Abstract
Planned large-scale offshore wind developments in the North and Baltic Seas are set to transform these regions into major energy hubs for northern Europe. Offshore wind farms are expected to influence the marine ecosystem through two mechanisms: a wind wake effect inducing increased stratification, and a drag effect increasing mixing around monopiles. We applied 3D numerical models that, for the first time, resolve both atmosphere-controlled wind wake effects and monopile drag to quantify their combined impacts on stratification and primary production. The North Sea showed spatially heterogeneous changes, whereas the Baltic Sea exhibited stronger stratification and reduced primary production driven by the wake effect. At current speeds >0.25 m s−1, drag-induced mixing dominated over wake-driven stratification in both seas. The provided knowledge that ecological effects of offshore wind farms depend on prevailing current speeds and baseline stratification can be used to minimize impacts on the environment in future spatial planning.