My PhD project within the GB2 project of the MARUM aims to investigate the interaction between nutrient availability and microbial activity in the benthic boundary layer (BBL) and the surface sediments of continental margins. The BBL and the surface sediments are a system with strong chemical gradients in which the BBL acts as a transition zone between the water column and the sediments. Due to the enhanced supply of substrate, microbial activity in the BBL may increase towards the sediment. However, the microbial communities and the abundances of different groups within these communities are, so far, poorly described. One aim of the PhD project is to combine incubation experiments using tracers such as stable (15N, 57Fe) as well as radioactive isotopes (33P) and molecular analyses (qPCR, FISH) to link microbial activity with the microbial community. Since to date it is not known how the iron and phosphate fluxes into the water column depend on redox and hydrodynamic conditions in the BBL, another aim is to investigate the fate of iron and phosphate as well as the different processes of the nitrogen cycle in the BBL and surface sediments that underlie bottom waters of different oxygen concentrations.