It is in the very nature of coastal ecosystems to respond rapidly to external driving factors from land, sea, and man. Human-induced environmental change from local to global scale, and introduced species in particular, has accelerated ecological transformation at many coasts worldwide. A series of major events has fundamentally changed the macrobenthos in a tidal bay of the Wadden Sea (Königshafen, south-eastern North Sea, Atlantic Europe) since 1924. (1) Overexploitation extirpated native oysters. (2) The once dominating seagrasses declined dramatically in the 1930s due to an introduced pandemic disease. This was followed by a loss of fine mud, and an expansion of bioturbating lugworms on sandy flats. (3) In the 1980s–2000s, seagrasses declined further and massive green algal mats spread due to eutrophication. (4) Mussel beds gradually expanded in spite of recurrent resets caused by severe winters. (5) Since the 2000s, introduced Pacific oysters dominated mussel beds. With the spread of the mixed reefs of native mussels and Pacific oysters, the extent of mud flats increased again. (6) Since the last three decades, many more non-native species, that are well adapted to a warmer climate, have increased benthic diversity. The ecological developments in this small bay may anticipate change on larger scales and could serve as a model site for learning about the role of contingent history for the functioning of a novel benthic ecosystem.

In an era of accelerating biological globalization and climatic warming, it is vital to understand how introduced species integrate. Pacific Manila clams Ruditapes philippinarum were introduced for aquaculture to Europe in the 1970s, spread and became harvested from the Mediterranean Sea to British coasts. From the Dutch Rhine Delta, Manila clams immigrated stepwise into the southern Wadden Sea, and by long distance jump dispersal they arrived in the northern Wadden Sea in the 2010s. Encounters remained few until 2021, when live clams were often found in intertidal seagrass beds and around mixed beds of Pacific oysters with native mussels. Shell lengths reached up to 74 mm with a longevity of 7-8 years. Adult abundances remained low (<10 clams m(-2)), although larval numbers substantially increased in 2022 and 2023. Strong predation on spat may constitute ecological resistance to the immigrant. Haplotype diversity at mtDNA COI gene fragments is high and its composition is mixed from dominant haplotypes of other European sites, suggesting multiple introductions or an unknown source with an already mixed population. Currently, this is the most northern (55 degrees N) population of R. philippinarum in continental Europe, but with the rapid expansion of this genetically variable population, further northward spread can be expected.

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.

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.

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.

Vaucheria species have previously been restricted to upper shore habitats in the Wadden Sea (eastern North Sea, European Atlantic). In contrast to these previous observations, we have now found Vaucheria longicaulis and a distinct plastid lineage of V. velutina spreading along the lower shores, forming extensive turfs at and below low tide level under fully marine conditions near the Island of Sylt. Species and populations were identified by morphological features and reconfirmed by sequencing the plastid-encoding gene rbcL and the psbA-rbcL spacer region. We needed to modify primer sequences to successfully amplify the rbcL region of marine and brackish Vaucheria species. Due to distinct phylogenetic grouping, we can reject the possibility of niche expansions from the upper shore and propose recent introductions as the cause of the newly formed populations at the lower shore.

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, 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, 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 exotic Vaucheria may have the potential for drastically altering the ecological web at the lower shore.

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.

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.

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.

A shallow sea and coastal plains constitute a natural entity on the Wadden Sea Coast. A highly dynamic history of natural processes and events as well as human intentions and interventions have shaped this unique region. Today, a divergent course has led to the separation of sea and land. This is based on a deep-rooted nature-culture divide in our mentality. However, this old dualism needs to be overcome, especially on a coast challenged by globalisations and a sea level rise of some metres. An approach that keeps nature protection (the sea), coastal protection (along the shore), and the protection of cultural values (in the landscape) separate from each other is bound to run aground. The natural and cultural sciences should join forces together with societies and policymakers in a transdisciplinary approach, developing new concepts for the protection of natural and cultural diversity and for coastal identity.

Global warming will ultimately entail a sea level rise of several metres.However, the rate at which it will rise in the future is still uncertain.The question arises whether dikes built to protect against storm surge flooding will also suffice against a long-term sea level rise on the Wadden Coast.Although much higher dikes are technically possible, costs are likely to escalate.Therefore, no-regret adaptations relieving the outer dike line should be considered as supplementary precaution.Seaward of the dikes, sand nourishments from offshore to inshore could stabilise island positions, supplement sediment accretion rates on tidal flats, and replace stone revetments and brushwood groins.In addition to resulting in wave attenuation, these coastal defence measures would integrate with the landscape and support ecological or touristic functions.Landward of the outer dikes, housing should be above level ground by restoring traditional dwelling mounds, building stilt houses and floating homes.Roads should run on dams.Polders could be permanently inundated, either seasonally or tidally to stop soils from further sinking.More water would diversify the landscape as well as the regional economy with aquaculture, floating gardens and greenhouses.Living with more water could also attract more visitors enjoying water sports and nature.Together, this would spread the risk during disastrous flooding and initiate stepwise the sustainable transformation of the Wadden Coast faced with an unprecedented sea level rise.

Unintended species introductions may offer valuable insights into the functioning of species assemblages. A spectacular invasion of introduced Pacific oysters Magallana (formerly Crassostrea) gigas in the northern Wadden Sea (eastern North Sea, NE Atlantic) has relegated resident mussels Mytilus edulis on their beds to subtenant status. At the beginning of feral oyster establishment, mussel beds offered suitable sites with ample substrate to settle upon. After larval attachment to mussels, the fast-growing M. gigas overtopped and smothered their basibionts. With increasing Pacific oyster abundance and size, oyster larvae preferentially settled upon oysters, and the ecological impact of the invaders on the residents changed from competitive displacement to accommodation of mussels underneath a canopy of oysters. Oysters took the best feeding positions while mussels received shelter from predation and detrimental epibionts. The resident's mono-dominance has turned into co-dominance with an alien, persisting in novel, multi-layered mixed reefs of oysters with mussels, which we term "oyssel reefs." The first 26 yr of the Pacific oyster's conquest of mussel beds in the northern Wadden Sea may question the overcome notions of natural balance, superiority of pristine over novel species combinations, and of introduced alien species threatening biodiversity and ecosystem stability in general.