Lugworm exclusion experiment: Responses by deposit feeding worms to biogenic habitat transformations

On six 400 m² plots over 3 years, we excluded the sessile subduction and conveyer-belt feeding polychaete Arenicola marina which generates a pit-and-mound topography at the sediment surface from intertidal sands near the island of Sylt, Germany. This experiment was used to test whether other abundant deposit feeding polychaetes (the discretely motile and surface feeding ragworm Nereis diversicolor and the subsurface-feeding, motile orbiniid polychaete Scoloplos cf. armiger) benefit from competitive release. Ragworms took advantage from the absence of lugworms. Presumably they responded to a more stable and nutritious surface layer at lugworm exclusion plots (relief from inhibitive bioturbation). Contrary to this, S. cf. armiger was negatively affected by the exclusion of A. marina. It may have suffered from higher sulfide concentrations in the less irrigated and less permeable sediment where lugworms were absent. For adult worms of both species these results were consistent in 2 out of 3 years examined. Recruitment by N. diversicolor was highly variable between years and occurred either irrespective of experimental treatments or the response was inconsistent. Juveniles of S. cf. armiger benefited from the presence of A. marina and aggregated near lugworm tail shafts where inflow of oxygen rich water was high and sulfide concentrations were low. Biogenic habitat mediated effects of lugworms on both deposit feeders were in the same order of magnitude as abundance variation in space and time. Thus, A. marina was one of the key factors structuring the deposit feeding community. It is suggested that arenicolids modify the composition of the associated polychaete assemblage primarily through habitat transformation.