High abundance of small zoobenthos around biogenic structures in tidal sediments of the Wadden Sea

Abstract

On the tidal flats of the island of Sylt (eastern part of the North Sea) the quantity of micro- and meiofauna associated with shoots of seagrass(Zostera noltii), with infaunal bivalves(Macoma balthica), and with tubes and burrows of polychaetes(Pygospio elegans, Pectinaria koreni, Nereis diversicolor, Nereis virens, Arenicola marina) was found to add up to 5 to 33 % of the overall abundance. These structures, taken together, account for 10 to 50 % of the faunal abundance on an average tidal flat at Sylt. The quantitative effect of biogenic structures at the sediment surface (casts and funnels) is small compared to that of tubes and burrows penetrating the anaerobic subsurface layer. In providing stable oxic microenvironments these elite structures frequently bring together more individuals than occur in the entire reducing sediment below surface. Faunal composition of irrigated dwellings of large infauna is different from that of the oxic surface sediment. The common denominator of all elite structures of the subsurface is an oxic halo. Burrows without such a halo are unattractive. There is no evidence that owners of burrows prey on their smaller inmates.