Spatiotemporal variability in the eelgrass Zostera marina L. in the north-eastern Baltic Sea: canopy structure and associated macrophyte and invertebrate communities

Seagrasses are marine angiosperms fulfilling important ecological functions in coastal ecosystems worldwide. Out of the 66 known seagrass species only two inhabit the Baltic Sea and only one, Zostera marina L., is found in its NE part. In the coastal waters of Estonia, where eelgrass grows at its salinity tolerance limit, only scarce information exists on the Z. marina community and there are no data on eelgrass growth. In the current study the community characteristics and growth of eelgrass were studied at four sites: Ahelaid, Saarnaki, and Soru in the West-Estonian Archipelago Sea and Prangli in the Gulf of Finland. Fieldwork was carried out from May to September in 2005. The results showed that eelgrass grew between 1.8 and 6 m with main distribution at 2-4 m. The eelgrass bed had a considerably higher content of sediment organic matter compared to the adjacent unvegetated areas, but this difference was statistically significant only in areas where the movement of soft sediments is higher. The results also showed that altogether 19 macrophytobenthic and 23 invertebrate taxa inhabited the eelgrass stand. The prevailing vascular plants were Stuckenia pectinata and Potamogeton perfoliatus. Besides attached macrophytes, drifting algae were recorded within the eelgrass communities throughout the study period. Most common invertebrate species inhabiting eelgrass stands were Peringia ulvae, Cerastoderma glaucum, Mytilus trossulus, Macoma balthica, Mya arenaria, Theodoxus fluviatilis, and Idotea chelipes. Irrespective of the studied basin, the increasing eelgrass density supported an elevated diversity and abundance of benthic invertebrates. The eelgrass shoot biomass showed a constant increase from May to September. The overall growth pattern was similar for all studied areas but varied among depth strata. The average shoot density of eelgrass was 50-1300 shoot per m 2 , average biomass ranged from (0.75) 7.8 to 37.31 g dw m -2 in spring and from 18.42 to 68.59 g dw m -2 in autumn.