Geological and faunistic characterisation of differentially structured habitats in offshore areas of the North Sea

Abstract

Reefs are ecologically valuable habitats in coastal oceans, acting as ground for settlement, feeding, nursery and shelter for numerous species. Around 80% of the reefs worldwide are threatened by human activities. The reefs of the North Sea are mostly of geological origin. In the German Bight the rocky reefs are protected by the European Union as part of the Natura 2000 Network. In this study, the Sylt Outer Reef, located in the south-eastern part of the North Sea, was described geologically and faunistically using digital videos from a towed underwater camera. A methodology was established to evaluate the distribution of seafloor substrata and to identify the reef fauna. Additionally, the fauna was quantified and the relationships between geological and faunistic variables were explored. A total of 11 video transects were analysed. At all stations, the benthic habitats were dominated by sandy sediment followed by lag deposit, while only small fractions of the seafloor were covered by stones. The sizes of the stones varied considerably within and between the stations with a maximum size of almost 60 cm. The benthic epifauna of the Sylt Outer Reef comprised 20 taxa (mostly species) from nine higher taxonomic groups. The Shannon-Wiener diversity of the epifauna varied between the stations from 0.69 to 2.01. The number of taxa at the stations depended on the habitat structure and declined with an increasing amount of the seafloor being covered by sand. Conversely, the number of taxa increased with stone size. Only the smallest stones were entirely uncolonized. In total, three different clusters of the epibenthic fauna could be distinguished inside the protected area of the Sylt Outer Reef. Although only a relatively small fraction of the benthic fauna can be captured by optical methods, the results generated by this noninvasive method allowed for a differentiated description of the epibenthic habitats of the reef areas of the Sylt Outer Reef.