Offshore wind farms are projected to impact primary production and bottom water deoxygenation in the North Sea

Abstract

The wind wake effect of offshore wind farms affects the hydrodynamical conditions in the ocean, which has been hypothesized to impact marine primary production. So far only little is known about the ecosystem response to wind wakes under the premisses of large offshore wind farm clusters. Here we show, via numerical modeling, that the associated wind wakes in the North Sea provoke large-scale changes in annual primary production with local  changes of up to ±10% not only at the offshore wind farm clusters, but also distributed over a wider region. The model also projects an increase in sediment carbon in deeper areas of the southern North Sea due to reduced current velocities, and decreased dissolved oxygen inside an area with already low oxygen concentration. Our results provide evidence that the ongoing offshore wind farm developments can have a substantial impact on the structuring of coastal marine ecosystems on basin scales.

Partial Summary of Findings

Ecosystem impacts. In the southern North Sea, areas with particularly high primary production are co-located with the frontal belt off the coast and around Dogger Bank (Fig. 2a, insert). The majority of future OWF installations are planned in exactly those highly productive areas, which are known to be ecologically highly important33. Our model results show that the systematic modifications of stratification and currents alter the spatial pattern of ecosystem productivity (Fig. 2a). Annual net primary production (netPP) changes in response to OWF wind wake effects in the southern North Sea show both areas with a decrease and areas with an increase in netPP of up to 10%. Most obvious is the decrease in the center of the large OWF clusters in the inner German Bight and at Dogger Bank, which are both clearly situated in highly productive frontal areas, and an increase in areas around these clusters in the shallow, near-coastal areas of the German Bight and at Dogger Bank. The latter might be fueled by nutrient supply from subsurface waters as a consequence of the upwelling and downwelling dipole as suggested in earlier studies20. Additionally, we also find changes in netPP in areas further away from the OWF clusters, such as a decrease along the fresh water front of the German and Danish coasts and an increase south-east of Dogger Bank at Oyster Grounds, which is typically seasonally stratified and shows lower productivity. Identifying the robustness of these patterns with respect to different weather conditions and interannual variations requires additional analysis and simulations. When integrated over a larger area, the estimated positive and negative changes tend to even out. Regional averages for the whole North Sea (model area with longitude <9°E) as well as for the southern North Sea area (as in Fig. 2a) and the German Bight (latitude: 53.5–55.5°N; longitude: 4–9°E) only show reductions down to −0.5%, while the average reductions in netPP directly at the OWF locations adds up to −1.2 %. The direct response of the ecosystem at the OWF sites can be assigned to the changed hydrodynamic conditions. This includes, on the one hand, the clearly defined upwelling and downwelling patterns (Supplementary Fig. 2), which have been hypothesized to play a major role in the changes OWFs provoke in marine ecosystems10,20. Those patterns depend on the wind direction and can be expected to modify the nutrient exchange at the thermocline, as has been shown for temperature and salinity9, at and around the OWF clusters. On the other hand, the production changes are directly related to the changes in stratification. A closer look at the vertical distribution of netPP change (Fig. 2b) averaged over the areas with OWF installations (partitioned spatially into OWFs at strongly stratified and less stratified regions and temporally into spring and summer periods) shows that OWFs in clearly seasonally stratified waters experience an upward shift of the vertical production maximum, which occurs typically at the mixed layer depth in summer. This is a consequence of the shallower mixed layer depth, due to reduced wind mixing. This signal is more prominent in summer than in spring. In contrast, OWFs in less stratified and frequently mixed waters show a decrease in production in the upper 20m of the water column in spring and at the depth of the thermocline in summer.