Contrasting Responses of Stratification and Primary Production to Offshore Wind Farm Development between the North Sea and the Baltic Sea

Large-scale offshore wind farm developments are planned for the North Sea and the Baltic Sea becoming Green Power Plants for northern Europe. Offshore wind farms are expected to influence the physical and biogeochemical environment through two mechanisms: decreased wind stress at the sea surface (wake effect) and increased friction (drag effect) around monopiles. We applied 3D numerical models to study impacts of offshore wind farms on stratification and primary production in the North Sea and western Baltic Sea. The future Y2030 scenario, defined by the Danish Energy Agency, projects an increase of 50 GW for onshore wind farms and 158 GW for offshore wind farms. The changes were compared to a reference scenario without any onshore and offshore wind farms.

Our findings revealed contrasting responses between the two seas due to differences in current speeds and levels of stratification. The North Sea showed a complex spatial pattern with both positive and negative changes in stratification and primary production depending on the current speed, stratification, and wind farm capacity. The Baltic Sea showed far-field effect with stronger stratification and lower primary production. Generally, the monopile drag effect dominated over the wind wake effect at current speeds above 0.25 m/s. Hence, offshore wind can affect the marine environment differently depending on the underlying hydrodynamics.