Macrofaunal diversity on beds of the Pacific oyster (Crassostrea gigas) in the Oosterschelde estuary

The Pacific oyster (Crassostrea gigas) is a marine exotic species that has successfully been introduced into the Oosterschelde estuary in the Netherlands in the 1960's. It is still spreading, and can now be considered a part of the local ecosystem. Along with C. gigas other exotic species have been introduced. C. gigas influences its environment through the formation of oyster beds. An oyster bed alters the local environment by influencing current flow, filtering food particles from the water column, by providing structural complexity and by locally enriching the sediment through faeces and pseudofaeces deposition. The aim of this study was to investigate how the presence of a Pacific oyster bed affects local macrofaunal diversity. Along a transect from the center of the oyster bed to the adjacent mudflat, sediment and oysters were sampled to determine the occurrence of species and their abundances. Two oyster beds were sampled. In total, 38 species were found. Polychaeta dominated, and together with Bivalvia and Malacostraca comprised 76% of all specimens identified to species level. Considerably higher biodiversity and abundances per unit area were found on the oyster beds than on the mudflats. This may be due to the structural complexity of the oyster beds, providing many microhabitats, and by organic enrichment of the sediment. The total number of specimens found increased from the center of the oyster bed to the transition zone, where the oyster bed and mudflat overlap. Both depositlsuspension feeders and carnivore/omnivores showed this pattern. This may be explained by the heterogeinity in habitat types of the transition zone. The transition zone may provide an optimal compromise between nutrient availability, shelter and living space for deposit/suspension feeders. Carnivore/omnivores may feed on these, and thus show a similar pattern. Total diversity also increased from the center to the transition zone on one oyster bed, but this was not shown conclusively for the other oyster bed. The implications of the spreading of the Pacific oyster in Dutch waters may be profound. Locally the spreading of Pacific oyster considerably increased biodiversity and abundances. However, the extent of the implications of this to the ecosystem as a whole remain to be investigated.