Karsten Reise

Introduced marine species of the North Sea coasts

About 80 non-indigenous species are assumed to have been introduced into the North Sea by transoceanic shipping and aquaculture. The number is certainly underestimated as most small organisms received insufficient attention at the species level. Also, the seafaring tradition of the North Sea countries is much longer than our biological surveys are. Most exotic invertebrates originate from the western Atlantic and were introduced by shipping, while most algae stem from the Pacific and came with the introduced oysters. A peak of newcomers was observed in the 1970s. Most of the arrivals became established in brackish environments, at harbor sites and in the vicinity of oyster farms, fouling on hard substrates or living as epibionts. A few live in sediments, are holoplanktonic or are parasites. At the open coast, approximately 6% of the macrobenthic species are exotics, while in estuaries their share is up to 20%. Most exotics have been encountered in the southern North Sea first, and many did not spread further north. About 25% of the established non-natives are widespread and attain locally high abundances. As a consequence, some inshore habitats are entirely dominated by exotics. The overall effect on the ecosystem seems to be more additive than one of displacement. This suggests that the coastal biota of the North Sea are quite capable of accommodating newcomers. However, this is no guarantee that the next introduced species may not cause severe ecological change or economic harm. There is a need to minimize the risk of unintentional introductions by ballast water treatment and by adhering to quarantine procedures in aquaculture. Current research on exotics in the North Sea is regarded as inadequate for proper evaluation and management requirements.

Historical changes in the benthos of the Wadden Sea around the island of Sylt in the North Sea

The situation regarding the distribution and abundance of seagrass, macroalgae and benthic fauna near the island of Sylt in the south-eastern North Sea during the period 1923 to 1940 is compared with that of the 1980s. Evidence of organic enrichment in recent times is provided by (1) massive growth of green algal mats on sheltered tidal flats, (2) a decline of red algae in the subtidal zone, (3) an expansion of mussel beds along low water line and down to 20 m depth, (4) increased abundance of polychaetes inhabiting intertidal and subtidal sandy bottoms. Seagrass beds have undergone complex changes which remain unexplained. Intensified erosion has contributed to the loss of habitats in the intertidal zone, and probably affected sessile epifauna in the deep channels. Here, direct removal and disturbance by the bottom-trawling fishery may also have contributed to the observed species impoverishment.

Macrobenthos of the subtidal Wadden Sea: revisited after 55 years

During the years 1923–1926 Hagmeier & Kändler (1927) sampled the macrofauna of subtidal shallows and channels of the Wadden Sea close to the Island of Sylt (German Bight, North Sea). Reinvestigating this study area in 1980, a substantially altered faunal composition was recorded. An approach is made to quantify the comparison in terms of abundance, species richness and diversity of invertebrate taxa. Human interference is assumed to be responsible for the major changes. Natural oyster beds have been overexploited and the local population ofOstrea edulis has been driven to extinction. Subsequently, mussels(Mytilus edulis) spread in the entire region, promoted by shell fishery. Particularly barnacles and many polychaetes took advantage of the expansion of mussel banks which is substantiated by correlation analysis. Reefs of the colonial polychaeteSabellaria spinulosa stood in the way of shrimp trawling and became destroyed together with the associated fauna. A subtidalZostera marina bed was wiped out in 1934 by a natural epidemic disease but never succeeded in reestablishing itself. The associated fauna disappeared. Large epibenthic predators and scavengers (crabs, snails and starfish) survived all these changes. The total number of species remained approximately at the same level but molluscs experienced losses and polychaetes diversified. Overall abundance increased with a disproportionately large share of a few species(Mytilus edulis, Balanus crenatus, Cerastoderma edule, Scoloplos armiger). The subtidal fauna of the Wadden Sea proved to be vulnerable to human disturbance; thus, the present community can no longer be viewed as the outcome of entirely natural processes.

High abundance of small zoobenthos around biogenic structures in tidal sediments of the Wadden Sea

On the tidal flats of the island of Sylt (eastern part of the North Sea) the quantity of micro- and meiofauna associated with shoots of seagrass(Zostera noltii), with infaunal bivalves(Macoma balthica), and with tubes and burrows of polychaetes(Pygospio elegans, Pectinaria koreni, Nereis diversicolor, Nereis virens, Arenicola marina) was found to add up to 5 to 33 % of the overall abundance. These structures, taken together, account for 10 to 50 % of the faunal abundance on an average tidal flat at Sylt. The quantitative effect of biogenic structures at the sediment surface (casts and funnels) is small compared to that of tubes and burrows penetrating the anaerobic subsurface layer. In providing stable oxic microenvironments these elite structures frequently bring together more individuals than occur in the entire reducing sediment below surface. Faunal composition of irrigated dwellings of large infauna is different from that of the oxic surface sediment. The common denominator of all elite structures of the subsurface is an oxic halo. Burrows without such a halo are unattractive. There is no evidence that owners of burrows prey on their smaller inmates.

Predator exclusion experiments in an intertidal mud flat

The intensity of predation pressure exerted on the macrofauna in muddy sediments was investigated in Königshafen (island of Sylt, eastern North Sea). Tests in aquaria revealed shrimpCrangon crangon, juveniles of the shore crabCarcinus maenas, and gobiid fishPomatoschistus microps as the most important local predators. Their high abundance from July to September led to the hypothesis, that predation pressure on the infauna will be most intense during that period. Cages were set up to protect the infauna against these predators. No protective effect was achieved with cages constructed of 20-mm mesh nylon net; however, cages with screen wire and gauze of 5-mm mesh and smaller resulted in a considerable increase of the infauna. In cages of 1-mm mesh gauze, set up from March to June, the macrofauna reached an abundance of four times the control density. In the period from July to October, the factor of increase was as high as 23. Species density was 28 400 cm−2, as compared with only 7 in the control. Species which colonized the mud flat during spring and early summer established dense settlements inside the cages, but failed to do so in uncaged mud-flat areas. It is concluded that predation by young crabs, shrimp and gobies determines to a large extent the structure and dynamics in the local intertidal macrofauna.

Moderate predation on meiofauna by the macrobenthos of the Wadden Sea

The role of meiofauna in the trophic web of marine ecosystems is a controversial topic. During an experimental study on tidal flats in the eastern part of the North Sea, potential predators on meiofauna were kept in enclosures. A gobiid fish, shrimp, crabs, an amphipod, four polychaete worms and a nemertine were tested for their ability to decimate nematodes, turbellarians, ostracods and benthic copepods. Only few macrobenthic species preyed heavily on permanent meiofauna: juveniles of the shore crabCarcinus maenas, the hermit crabPagurus bernhardus with the colonial hydrozoanHydractinia echinata on its shells, and the rag wormNereis diversicolor. When benthic infauna was protected from these predators with exclosures, juvenile macrofauna responded with a marked increase in number, while the permanent meiofauna remained unaltered except for a limited increase in nematodes. It is concluded that in the Wadden Sea abundance of permanent meiofauna (Nematoda, Turbellaria, Ostracoda, Copepoda) is only locally or temporarily regulated by macrobenthic predators.

Human transformations of the Wadden Sea ecosystem through time: a synthesis

Today’s Wadden Sea is a heavily human-altered ecosystem. Shaped by natural forces since its origin 7,500xa0years ago, humans gradually gained dominance in influencing ecosystem structure and functioning. Here, we reconstruct the timeline of human impacts and the history of ecological changes in the Wadden Sea. We then discuss the ecosystem and societal consequences of observed changes, and conclude with management implications. Human influences have intensified and multiplied over time. Large-scale habitat transformation over the last 1,000xa0years has eliminated diverse terrestrial, freshwater, brackish and marine habitats. Intensive exploitation of everything from oysters to whales has depleted most large predators and habitat-building species since medieval times. In the twentieth century, pollution, eutrophication, species invasions and, presumably, climate change have had marked impacts on the Wadden Sea flora and fauna. Yet habitat loss and overexploitation were the two main causes for the extinction or severe depletion of 144 species (~20% of total macrobiota). The loss of biodiversity, large predators, special habitats, filter and storage capacity, and degradation in water quality have led to a simplification and homogenisation of the food web structure and ecosystem functioning that has affected the Wadden Sea ecosystem and coastal societies alike. Recent conservation efforts have reversed some negative trends by enabling some birds and mammals to recover and by creating new economic options for society. The Wadden Sea history provides a unique long-term perspective on ecological change, new objectives for conservation, restoration and management, and an ecological baseline that allows us to envision a rich, productive and diverse Wadden Sea ecosystem and coastal society.

Coast of change: habitat loss and transformations in the Wadden Sea

In the southern North Sea, coastal people commenced with habitat conversions 1,000xa0years ago. Partly interrupted in late medieval times by large-scale inundations of marshland, progressive embankments transformed the landward half of the amphibic transition zone between a limno-terrestric and a brackish-marine ecosystem into arable land and freshwater lakes. Sea walls rigidly separated the land from the sea. Dynamic transitional habitats have vanished. Areal loss has diminished the capacity of the Wadden Sea to dissipate wave and tidal energy. A coastal ecosystem once rich in marsh plants, seagrass and diatoms on mud flats became transformed into one with less autochthonous phototroph production, dominated by sandy tidal flats, and dependent primarily on allochthonous plankton supply. The large estuaries have been dredged to serve as shipping canals, and have lost most of their former retention and filter capacity. Riverine loads are now flushed right into the North Sea. Symptoms of a syndromatic coastal habitat degradation are diagnosed, leading to a decline in natural habitat diversity. The conventional on-line coastal protection may not achieve a sustainable coastal habitat configuration. At sedimentary coasts immobilised by dikes and petrified shores, a more flexible response to sea level rise is recommended.

Macrobenthic turnover in the subtidal Wadden Sea: The Norderaue revisited after 60 years

The benthic macrofauna of a tidal inlet in the northern Wadden Sea was sampled with grab and dredge in 1924–1926 (Hagmeier & Kändler, 1927), and again in 1985 and 1986. The comparison of surveys from consecutive years, as well as observations from an adjacent area, are employed to separate spurious from real long-term changes. Several epibenthic species of the 1920s became rare or absent in the 1980s. Oyster beds and reefs of the colonial polychaeteSabellaria spinulosa have disappeared completely. On the other hand, mussel beds have extended their range, and the abundance of mobile infauna has increased. The total number of species has remained approximately the same. Compared to surveys from consecutive years, the 60-year interval has doubled the species turnover rate, and has decreased the similarity in relative abundances by one third. The observed losses are best explained by the impact of dredging and trawling on the benthic fauna, while gains seem to indicate coastal eutrophication.

Epibenthic fauna dredged from tidal channels in the Wadden Sea of Schleswig-Holstein: spatial patterns and a long-term decline

The epibenthic fauna dredged in the channels of the Wadden Sea of Schleswig-Holstein is dominated by crustaceans. Mean species richness is lower in a southern region (7.6±2.2 species haul−1) which is part of the Elbe estuary and lacks protective barrier islands. A northern region is more marine and is protected by a seaward barrier of high sands. Here mean species richness is 10.9±2.8. Within channels, there is no significant inshore-offshore gradient, and species number does not increase with depth. Stratified sampling near the island of Sylt revealed that abundance and diversity are high along the gentle upper slope (5 m depth) of channels when mussels are present, but low in the absence of mussels. Abundance and diversity are also low along the steeper slope below (at 10 and 15 m depth). In a channel with scattered stones and boulders, highest diversity (16.0±5.1) and dense epibenthic assemblages were encountered along the upper and middle slopes (5 and 10 m). Comparisons with historical surveys suggest that a decline of nearly fifty percent of all epifaunal species within the last hundred years may be attributed to fishery disturbances.