Patches of the mussel Mytilus sp are islands of high biodiversity in subtidal sediment habitats in the Baltic Sea

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 Asko 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.