Seafloor communities and habitat disturbances in the North Sea

Many different organisms live at the seafloor. These organisms are a crucial component of marine ecosystems, as they play an important role in the food web. Their abundance is strongly regulated by the environment, as each species is adapted to certain conditions. Whereas some species thrive in deep and calm waters, others flourish under more dynamic conditions with waves and currents. The environment of a species also affects the impact of human activities, like demersal fisheries. Demersal fisheries target groundfish, but damage other species and the structure of the seafloor in the catching process. The impacts of this are larger for species in calm environments than for species in dynamic areas. In this thesis, I combined detailed observations of organisms at the seafloor with fisheries data and environmental conditions. I demonstrated that environmental conditions structured the distribution of seafloor-disturbing fisheries, and that such fisheries impact species differently depending on their way of life. The discovery of ross worm reefs in the southern North Sea revealed the importance of seafloor structures as environmental variables in the structuring of both fisheries and organisms. Here, seafloor structures like large sand dunes and smaller sand waves and megaripples create a very heterogeneous landscape. The distribution of the ecological valuable reefs matched the pattern created by megaripples. The troughs of different seafloor structures had higher organism abundancies, which may also explain the higher fishing intensity in the sand dunes troughs. I therefore argue that management should account for such local environmental differences