Persistent effects of sand extraction on habitats and associated benthic communities in the German Bight

Abstract. Sea-level rise demands for protection measures of endangered coastlines crucial for the local population. At the island of Sylt in the SE North Sea, shoreline erosion is compensated by replenishment with sand dredged from an offshore excavation site. We studied the long-term effects of sand extraction on bathymetry, geomorphology, habitats, and benthic fauna. Hydroacoustic surveys revealed that changes of bathymetry and habitat characteristics caused by sand extraction can be still detected after > 35 years while the investigation of grab samples revealed persistent changes in sediment composition and benthic faunal composition. The comparison of recently dredged areas (< 10 years ago), recovery sites (dredging activity > 10 years ago) and undisturbed sites exposed significant differences in the number of individuals and species of macrozoobenthic organisms as well as in the mud content, indicating a persistent successional stage of the communities in the dredged areas. The slow backfill of the dredging pits results from low ambient sediment availability and relatively calm hydrodynamic conditions, despite high wave energy during storms. Based on current sedimentation rates, we conclude that a complete backfill of the deep excavation sites and re-establishment of the benthic communities is likely to take centuries in this area. Since re-establishment of the benthic communities depends on previous re-establishment of habitat characteristics, habitat mapping with remote sensing techniques is suggested as a cost-effective means to monitor the state of regeneration.