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.), an d its importance for associated faunal communities in the coastal area s of the northern Baltic Sea are poorly known. The spatial distributio n of the fauna associated with Zostera was studied at five localities in SW Finland in 1993-94. Zostera was common on all localities, but th e beds varied in terms of area (1-5 m diameter), density (50-500 shoot s/m(2)) and blade length (20-110 cm). A total of about 40 species or t axa were recorded. The zoobenthic infauna showed significant spatial d ifferences, and total abundance and species diversity were significant ly higher in the Zostera beds than in adjacent bare sand. The total ab undance in Zostera ranged from 25 000 to 50 000 ind/m(2) and in sand f rom 2500 to 15 000 ind/m(2). The mean number of species in Zostera ran ged from 5.9 to 8.8 spp (H' = 1.76-2.54) and in sand from 2.2 to 5.5 s pp (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 diversit y 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 ambien t sand-bottom habitats are presented.