Reefs, sand and reef-like sand: A comparision 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 forbiodiversity conservation. High densities of the habitat building annelid Lanice conchilega also increase localbiodiversity. This study describes the benthic biodiversity of a rocky reef and its surrounding sand bottom withdense L. conchilega beds in the Borkum Reef Grounds, north of the island of Schiermonnikoog in the DutchNorth Sea. A side-scan sonar survey revealed distinct seabed areas with high acoustic reflections, indicatingthe presence of hard substrate on the sandy seafloor. To ground truth the side-scan sonar data and make aninventory of the biodiversity of the observed habitats, amulti-method sampling approach (box corer, SCUBA airliftsampler and visual transects, drop-down camera) was used. This revealed (1) rocky reefs: a combination ofgravel, stones and rocks; (2) individual rocks in a sandy environment; (3) sand with dense L. conchilega beds (N1500 ind·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; p b 0.001). Including denseL. conchilega beds as an additional habitat showed that these held a higher species richness than the rocky reefs (NB-GLM; p = 0.002), while sand without dense L. conchilega beds did not (NB-GLM; p = 0.14). Since the rocky reefs were present on a sandy bottom, the local biodiversity more than doubled with the presence of rocky reefs. The nMDS plot clearly separated the sand and rocky reef communities and also showed a clustering of dense L. conchilega beds within the sand samples. Each method detected unique species, demonstrating the value of a multi-method approach compared to e.g. box coring alone. This study identified several species previously unknown to the BorkumReefs Grounds area. The total area of rocky reefs in the southern part of the Dutch BorkumReef area is estimated to be 9.8km2 and of L. conchilega bedswith densities N1500 ind·m-2 to be 74 km2. Further research should focus on the possible function of L. conchilega as an ecosystemengineer creating intermediate sand-reef systems. Formapping these L. conchilega beds,we advise using side-scan sonar imaging combined with ground truthing by drop-down cameras