Spatially extended ecosystem engineering effect of Crassostrea gigas reefs on intertidal macrozoobenthos assemblage

Supervisors: Brenda Walles, Tom Ysebaert (Royal Netherlands Institute for Sea Research (NIOZ) and Utrecht University)
Reef-building bivalves can change the surrounding sediments, beyond the boundaries of their reef structures, through their physical structures and filter-feeding activity. A general understanding of the effect of these ecosystem engineers on macrozoobenthos assemblages in intertidal areas is however still lacking. In this study, we investigated the effect of Pacific oyster (Crassostrea gigas) reefs on lugworm (Arenicola marina), sand-mason worm (Lanice conchilega) and bivalves (cockle Cerastoderma edule, Baltic tellin Macoma balthica, and venus clams Venerupis spp.) abundances in the Eastern Scheldt estuary (the Netherlands). Areas of influence and non-influenced by six reefs, with different sizes, were compared in terms of biota density, emersion time, distance from the reef, silt and coarse sand content. Influenced areas of reefs (i.e. areas that showed an elevated zone) harbored significantly higher A. marina densities but lower L. conchilega densities, compared to non-influenced areas. Bivalves were present in too low numbers to observe any effect of the reefs. Our results suggest that sedimentary conditions and the higher elevation of reef influenced areas is favored by A. marina but not L. conchilega. The opposing densities of A. marina and L. conchilega in the influenced area might suggest a spatial-exclusion competition between both species in the influenced area. (194 words).