relevant ecosystem functions and services as a keystone filter feeder and habitat forming species across the North Sea. At the turn of the last century, the species suffered a severe decline following a brief period of massive economic exploitation. Today, a few local populations still exist in e.g. the UK, the Netherlands, and Denmark. In the German North Sea, O. edulis is classified as functionally extinct.The Sylt-Rømø Bight is one of the largest tidal back-barrier environments of the European Wadden Sea and is located on the border between Germany and Denmark. The environment underwent significant changes over the last millennia driven by natural landscape dynamics and, more recently, by anthropogenic claims to land and coastal resources. Despite its demise as a living organism, O. edulis shells are still widely found in the area and remain a part of the sedimentary environment that potentially bears evidence of past ecological and geomorphological change. Written and graphic accounts on the past state, composition and distribution of Ostrea reefs are rare and unreliable.In this case study, we explore shell assemblage counts, geospatial data, and radiocarbon dating as tools to better understand ecological change over the past few centuries. More than a hundred dredge profiles were collected in the German part of the Sylt-Rømø Bight during two campaigns in summer 2021 and 2022. The results show marked spatial differences in the shell material regarding total dredged volume, overall species composition, size distribution, and taphonomic state. These differences can partly be explained by landscape dynamics and the economic history of the area (blue mussel cultivation). Radiocarbon dating of Ostrea shells (n=42) from three locations suggests that Ostrea banks at more exposed sites were short-lived and dynamic features, while ages from a less exposed site suggest the continuous presence of Ostrea over (at least) three thousand years.This example showcases that methods from geoarchaeological and geoscientific contexts can help to shed light on the more recent paleoecology of coastal environments over historical timescales, where sufficient depth of information is lacking from other sources. Beyond pure curiosity, the information has a clear value as benchmark to define aims for marine conservation and restoration efforts.

For about a century, biodiversity in the tidal Wadden Sea (North Sea, European Atlantic) has increased by more than one hundred introduced species from overseas. Most originate from warmer waters and could facilitate the transformation of this coastal ecosystem to comply with climate warming. Some introduced species promote sediment stabilization and mud accretion. This could help tidal flats to keep up with sea level rise. Although some introduced species also entail negative effects, introductions have diversified lower food web levels, and may benefit foraging birds. So far, no resident populations have gone extinct because an introduced species had established. Rather than degrading the ecosystem, the establishment of introduced species seems to have raised the capacity to follow environmental change. We support increasing efforts against introductions to avoid risk. However, once species are integrated, the common condemnation attitude against “non-natives” or “aliens” ought to be reconsidered for tidal ecosystems of low biodiversity.

In the wake of biological globalization, translocated species of high bio-engineering capacity increasingly change bottom topography of sedimentary coasts. A Vaucheria-taxon (Xanthophyceae) of unknown origin is spreading at the transition between intertidal and subtidal zones, while resident Vaucheria-species are confined to the upper shore in the Wadden Sea (European Atlantic). Near the island of Sylt, dense turfs of green filaments rapidly expanded over an area of 180 ha within 3 years. The unicellular filaments reach about 5 cm out of and 5 cm into the sediment. Felted rhizoids provide firm anchorage. Dry phytomass (up to 208 g m-2) was similar to that of intertidal seagrass beds. Residual filaments overwinter in the sediment and give rise to renewed growth in late spring. In addition, oospores germinate. Fine particles are trapped by the turf during summer, generating laminated cohesive mud. Muddy hummocks arise up to 20 cm above ambient sand flats, alternating with troughs but gradually merge into coherent and pertinacious plateaus of mud. This shift in bottom topography and sediment composition may potentially change the mud balance of tidal basins, and the capacity of tidal flats in catching up with accelerating sea-level rise.

Bare sandy flats at and below low tide level of the Wadden Sea (eastern North Sea, European Atlantic) were observed in 2020 to have been invaded by an introduced grass-like alga,Vaucheriacf.velutina(Xanthophyceae). A dense algal turf accumulated and stabilized mud, where resident seniors of the lugwormArenicola marinahad reworked rippled sand.Algae and worms were incompatible. Initially, rising patches with algal turf alternated with bare pits where lugworms crowded. Their bioturbation inhibited young algae, while the felt of established algal rhizoids clogged feeding funnels of worm burrows. Eventually, a mosaic pattern of competitors gave way to a coherent algal turf without lugworms. Concomitantly, a rich small-sized benthic fauna took advantage of the novel algal turf. This exoticVaucheriamay have the potential for drastically altering the ecological web at the lower shore.

Dune spits of the island of Sylt (North Sea) persisted through millennia by mobile dunes transferring sand from exposed to sheltered shores. However, 150 years of systematic planting on washovers, blowouts and migrant dunes have almost completely stopped this sand conveyor belt. Spits lost sand until sand replenishments to the exposed side compensated for losses during the last three decades. Based on maps since 1878, aerial images from the 1930s onward as well as botanical ground surveys, we document a long-term shift from loose grass cover to dense heather dominated vegetation. Bare sand areas almost vanished and high shrubs proliferated. Only three dunes continue migrating with about 3 m a-1 since 1936. Mainly artificial dune stabilizations and introduced plants have facilitated succession to an almost complete plant cover. Thus, revitalization of aeolian dynamics would be desirable, and pilot projects may acquire the appropriate knowledge. For a 1.7 × 2.5-km corridor across the island spit, we therefor discuss two suggested experiments: (1) intensify sand nourishments to the eroding beach and stop stabilization measures to allow excess sand blown inland and washovers entering deflation plains, and (2) examine the potential of tunnels for roads to let mobile dunes passing over. Such experiments may not only contribute to dune biodiversity but also to long-ranging adaptations of crowded barrier islands to accelerating sea level rise.

Bare sandy flats at and below low tide level were observed in 2020 to have been invaded by an introduced grass-like alga, Vaucheria cf. velutina (Xanthophyceae). A dense algal turf accumulated and stabilized mud where resident seniors of the lugworm Arenicola marina had reworked rippled sand. Algae and worms were incompatible. Initially, rising patches with algal turf alternated with bare pits where lugworms crowded. Their bioturbation inhibited young algae, while the felt of established algal rhizoids clogged feeding funnels of worm burrows. Eventually, the mosaic pattern of competitors gave way to a coherent algal turf without lugworms. Concomitantly, a rich small-sized benthic fauna took advantage of the novel algal turf. This exotic Vaucheria has the potential for taking over at the lower shore of the Wadden Sea (eastern North Sea, European Atlantic).

The kelp Undaria pinnatifida, native to East Asian shores, was unintentionally introduced with Pacific oysters into the Mediterranean in 1971. Intentional introduction from there to the French Atlantic coast 12 years later led to a gradual spread to the British Isles and the North Sea. Here, we report on the northernmost established population in continental Europe, and suggest a further spread into Scandinavian waters to be almost inevitable. In 2016, several thalli were found washed ashore at the eastern side of the island of Sylt in the northern Wadden Sea (German Bight, Eastern North Sea). Most specimens bore fertile sporophylls and thallus lengths of >1 m were common. In June 2017, 91 sporophytes were found attached to a mixed bed of Pacific oysters and native blue mussels, located just below low tide level in a moderately sheltered position. Mean thallus length was 0.2 m and the longest 0.7 m. Most had distinctive sporophylls and released spores in the laboratory. From sporophylls collected in the previous year, we successfully reared a new generation, demonstrating the kelp’s potential for further spread by natural means or human vectors.

Based on the past 150 years of research and ongoing time-series observations we give a comprehensive overview of marine species composition around the island of Sylt in the eastern North Sea. A total of 2758 species is listed according to the categories microplankton (591 species), zooplankton (137), nekton (118), benthic microflora (158), benthic macroflora (125), benthic micro-and meiofauna (1204), benthic macrofauna (509), birds and mammals (91), and neobiota (39). Plants account for a third of the species, most (85%) of them are microscopic Chromista. Among animals, 60% of the species are micro- and meiofauna though this faunal component is still insufficiently known. These figures are similar to records from the southern North Sea and therefore may by typical for temperate climate sedimentary coastal areas. A comparison with the total of marine species suggests that the small benthic fauna may be severely understudied over most of the world. Analysis of global change depends on sound baseline data and species inventories like this can assist in the detection of biodiversity changes. They emphasise rare species and the full range of local habitats while time-series measurements usually rely on a few selected habitats and biotic components to generate a very general picture of the state of an ecosystem.

With globally growing aquaculture activities, the co-introduction of parasites alongside large-scale movements of commercial species poses an increasing risk for marine ecosystems. Here, we present the first record of the shell-boring polychaete Polydora websteri Hartman in Loosanoff and Engle, 1943 in invasive Pacific oysters Crassostrea (Magallana) gigas (Thunberg, 1793) in the European Atlantic Ocean. In October 2014, mud blisters in the shells of wild Pacific oysters and specimens of a spionid polychaete were observed in close proximity to a commercial oyster farm at the island of Sylt (Germany) in the European Wadden Sea. Subsequent investigations indicated that these blisters only occurred near the farm and that no other mollusc species were affected. Morphological and molecular analysis identified the polychaete as Polydora websteri, a species that nowadays widely occurs around the globe, but likely is native to the Asian Pacific. Later sampling activities detected P. websteri also at other locations around Sylt as well as in the Dutch part of the Wadden Sea at the island of Texel. The number of polychaetes in the oysters was, however, relatively low and mostly below 10 individuals per oyster. Together, this evidence suggests that P. websteri is currently extending its range. As the introduction of P. websteri may have severe ecological and economic implications, this study aims to alert others to look for P. websteri at Western European coasts within farmed or wild Pacific oysters to further document its spread.

Owing to global warming, imminent losses at polar ice sheets will entail several meters of highly unpredictable sea level rise. Merely carrying on with bulwarks for defense against a rising sea will exacerbate the fate of coastal flatlands such as the subsiding North Sea Wadden Coast. The mighty dimension of the rising sea requires debates on a wider conceptual horizon. It is time to initiate stepwise coastal transformations toward growing with a rising sea.

The Wadden Sea Region is comprised of the embanked coastal marshes and islands in the Wadden Sea near Denmark, Germany, and the Netherlands. This area retains an exceptional common history in all its aspects: archaeologically, economically, socially, and culturally. Its settlement history of more than two thousand years is unrivalled and still mirrored in the landscape. Even though it has never constituted a political unity, it still shares a landscape and cultural heritage. For example, the approaches to water management and associated societal organization developed in the region during the last millennium have set significant world standards. This book offers an overview of current research on history, landscape and cultural heritage of the Wadden Sea region.

Invasion trajectories of introduced alien species usually begin with a long establishment phase of low abundance, often followed by exponential expansion and subsequent adjustment phases. We review the first 26 years of feral Pacific oysters Crassostrea gigas around the island of Sylt in the Wadden Sea (North Sea, NE Atlantic), and reveal causal conditions for the invasion phases. Sea-based oyster farming with repeated introductions made establishment of feral oysters almost inevitable. Beds of mussels Mytilus edulis on mud flats offered firm substrate for attachment and ideal growth conditions around low tide level. C. gigas mapped on to the spatial pattern of mussel beds. During the 1990s, cold summers often hampered recruitment and abundances remained low but oyster longevity secured persistence. Since the 2000s, summers were often warmer and recruitment more regular. Young oysters attached to adult oysters and abundances of >1000 m−2 were achieved. However, peak abundance was followed by recruitment failure. The population declined and then was also struck by ice winters causing high mortality. Recovery was fast (>2000 m−2) but then recruitment failed again. We expect adjustment phase will proceed with mean abundance of about 1000 m−2 but density-dependent (e.g., diseases) and density-independent (e.g., weather anomalies) events causing strong fluctuations. With continued global warming, feral C. gigas at the current invasion fronts in British estuaries and Scandinavian fjords may show similar adjustment trajectories as observed in the northern Wadden Sea, and also other marine introductions may follow the invasion trajectory of Pacific oysters.

Salt marshes in the Wadden Sea constitute about 20% of all salt marshes along European coasts. They are of immense importance for coastal protection reasons and as habitat for coastal plant, bird, and invertebrate species. The Wadden Sea is a coastal sedimentary ecosystem in the southeastern North Sea. Besides salt marshes, it is composed of tidal flats, high sands, and sandy shoals, dissected by (sub)tidal channels and located behind barrier islands. Accelerated global sea-level rise (SLR) and changes in storm climate have been identified as possible threats for the persistence of the Wadden Sea ecosystem including its salt marshes. Moreover, it is known that the amount and composition of the sediment available for salt marshes are the most important parameters influencing their ability to adapt to current and future SLR. Assessing these parameters requires a thorough understanding of the sedimentary system of the salt marshes and the adjacent tidal basins. In the present review, we investigate and unravel the interactions of sedimentary processes in the Wadden Sea with the processes taking place on the salt marshes. We identify the most crucial processes and interactions influencing the morphological development of salt marshes in the Wadden Sea. A conceptual model is proposed, intended as a framework for improved understanding of salt marsh development and for incorporation into new salt marsh models. The proposed model may also be applicable to regions other than the Wadden Sea.

Conspicuous macrozoobenthos and vegetation of intertidal sandflats in Konigshafen (Island of Sylt, SE North Sea) were mapped in 1932, 1988 and 2008. Higher water levels since the 1930s with a concomitant increase in tidal dynamics are assumed to have weakened sediment stability. This dissolved the distinctly banded macrobenthic zonation of the 1930s. Near high water level, cyanobacterial mats with associated beetles, belts of the mudshrimp Corophium volutator and the seagrass Zostera noltii have vanished, while the range of the lugworm Arenicola marina has extended towards the shore. Near low water level, sandy elevations have become permanently submerged because a tidal creek has widened its bed. In 1988, extensive green algal mats and the almost complete absence of seagrass are attributed to peak eutrophication. This partially reversed until 2008. The mussel Mytilus edulis had strongly extended its beds along the creek in 1988. These were taken over by introduced Pacific oysters Crassostrea gigas in 2008. Also in 2008, the cordgrass Spartina anglica, another introduced species, grew into large tussocks where cyanobacterial mats and a Corophium-belt had been mapped in the 1930s. Former benthic patterns may have little chance of resurrection by conventional nature protection because these small-scale shifts represent responses to regional and global change.

In contrast to the global crisis of seagrass ecosystems, intertidal Zostera- beds in the Northfrisian Wadden Sea (coastal North Sea) have recovered recently. Present areal extent resembles that of the mid 1930s. In spite of an intermittent loss in area by about 60% in the 1970s to 1990s, beds have maintained their general spatial distribution pattern. Aerial photographs from parts of the region in 1935–37, and the total region in 1958–59 and 2005 were visually analysed, and seagrass beds were recorded and quantified with a geographic information system (GIS). Data from direct aerial mapping were added to extend the survey until 2010. From the mid 2000s to 2010, intertidal seagrass areas estimated from these records range between 84 and 142 km 2 (10–16% of the intertidal area), while records from the 1970 to 90s merely range between 30 and 40 km 2 (3–5%) (Reise and Kohlus, 2008). Despite variation in size, core positions of individual seagrass beds were identified and they shifted very little over the last decades. Most beds occur in the upper intertidal zone and where barrier islands offer shelter against swell from the open sea. While land claim activities since the 1930s have irreversibly eliminated at least 11 km 2 of seagrass beds, we suggest that intermittent losses of seagrass area were mainly caused by sediment dynamics and a phase of elevated eutrophication.

Two Northwest Pacific crabs have almost simultaneously invaded Northeast Atlantic shores. In the Wadden Sea, Hemigrapsus sanguineus and H. takanoi have become established on artificial-boulder shorelines. Peak densities of >100 crabs (>5 mm carapace width) per m² were attained within 2–3 years of arrival. The invaders segregated by microhabitat, with H. sanguineus dominant on exposed shores and H. takanoi dominant on sheltered shores as well as colonizing beds of mussels and oysters on mudflats. A field experiment indicated H. sanguineus displaced juvenile native shore crabs Carcinus maenas from beneath-boulder substrate. However, this displacement appeared restricted to armored shorelines, with little to no effect on large populations of C. maenas that occur widely in shallow, soft bottom, habitats of the coastal zone of the North Sea.

Die Vielfalt der Kustenokosysteme lasst sich am besten entlang von funf Gradienten ordnen: vom Land ins tiefe Wasser, vom Wellenschlag zur stillen See, vom Fels zum Schlamm, vom Salz- zum Suswasser und von den Polen zum Aquator. Die vom Menschen stark umgeformten Kusten benotigen fur eine nachhaltige Entwicklung unbedingt auch biologische und okologische Forschungen.

An inventory in the Dutch-German-Danish Wadden Sea revealed a total of 66 nonnative (alien) taxa including 17 tentative cryptogenics in the brackish-marine macrobenthos until 2010, which is close to average compared with similar inventories from other coasts. Although the Wadden Sea is known for the largest sedimentary tidal flats in the world, most aliens encountered were fouling at harbor walls, pontoons in marinas, at hard structures for coastal defense but also in epibenthic mussel and oyster beds. Recent qualitative rapid assessments focusing on port localities have substantially improved knowledge on introduced species in the Wadden Sea. Nonnative species have pervaded algal and invertebrate communities, and the guild of suspension feeders became particularly strengthened by aliens which took advantage of a recent phase with relatively warm years. Most alien species were not directly introduced into the Wadden Sea but have spread secondarily from adjacent coasts with more active harbors or shellfish cultures. We suggest that aliens which have already established in the Wadden Sea ecosystem should be tolerated to avoid ongoing manipulations in eco-evolutionary developments in this protected nature area. Mitigation of the advancing tide of invasive aliens should focus on vector control (shipping and open aquacultures) as well as on early detection with attempts of eradication during the initial phase of invasions on a scale of the entire European Atlantic coast of which the Wadden Sea ecosystem is an integral part.

The scale of human pressures on marine ecosystems gradually increased from local to global effects during the last centuries. Global warming, sea level rise and trans-oceanic spread of alien species are now or will be major drivers of ecological change in the Wadden Sea. Eutrophication may rank as a superregional effect as the organic matter turnover in the Wadden Sea is driven by import from the North Sea. These effects will not necessarily be uniform across the entire Wadden Sea. A major challenge will be to understand the interactions between global factors with local conditions, both changed by human interference. A conceptual understanding of factors leading to regional differences within the Wadden Sea is only slowly evolving. Based on our understanding of the eutrophication process and a regional comparison of the eutrophication status of the Wadden Sea we propose that wider tidal basins (with a large distance between barrier islands and the mainland) generally have a lower eutrophication status than narrower tidal basins (with a short distance between barrier islands and mainland). This may be explained by a “dilution” of the imported organic matter in wider tidal basins. The interactions between global and super-regional effects on local ecological developments in the Wadden Sea will be complex and pose a major challenge to science, Wadden Sea management and the assessment of ecosystem health as required by EU legislation. The implications of integrating a North Sea view into Wadden Sea management are discussed and we conclude that the river basin approach of the EU Water Framework Directive is not appropriate for the Wadden Sea.

We reconstruct past accretion rates of a salt marsh on the island of Sylt, Germany, using measurements of the radioisotopes 210Pb and 137Cs, as well as historical aerial photographs. Results from three cores indicate accretion rates varying between 1 and 16 mm year−1. Comparisons with tide gauge data show that high accretion rates during the 1980s and 1990s coincide with periods of increased storm activity. We identify a critical inundation height of 18 cm below which the strength of a storm seems to positively influence salt marsh accretion rates and above which the frequency of storms becomes the major factor. In addition to sea level rise, we conclude that in low marsh zones subject to higher inundation levels, mean storm strength is the major factor affecting marsh accretion, whereas in high marsh zones with lower inundation levels, it is storm frequency that impacts marsh accretion.

As a result of aquaculture activities Pacific oysters Crassostrea gigas (Thunberg, 1793) have invaded the European Wadden Sea. Using a variable noncoding mitochondrial marker, we show that the invaded range is the result of two independent invasions. Haplotype frequencies point towards two separate groups, one in the southern and the other in the northern Wadden Sea. We found virtually no genetic differentiation throughout the southern range and the putative source from British Columbia, Canada, suggesting that the Southern region can be considered as a closed population. In the North, mismatch distributions, haplotype ordination and isolation-by-distance analysis suggest a stronger, persistent impact of aquaculture on invasive populations. Due to the ongoing supply of new genetic material from hatchery production the northern invasive populations can therefore be considered as an open population highlighting the importance of aquaculture practice on the genetics of this keystone invader in the Wadden Sea. Highlights: ► Invasions of Pacific oysters into the Wadden Sea are genetically distinct. ► Aquaculture provides constant input of new genetic material into the northern range. ► No significant genetic differentiation was found throughout the southern range. ► Aquaculture practice has strong effects on population demographic parameters.

Generally, estuarine intertidal mudflats constitute important nurseries for fish and foraging grounds for coastal birds by providing a plenitude of mollusks, worms, and crustaceans as prey, which in turn mostly feed on suspended and benthic microalgae, bacteria, and detritus. Despite the high productivity of such habitats, pronounced variability in both salinity and temperature results typically in low diversity. The only sea anemone reported from estuarine mud is the edwardsiid Nematostella vectensis Stephenson, 1935 . It occurs widely in the northern hemisphere, and occasionally in extremely high density. Here we document another sea anemone from estuarine mud and muddy sand found in Southern Chile which has similar ecological attributes. Taxonomic confusion has impeded the reporting on this small but prominent member in a macrozoobenthic assemblage, the brooding Anthopleura hermaphroditica (Carlgren, 1899; Anthozoa: Actiniidae). It differs from N. vectensis by the presence of symbiotic algae. Average density under poly- to euhaline conditions in mud and muddy sand at around mid tide level was about 3 actinians per cm 2 . An average abundance of 11,000 m − 2 , a biovolume of 487 cm 3 m − 2 , and a biomass of 35.5 g dry organic weight m − 2 were found in mud and muddy sand in two surveys 20 years apart. The mean fishing area of fully expanded individuals covers 42 ± 25 mm 2 , corresponding to a circular area with a diameter of 7.3 ± 5.7 mm. Preliminary experiments indicate that associated benthos may be relegated to life below surface by the net of tentacles above the sediment. As no predators on A. hermaphroditica could be found on the mudflat, the success of this mixotrophic sea anemone may entail a trophic dead end.

Background and Aims The dwarf eelgrass, Zostera noltii, is a predominant inhabitant of soft-bottom intertidal regions along the coasts of northern Europe. It is a monoecious, protogynous angiosperm in which the potential for self-fertilization and inbreeding are high, especially if clone sizes exceed pollen dispersal distances. The aim of the present study was to determine the relationship between mating system and clonal structure, examine the relative roles of geitonogamous selfing and biparental inbreeding, measure pollen availability (multiple paternities) and estimate pollen dispersal.Methods A 100-m(2) plot was established in a large, intertidal Z. noltii meadow on the island of Sylt in the German Wadden Sea. A total of 256 adult shoots was sampled: one from the centre of 100 fixed 1-m(2) quadrats (large scale resolution) and an additional 156 from within eight randomly selected 1-m(2) sub-quadrats (small-scale resolution). DNA was extracted from seeds and leaf tissue of all samples and genotyped with nine microsatellite loci.Key Results Mating system analysis revealed high multilocus and single locus outcrossing rates. Average pollen dispersal distance was nearly the same as the average genet (clone) size. Multiple paternity was common and 20-30% of mature seeds originated from matings within the plot. Among inbred seeds, most resulted from geitonogamy rather than biparental inbreeding.Conclusions Moderate disturbances intrinsic to the intertidal habitat appear to facilitate seed recruitment by gap formation. Pollen dispersal distances are sufficient to maintain outcrossing and high clonal diversity.

Feeding pits dug by waterfowl in Zostera noltii meadows are thought to promote seedling recruitment by accumulating seeds and enhancing germination. We tested the latter hypothesis by creating a series of "treatment pits" (resembling natural feeding pits) in the center and at the edge of two meadows near the Island of Sylt (Germany). Seedling density was monitored from the autumn seed set until the following spring. Seedling density (mean, SE) in treatment pits was significantly higher (4.4, 5.3) than in manipulated (2.4, 1.9) and unmanipulated controls (1.4, 0.4), as well as significantly higher in center (2.8, 0.5) relative to edge (2.5, 1.1) locations. Results confirm a facilitating effect of waterfowl grazing on seedling recruitment in spring due to seed accumulation in feeding pits in autumn. The mechanism could provide a valuable tool for the conservation of intertidal Z. noltii meadows in the Wadden Sea. (C) 2010 Elsevier B.V. All rights reserved.

Neue Untersuchungen im Wattenmeer decken eine Uberflutung durch eingeschleppte Arten auf, die sich in den vergangenen Jahren vor allem im Flachwasserbereich direkt unterhalb der Gezeitenzone ausbreiten konnten. Hier sind sie Nutznieser der durch den globalen Klimawandel verursachten ansteigenden Wassertemperaturen. Entstanden ist eine diverse Lebensgemeinschaft von Algen und Wirbellosen, die sich fortlaufend weiter verandert. Neben der strukturellen Komplexitat der biogenen Habitate hat besonders die funktionelle Gruppe der filtrierenden Organismen zugenommen. Insgesamt ist das gesamte Okosystem in einen Umbruch geraten, der nicht mehr umkehrbar ist. Globalization under water: Alien species in the Wadden Sea World Heritage Recent investigations reveal an increasing number of non-native species in the Wadden Sea which profit from warmer water temperatures caused by global change. These exotic species achieve highest occurrence and densities in shallow waters near the low tide water level. In this tidal zone, a highly diverse species community of algae and invertebrates became established and will continue to alter in composition. This leads to enhanced complexity of biogenic habitats and to a prevalence of filter feeding organisms. Thus, we observe a fundamental change of the whole Wadden Sea ecosystem which is without return.

article i nfo On aerial photographs, sandy tidal flats display (1) large sandy bedforms (N10 m long, N 3 m wide), indicating effects of strong hydrodynamics on sediment relief, and (2) beds of seagrass and mussels, indicating stable sediment conditions. These physical and biogenic structures have been mapped from aerial photographs taken in a back-barrier tidal basin of the North Sea coast at low tide between 1936 and 2005. Fields of large intertidal sandy bedforms show a consistent spatial distribution in the central part of the basin, and have increased in area from 7.2 to 12.8 km 2 , corresponding now to 10% of the tidal flats. Areal expansion may be linked to a rise in average high tide level and an increase of the expansion rate from the 1960s to the mid 1990s might be traced back to an increased frequency of storm tides during this period. It is shown that expanding fields of large sandy bedforms have replaced mussel beds in the low tidal zone and displaced seagrass beds in the mid tidal zone. Fields of intertidal large sandy bedforms are expected to expand further with an accelerating rise in sea level, and it is recommended to monitor these physical indicators of sediment instability and disturbance of biogenic benthic structures by analysing aerial photographs.

Genotypic structure and temporal dynamics of the dwarf seagrass, Zostera noltii, were studied in an intertidal meadow that has persisted since prior to 1936 near the Wadden Sea island of Sylt. Samples were collected from two 10 x 10 m plots separated by 250 m from May 2002 to June 2005 and from four 1 x 1 m plots from June 2003 to September 2004. All the samples were genotyped with nine microsatellite loci. No genotypes were shared between the plots separated by 250 m. Genetic diversity was higher in the Wadden Sea than in the other regions of its geographic range. The average clone size (genets) (SD) in the two plots was 1.38 (0.26) and 1.46 (0.4) mA(2), respectively, with a range up to 9 mA(2) and 4 years. A high genetic and genotypic diversity was maintained by annual recruitment of seedlings despite a dramatic decrease in ramet density that coincided with the severe heat stress event of 2003. Fine-scale (1 mA(2)) analysis suggested that extensive loss of seagrass cover precluded space competition among the genets, while a persistent seed bank prevented local extinction. Long-term persistence of Z. noltii meadows in the intertidal Wadden Sea was achieved by high genet turnover and frequent seedling recruitment from a seed bank, in contrast to the low diversity observed in large and long-living clones of Z. noltii and other seagrasses in subtidal habitats.

The intertidal dwarf eelgrass Zostera noltii is a dominant species in the Dutch and German Wadden Sea. Although numerous studies of its reproductive ecology have been conducted, few have examined the importance of seeds and seed banks for meadow maintenance. We investigated the contribution of a seed bank (size, genetic potential and persistence) to annual recruitment of dwarf eelgrass in the German Wadden Sea using temporal sampling of seeds from the sediment and genetic assignment tests of seedlings to populations of adult shoots from previous years. Annual sediment seed density (SD) was 487.5 m(-2) (269.4) and 367.3 m(-2) (95.5) in 2004 and 2005, respectively, and distribution of seeds in the sediment was highly aggregated. The proportion of over-wintering seeds that germinated under laboratory conditions was 16 to 25%, and field-germination revealed a 12% survival to the seedling stage. Nearly 20% of all shoots present in May 2004 were seedlings. Using 9 microsatellite loci, seedlings sampled in 2004, 2005 and 2006 were compared with adults sampled in 2002, 2003 and 2004; results revealed that 7 to 33% of seedlings could be assigned to the local adult population in Current or previous years. Although new recruitment plays an important role in the maintenance of these meadows, considerable new recruitment comes from within the meadow., itself. Seeds are viable for at least 3 yr, thereby forming a relatively short-term, but persistent, seed bank.

Bioturbating lugworms (Arenicola marina) were excluded from 400 m² plots of intertidal sand which initiated sequences of direct and indirect changes in the structure of the benthic community. The sessile, tube-building species Polydora cornuta and Lanice conchilega took advantage of the absence of lugworms and settled preferentially on lugworm exclusion plots. The protruding tubes provided attachment for an ephemeral development of algal tufts (Berkeleya colonies and Enteromorpha thalli) which in turn enhanced settlement of the juvenile drifting clams Mya arenaria and Macoma balthica. This causal chain of enhanced bivalve settlement in the presence of above-ground structures, like animal tubes and algae, on lugworm exclusion plots occurred in 2 years at different tidal zones with different tube builders, algae and juvenile clams. A significant response of L. conchilega in a year with relatively low lugworm abundances at the entire site suggests that not only the actual absence of large bioturbators was responsible for the establishment of tube-dwelling species, but also a cumulative change of the sediment in exclusion plots since the onset of the experiment. While the sediment on lugworm plots remained permeable, fine particles and organic matter accumulated at exclusion plots. It is suggested that these differences in sediment characteristics were the product of divergent benthic engineering by sediment destabilizing lugworms on control plots and sediment stabilizing species on exclusion plots. Cumulative changes of the sedimentary habitat and cascading effects in the benthic community may explain the persistence of patches that are dominated by either sediment stabilizing or destabilizing species in the assemblage mosaic of intertidal sediments.

Shallow waters and lowland meet at the same level in the Wadden Sea, but are separated by walls of coastal defense. What are the prospects of this coastal ecosystem in a warmer world? We focus on tidal waters and inshore sedimentary bottoms, expect nutrient supply from land to decline and species introductions, temperature and sea level to rise. The effects are interrelated and will have an increasing likelihood of abrupt and irreversible developments. The biotic interactions are hardly predictable but we anticipate the following changes to be more likely than others: blooms of phytoplankton will be weak mainly because of increasing pelagic and benthic grazing pressure, both facilitated by warming. Possibly birds feeding on mollusks will encounter decreasing resource availability while fish-eaters benefit. Extensive reefs of Pacific oysters could facilitate aquatic macrophytes. Sea level rise and concomitant hydrodynamics above tidal flats favor well-anchored suspension feeders as well as burrowing fauna adapted to dynamic permeable sand. With high shares of immigrants from overseas and the south, species richness will increase; yet the ecosystem stability may become lower. We suggest that for the next decades invasions of introduced species followed by warming and declining nutrient supply will be the most pressing factor on the changes in the Wadden Sea ecosystem, and the effects of sea level rise to be the key issue on the scale of the whole century and beyond.

Where, since the 1980s, patchy and variable green algal mats are prevailing, distinct belts of an amphipod (Corophium volutator) and seagrass (Zostera spp.) had dominated in the 1930s. The zonation between tide marks has been mapped in a sheltered sedimentary bay in the Wadden Sea near the island of Sylt (coastal eastern North Sea). Maps on vegetation from 1924 and on selected macrobenthos from 1932 and 1934 are compared with biannual surveys conducted from 1988 to 2006. Rising high water levels and eutrophication are suggested to be major causes of the observed long-term changes. In front of a saltmarsh, a sandy beach developed and partly displaced former cyanobacterial mats. Advancing sandiness may have inhibited C. volutator and facilitated lugworms, Arenicola marina, in the upper tidal zone. A variable occurrence of green algal mats arising in the 1980s affected infauna and seagrass by smothering the biota underneath. This dissolved a coherent belt of Zostera noltii. In the lower tidal zone, natural disturbances had lasting effects on the occurrence of mussels with attached fucoid algae. The spectrum of species became enriched by alien species (13% of macrobenthic taxa). A reversal to habitat structure and biotic zonation of the 1920–1930s does not seem possible. Aliens, in combination with climate change, are expected to further divert the ecological pattern to new configurations.

Aerial surveys on seagrass (Zostera spp.) indicate a three to fourfold increase in bed area from 1994 to 2006 with up to 100 km2 or 11% of intertidal flats in the Northfrisian Wadden Sea (coastal eastern North Sea), observed at seasonal maximum in August when flying during low tide exposure 300 to 500 m above ground. When viewed from the air, difficulties in distinguishing between seagrass and green algae and a lack of contrast on dark-coloured mudflats are sources of error in areal estimates. Particularly the positioning of beds remote from shores was imprecise. However, the consistency in method over time gives confidence to the inferred positive trend which is opposite to the global pattern. Both, the spatial pattern and a recent decrease in storminess suggest that sediment stability is the key factor for seagrass dynamics in this tidal area. On exposed sand flats, high sediment mobility may be limiting and along the sheltered mainland shore land claim activities with high accretion rates may cause a scarcity of seagrass. The potential area of seagrass beds may be twice as large as the realized maximum in 2006 but eventually the rising sea level will reverse the observed seagrass expansion.

The lugworm Arenicola marina was excluded from sandy sediment areas in the mid and low intertidal zone of the Wadden Sea. Exclusion, control and ambient plots were 400 m 2 each, replicated six times and sampled in August of three consecutive years. Responses were analysed with respect to functional trait groups in the associated polychaete assemblage using uni- and multivariate statistical techniques. Tube-building worms and predacious worms were most abundant in exclusion plots, while subsurface deposit feeders tended to dominate in the presence of lugworms. Lugworm effects were stronger in low intertidal fine sand than in mid intertidal medium sand. In the third year, lugworm densities strongly decreased at the study site. Nevertheless the polychaete functional group composition in lugworm exclusion plots still significantly differed from that in control and ambient plots. We assume that the permanent exclusion of lugworms may have entailed a cumulative change in sediment properties in the exclusion plots. Overall, lugworm effects were highly dependent on space and time as well as on differential recruitment success in this intertidal polychaete assemblage. Sediment-mediated effects of an ecosystem engineer on associated species appear to be subtle and contingent in variable environments.

Introduced species are assumed to exert a variety of negative ecological effects in their new environments. However, rigid studies on such effects are still rare. Using a case study we exemplify pitfalls and obstacles for research on ecological effects of invaders and highlight the need for a concise framework. The suspension feeding gastropod Crepidula fornicata was accidentally introduced with American oysters to Europe and was soon after defamed as an ‘oyster pest’ although no evidence was provided in justification. Recently, small-scale experiments with C. fornicata and the Pacific oyster Crassostrea gigas failed to prove competition. As an epizootic, however, C. fornicata is impeding native mussels Mytilus edulis but at the same time provides protection against starfish predation. It also may serve as a sink for infectious trematode parasites and hence be beneficial for bivalve basibionts. Another positive effect of C. fornicata, especially at the coast of France where it is superabundant, may be that it causes a shift of phytoplankton blooms from toxic flagellates to diatoms. The multiple interactions with recipient coastal ecosystems result in a complex interplay of negative as well as positive effects of the invader on native biota. Positive effects of invaders might occur with the same frequency as negative ones, and the general prejudice that introduced species exert per se a negative effect may dictate the outcome of research. We argue that considering both, negative as well as positive effects of an introduced species is needed to eventually enable us to evaluate the overall effect of an invasion on recipient ecosystems. Besides pointing to the importance of positive effects, this case study also shows that research on effects of introduced species should (1) be species specific, (2) consider different spatial scales, (3) clarify which stage of an invasive process is under observation and (4) clearly distinguish between ecological effects, human-relevant impacts and the ethical judgment of both. A concise framework for research on ecological effects of introduced species – which still has to be developed – should incorporate these aspects.

At four intertidal sites near the island of Sylt (eastern North Sea), 13 metazoan parasite taxa were found in 1400 cockles investigated, with digenean trematodes being dominant. Almost all cockles were infected by parasites and most individuals harboured more than one parasite species. We observed four conspicuous patterns: (1) Adult cockles harboured a two times higher species richness (2003: 6.1 ± 0.7 species/host; 2004: 7.1 ± 0.7) than juveniles (2003: 2.9 ± 0.8; 2004: 3.4 ± 0.8) and total parasite community composition significantly differed between age groups. (2) Infection levels were 2–52 times higher in adult cockles than in juveniles both in trematode species and in non-trematode species. In the dominant trematodes, species utilising cockles as first intermediate host ( Gymnophallus choledochus, Labratrema minimus, Monorchis parvus ) only occurred in adult cockles, and prevalences were low (2–12%). Prevalences of up to 100% were reached by trematodes using cockles as second intermediate host ( Himasthla elongata , H. continua , H. interrupta, Renicola roscovita, Psilostomum brevicolle, Meiogymnophallus minutus, Gymnophallus gibberosus). Metacercariae of these species were segregated between body parts within cockles. (3) High spatial heterogeneity in parasite community composition and infection levels occurred between sampling sites. However, communities in juveniles were more similar than communities in adults. (4) Temporal variation in parasite community composition was low between two consecutive years, especially for adult cockles. The omnipresence of parasites in this dominant bivalve species has important implications for sampling designs and as a potentially confounding variable in e.g. physiological studies. It suggests strong and cumulative negative effects on the cockle hosts.

We provide a quantitative inventory of macroparasites in intertidal molluscs from a tidal basin in the Wadden Sea (eastern North Sea). Gastropods and bivalves contained a species rich macroparasite community consisting of trematodes (26 species), turbellarians (1), nematodes (1), copepods (2) and polychaetes (1) in 3,800 host individuals from 10 host species. Highest parasite burdens were observed in the gastropods Hydrobia ulvae and Littorina littorea and in the bivalves Cerastoderma edule and Mytilus edulis. In contrast, only one parasite species and no trematodes were found in Crepidula fornicata. The parasite community in the molluscs was similar to other Western European localities but some parasite species showed obvious differences, related to the large-scale distribution of intermediate and final hosts. Parasitism seems to be a common phenomenon in molluscs of the Wadden Sea and hence the detrimental effects observed in experiments can be expected to frequently happen in the field.

The introduced cordgrass Spartina anglica, a fertile hybrid of S. maritima and S. alterniflora, grows as a pioneer plant in the upper intertidal zone and has invaded most sheltered shorelines of the Wadden Sea. After its introduction in 1927 S. anglica has spread vigorously along the mainland shore and on some of the more southern islands. In contrast, it has later established on Sylt and spread at a lower pace. On the island of Sylt it occurs near at its northern limit in Europe. Due to rising sea level and storm frequency a decrease or steady state of S. anglica was expected because its niche is narrowing rather than widening and plants are more frequently eroded. Contrary to that, many new sites were colonized and dense monotypic swards have formed after 1985. This new spread coincided with a shift in the local temperature regime around 1987. The monthly mean temperature from January to April has increased significantly after 1987. Furthermore, the important physiological thresholds of 4 °C for germination and 7 °C for photosynthesis were more often exceeded during spring after 1987 than before. We suggest that warmer spring seasons since 1988 could have promoted germination, growth and the recent accelerated spread of this neophyte.

On six 400 m2 plots over 3 years, we excluded the sessile subduction and conveyer-belt feeding polychaete Arenicola marina which generates a pit-and-mound topography at the sediment surface from intertidal sands near the island of Sylt, Germany. This experiment was used to test whether other abundant deposit feeding polychaetes (the discretely motile and surface feeding ragworm Nereis diversicolor and the subsurface-feeding, motile orbiniid polychaete Scoloplos cf. armiger) benefit from competitive release. Ragworms took advantage from the absence of lugworms. Presumably they responded to a more stable and nutritious surface layer at lugworm exclusion plots (relief from inhibitive bioturbation). Contrary to this, S. cf. armiger was negatively affected by the exclusion of A. marina. It may have suffered from higher sulfide concentrations in the less irrigated and less permeable sediment where lugworms were absent. For adult worms of both species these results were consistent in 2 out of 3 years examined. Recruitment by N. diversicolor was highly variable between years and occurred either irrespective of experimental treatments or the response was inconsistent. Juveniles of S. cf. armiger benefited from the presence of A. marina and aggregated near lugworm tail shafts where inflow of oxygen rich water was high and sulfide concentrations were low. Biogenic habitat mediated effects of lugworms on both deposit feeders were in the same order of magnitude as abundance variation in space and time. Thus, A. marina was one of the key factors structuring the deposit feeding community. It is suggested that arenicolids modify the composition of the associated polychaete assemblage primarily through habitat transformation.

Inshore waters of European coasts have accumulated a high share of non-indigenous species, where a changeable palaeoenvironment has caused low diversity in indigenous biota. Also strongly transformed modern coastal ecosystems seem to assimilate whatever species have been introduced and tolerate the physical regime. Adding non-native species does not have any directional predetermined effects on recipient coastal ecosystems. The status of being a non-native rather refers to a position in evolutionary history than qualify as an ecological category with distinct and consistent properties. Effects of invaders vary between habitats and with the phase of invasion and also with shifting ambient conditions. Although aliens accelerate change in European coastal biota, we found no evidence that they generally impair biodiversity and ecosystem functioning. More often, invaders expand ecosystem functioning by adding new ecological traits, intensifying existing ones and increasing functional redundancy.