Effects of Arenicola marina on polychaete functional diversity revealed by large-scale experimental lugworm exclusion

The lugworm Arenicola marina was excluded from sandy sediment areas in the mid and low intertidal zone of the Wadden Sea. Exclusion, control and ambient plots were 400 m² each, replicated six times and sampled in August of three consecutive years. Responses were analysed with respect to functional trait groups in the associated polychaete assemblage using uni- and multivariate statistical techniques. Tube-building worms and predacious worms were most abundant in exclusion plots, while subsurface deposit feeders tended to dominate in the presence of lugworms. Lugworm effects were stronger in low intertidal fine sand than in mid intertidal medium sand. In the third year, lugworm densities strongly decreased at the study site. Nevertheless the polychaete functional group composition in lugworm exclusion plots still significantly differed from that in control and ambient plots. We assume that the permanent exclusion of lugworms may have entailed a cumulative change in sediment properties in the exclusion plots. Overall, lugworm effects were highly dependent on space and time as well as on differential recruitment success in this intertidal polychaete assemblage. Sediment-mediated effects of an ecosystem engineer on associated species appear to be subtle and contingent in variable environments.