Community structure and spatial variation of benthic invertebrates associated with Zostera marina (L.) beds in the northern Baltic Sea

The distribution and bed structure of eelgrass (Zostera marina L.), and its importance for associated faunal communities in the coastal areas of the northern Baltic Sea are poorly known. The spatial distribution of the fauna associated with Zostera was studied at five localities in SW Finland in 1993–1994. Zostera was common on all localities, but the beds varied in terms of area (1–5 m diameter), density (50–500 shoots/m2) and blade length (20–110 cm). A total of about 40 species or taxa were recorded. The zoobenthic infauna showed significant spatial differences, and total abundance and species diversity were significantly higher in the Zostera beds than in adjacent bare sand. The total abundance in Zostera ranged from 25 000 to 50 000 ind/m2 and in sand from 2500 to 15 000 ind/m2 The mean number of species in Zostera ranged from 5.9 to 8.8 spp (H′ = 1.76–2.54) and in sand from 2.2 to 5.5 spp (H′ = 1.67–2.31). The epifauna in Zostera was numerically dominated by grazing gastropods (Hydrobiidae) and copepods. The epifauna is an important community component, which contributes to the total diversity of the Zostera assemblage. These systems are among the most species-rich components of the shallow soft-bottom ecosystems in the northern Baltic Sea. The mechanisms structuring both the Zostera and the ambient sand-bottom habitats are presented.