Reefs, sand and reef-like sand: A comparison of the benthic biodiversity of habitats in the Dutch Borkum Reef Grounds

Reefs play an important role in the distribution of species associated with hard substrates and are of value for biodiversity conservation. High densities of the habitat building annelid Lanice conchilega also increase local biodiversity. This study describes the benthic biodiversity of a rocky reef and its surrounding sand bottom with dense L. conchilega beds in the Borkum Reef Grounds, north of the island of Schiermonnikoog in the Dutch North Sea. A side-scan sonar survey revealed distinct seabed areas with high acoustic reflections, indicating the presence of hard substrate on the sandy seafloor. To ground truth the side-scan sonar data and make an inventory of the biodiversity of the observed habitats, a multi-method sampling approach (box corer, SCUBA airlift sampler and visual transects, drop-down camera) was used. This revealed (1) rocky reefs: a combination of gravel, stones and rocks; (2) individual rocks in a sandy environment; (3) sand with dense L. conchilega beds (>1500ind·m−2) and (4) sand bottom habitat. A total of 193 taxa were found with many unique species per habitat. Species richness was significantly higher on sand when compared to the rocky reef (NB-GLM; p=0.006), caused by the presence of dense L. conchilega beds (Poisson GLM; p1500ind·m−2 to be 74km2. Further research should focus on the possible function of L. conchilega as an ecosystem engineer creating intermediate sand-reef systems. For mapping these L. conchilega beds, we advise using side-scan sonar imaging combined with ground truthing by drop-down cameras. •With a combination of methods we inventoried natural reefs in the