Community structure and bioturbation potential of macrofauna at four North Sea stations with contrasting food supply

We investigated the macrobenthic faunal composition, vertical distribution, biomass, abundance and trophic structure in 4 North Sea sediments with contrasting quantity and quality of organic matter and with different hydrodynamic environments. The vertical distribution of macrofauna biomass generally followed the TOC (total organic carbon) profiles. The trophic structure reflected differences in relative quality of the organic matter. In the high amount-high quality TOC sediment at the German Eight station, most organisms were found at shallow sediment depths (<2 cm), mainly feeding on freshly deposited or (re-)suspended material as interface or suspension feeders. Skagerrak sediments, containing a large amount of refractory organic matter, supported a small-sized deeply penetrating fauna (up to 20 cm) consisting mainly of deep-living deposit feeders and also endobenthic predators. Highest diversity of trophic groups, largest individual sizes and deepest distribution were found in sediment with organic matter of intermediate quantity and quality at the Frisian Front. The highly dynamic, coarse-grained Broad Fourteens sediments were very poorly inhabited, probably due to the extremely high hydrodynamic stress and low quantity of TOC. With the aid of a simple diagenetic model, we analysed the potential effect of bioturbation on the distribution of food in the investigated sediments. The model predicted for a given bioturbation coefficient that total mineralization rate at depth reaches an optimum when the arriving material is of intermediate quality, whereas it is not possible for macrofauna to redistribute the organic matter up to depth when the quality of the arriving material is high. The results of the diagenetic model agreed well with our characterization of the fauna community on the basis of principal bioturbation categories (species causing surface deposition, biodiffusion, conveyor belt transport). Strong mixing by biodiffusion was observed at the station which had organic matter of intermediate quality (Frisian Front) and minimal mixing at the German Eight station, which was characterized by the high quality of the arriving organic matter.