Pockmark ecology in fjords and offshore Norway

Pockmarks, described as crater like depressions on the sea-bed, are now widely recognised features throughout the world’s oceans. The recent increase in the use of multibeam sonar to map seabed topography with high resolution means such features are now being frequently discovered in many new areas. Our research looks at the effect of these structures on the biology of the seabed and also what the biology can tell us about the possible causes and formation of pockmarks. Any structures which increase seabed heterogeneity could be very important in generating niches and biodiversity in an environment frequently seen as paradoxically uniform for the number of species present. Vast fields of pockmarks occur in the Norwegian Sea around the Troll platform. Analysis of remotely operated vehicle video transects from this area has revealed concentrations of macrofauna inside pockmarks and depauperate fauna in the surrounding area. The distribution of fauna in the pockmarks surveyed was highly dependent on the sediment type and morphology. Large fragile gorgonian corals, e.g., Paragorgia arborea, were discovered in the centre of several of the complex pockmarks, indicating some protection from trawling. In the Inner Oslofjord, differences between fauna inside and outside pockmarks seem less marked, with accumulation of man made debris in the centre of many pockmarks playing a role in colonisation of fauna by creating artificial substrate. Within pockmarks, the geology and morphology play a role in determining the distribution of fauna due to the hydrodynamic and microhabitat effects. The deflection of bottom current patterns caused by the topography will affect the distribution of species within the pockmark. Upwelling and seeping of nutrients from active pockmarks could enhance species abundance and allow chemosynthetic organisms to thrive. The substrate coarsening within the pockmark caused by the removal of fine sediments and presence of authigenic carbonate allows the settlement of benthic larvae and provides protection and shelter in an otherwise soft sediment environment. In heavily trawled, and homogenous soft-sediment areas pockmarks may act as hotspots of life, acting as refuges for fauna that would otherwise be absent.