Habitat-modifying species such as Mytilus edulis strongly impact both community structure and ecosystem functioning through positive or negative interactions with other species and by changing physical and biological conditions. A study of natural patches of mussels showed that C and N content of sediment was higher in mussel patches compared to the surrounding sand community. Species richness and biomass of associated macrofauna and -flora was enhanced even by the presence of single mussels and increased rapidly with patch size up to about 150 cm 2 , while no change in sediment meiofauna was found. In order to separate the effects of structure and function of M. edulis, a manipulative field experiment was performed with constructed patches of cleaned live mussels or intact empty shells. After 3 mo colonisation, the number of associated species in both treatments approached those in natural mussel patches, indicating that species richness was mainly due to physical structure. Abundance and biomass of associated flora and fauna were higher if live mussels were present because mussel biodeposition and nutrient regeneration supplied limiting resources and increased carrying capacity. Species composition was also affected. Complexity and biodiversity increased with time, especially if Fucus vesiculosus plants established themselves. Measurements of community metabolism showed that the associated community found in mussel patches depends on mussel biodeposition for 24 to 31% of its energy demand. Thus, the energy balance of the community is dependent on the function of live mussels.

Mytilus sp. is a habitat-modifying species that can influence biodiversity by facilitation and inhibition of species. We investigated the relationship between size of Mytilus patches, sediment characteristics and species richness of associated macrofauna, meiofauna and macroalgae on sandy sediments at 7 m depth in the Askö area, northern Baltic proper. A total of 24 associated macrofauna species were identified, 11 of which were not present on bare sediment outside the patches. The bare sediment had on average 5 species, which increased to 6 associated species at a mussel patch size of 5 cm 2 and 17 species at 314 cm 2 , which was > 85% of the associated species in a nearby large mussel bed. The diversity of macro-infauna was higher in mussel patches compared to sediment. Macroalgae, being dependent on mussel shell as substrate or on attachment by byssus threads, were not found outside patches. Sediment contents of total carbon and nitrogen were significantly higher within patches than outside, mainly due to the mussels' filtration and biodeposition of small nutrientrich particles. Diversity of meiofauna did not change with patch size, but total abundance of Nematoda was higher within patches. As patch size increased, the effects of Mytilus on the sediment characteristics became stronger, but few negative effects of increased biodeposition were seen on species richness. The results show that mussel patches represent islands of high biodiversity in sediment areas and belong to the most species-rich habitats in the Baltic Sea, due to structural and functional effects.

The Pacific oyster, Crassostrea gigas, was introduced to Europe for aquaculture purposes, and has had a rapid and unforeseen northward expansion in northern Europe. The recent dramatic increase in number of C. gigas populations along the species’ northern distribution limit has questioned the efficiency of Skagerrak as a dispersal barrier for transport and survival of larvae. We investigated the genetic connectivity and possible spreading patterns between Pacific oyster populations on the southern Norwegian coast (4 localities) and Swedish and Danish populations by means of DNA microsatellite analysis of adult oysters, and by simulating larvae drift. In the simulations we used a 3D oceanographic model to explore the influence of recent climate change (1990–2010) on development, survival, and successful spreading of Danish and Swedish Pacific oyster larvae to Norwegian coastal waters. The simulations indicated adequate temperature conditions for development, survival, and settlement of larvae across the Skagerrak in warm years since 2000. However, microsatellite genotyping revealed genetic differences between the Norwegian populations, and between the Norwegian populations and the Swedish and Danish populations, the latter two populations being more similar. This patchwork pattern of genetic dissimilarity among the Norwegian populations points towards multiple local introduction routes rather than the commonly assumed unidirectional entry of larvae drifted from Denmark and Sweden. Alternative origins of introduction and implications for management, such as forecasting and possible mitigation actions, are discussed.

The habitat-modifying suspension-feeding mussel Mytilus edulis may have facilitating or inhibiting effects on seagrass meadows depending on the environmental conditions. We investigated the effects of M. edulis on sediment biogeochemistry in Zostera marina meadows under eutrophic conditions in Flensborg fjord, Denmark. Sediment and plant samples were collected at 5 stations with Z. marina (Eelgrass), 5 with Z. marina and M. edulis (Mixed), and at 2 unvegetated ones, 1 with mussels (Mussel) and 1 with sand (Sand). The Mixed sediment was enriched in fine particles (2 to 3 times), nutrients and sulphides compared to Eelgrass stations. Increased sediment nutrient availability at the Mixed stations was reflected in increased N and P content in eelgrass. However, the plant biomass did not differ significantly between stations, while shoot features (number of leaves and leaf area) were significantly reduced at Mixed stations, suggesting an inhibiting effect of M. edulis on Z. marina. Negative correlations between eelgrass measures and sediment sulphide at Mixed stations indicate that the presence of mussels increases sulphide invasion in the plants. A survey of 318 stations in Danish fjords suggests a threshold of 1.6 kg M. edulis m −2 beyond which no coexistence between Z. marina and M. edulis was found.

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