Importance of blue mussels for biodiversity and ecosystem functioning in subtidal habitats

Abstract: Blue mussels, Mytilus spp., occur on rocky and sedimentary shores worldwide where they often form dense aggregates. These assemblages change the local environment and create unique habitats. By extensive filter-feeding, the mussels regulate availability and flow of resources such as nutrients and organic material, and are thereby important in benthic-pelagic coupling. This thesis investigates the role of Mytilus spp. for the diversity of associated species, ecosystem functioning and services that the mussels provides. My results show that Mytilus habitats support highly diverse associated communities, especially in subtidal sediment habitats in the Skagerrak and Baltic Sea. The macrofauna species richness was shown to correlate with mussel patch size and biomass. Structural properties of Mytilus spp. provide substrate for attachment, shelter and increase habitat complexity in the system, which was found to enhance species diversity. Biological activities of the mussels, e.g. filter-feeding, biodeposition and nutrient regeneration, seemed to determine the abundance, biomass and functioning of the associated plant and animal communities. The communities are dependent on import of energy resources through the mussels filter-feeding from the pelagic system, which increase contents of organic carbon and nitrogen in sediments by biodeposition even in very small patches. At larger scale, the role of Mytilus spp. as a habitat-modifying species varied between substrate types. On soft substrate, where physical structure is scarce, Mytilus spp. had strong positive effects on the associated algal community, while on hard substrate much less influence was found. In Flensborg fjord, when coexisting with eelgrass, Zostera marina, presence of Mytilus edulis influenced the grain size and biogeochemistry of the sediment, which in turn, seemed to induce sulphide stress in the plants and change plant performance. Due to its biomass dominance and substantial water-filtering capacity, Mytilus sp. tends to counteract eutrophication and maintain water quality maintenance services in the Baltic Sea. The mussels' filtration of plankton and particulate organic material from the pelagic system improves the light climate for benthic algae and increase production of other benthic organisms. This promotes a shift from a turbid plankton dominated system to a highly diverse and productive benthic system. I conclude that Mytilus spp. is important for sustaining biodiversity and ecosystem functioning of subtidal habitats, especially in the Baltic coastal zone.