Long-term Changes of a Brackish-water Eelgrass (Zostera marina L.) Community Indicate Effects of Coastal Eutrophication

The distribution and importance of eelgrass (Zostera marina L.) meadows for associated faunal communities in the coastal waters of the Baltic Sea are still poorly known. In these low-saline (5–7), shallow coastal areas Z. marina grows at its limit of distribution, forming mostly patchy meadows. In June 1993, a seagrass locality (Tva¨rminne, SW Finland) thoroughly studied in 1968–71 was revisited in order to detect possible long-term changes in both vegetation structure (distribution, density, biomass) and benthic fauna (species composition, abundance, biomass, distribution and diversity patterns). The same sampling design as in the 1970s was used in both sparse (<20 shoots m 2) and dense (>150 shoots m 2) Z. marina. In addition, the feeding-efficiency of adult flounder (Platichtys flesus L.) on infauna was measured by counting feeding pits in vegetated and bare sand. The analysis shows that the shoot density had increased in sparse Z. marina, while dense Z. marina patches showed similar biomass values (20 g AFDW m 2) as in the 1970s. In contrast to the vegetation, where little apparent change could be recorded, the total abundance and biomass of zoobenthos has increased significantly between 1968–71 and 1993 in the dense Z. marina patches. These changes are mainly attributed to significant increases of the bivalve Macoma balthica L., mudsnails Hydrobia spp. and oligochaetes. In sparse Z. marina diversity in terms of number of taxa exhibited minor changes over time, whereas in dense Z. marina patches the mean number of taxa has increased from 16 to 20. The vegetation cover was sufficient to reduce significantly the predation effects of flounder on seagrass infauna. This study represents a rare example of long-term persistence of seagrass communities in an area where the negative effects of nutrient enrichment are evident. The faunal changes in the Z. marina community indicate increased food availability, which is associated with positive effects of coastal eutrophication.