Defining soft-sediment macrofaunal baseline conditions through large-scale diversity and biological trait analysis.

Soft-sediment macrobenthic communities are an essential part of the marine ecosystem, for which a healthy status is strived for. The patchy occurrence of benthic species across natural environmental gradients, together with the multiple human pressures acting on it, make it a challenge to manage the seafloor ecosystem. For sustainable management and reliable quality status evaluation, scientific sound knowledge on the ecosystem characteristics (both structural and functional) is of vital importance. Furthermore, baseline conditions have to be defined which serve as yardstick against which future status evaluation can be done. In this regard, it is important to gather and combine all appropriate data to investigate an areas’ baseline characteristics. For the Belgian North Sea, an extensive dataset was compiled and harmonised using most available macrobenthos and sedimentological data from targeted research projects, long-term monitoring and environmental impact monitoring. Based on species identity and abundance, we identified five soft-sediment benthic communities, each with their own structural characteristics, indicator species, sediment properties and spatial distribution. The coastal fine sand Abra alba community and offshore coarse sand Hesionura elongata community have the highest species richness and diversity values. The coastal muddy Limecola balthica community and medium sand Nephtys cirrosa community the lowest. However, looking at biological trait characteristics, the communities show some functional redundancy. In the coarser permeable sands, more free living mobile species were found causing diffusive mixing, whilst the finer sand and mud communities have more sessile, tube building and burrow dwelling species. With their contribution to bioturbation and bio-irrigation processes, the benthic fauna prove to be essential for the biogeochemical status of these finer sand sediments. We will demonstrate how this integrated evaluation of biodiversity and functional related characteristics allows for sound baseline setting, a more appropriate status assessment and an improved determination of habitat sensitivity to different human induced pressures.