Does the oceanographic response to wind farm wind-wakes affect the spring phytoplankton bloom?

Abstract

Large offshore wind farms (OWFs) will be deployed in the North Sea, potentially causing multiple effects on marine ecosystems some of which may be synergistic with climate change. This study modelled the oceanographic bio-physical response to anticipated atmospheric wind farm wakes in the north-west North Sea. The wind wake was included as a reduction of the wind speed into the atmospheric forcing of the Finite Volume Community Ocean Model (FVCOM) coupled with the European Regional Sea Model (ERSEM) from three OWFs using a year with available in situ data for model validation. The spatial distribution of physical variables and chlorophyll a (Chl-a) was compared between model runs with and without OWFs during three temporal subdivisions (pre-bloom, bloom and post-bloom) of the spring-summer period. Overall, across the entire period there was a 7 % decrease in Chl-a concentration, with the decrease being more pronounced during the bloom period. However, there was a slight increase in Chl-a in the post-bloom period. At higher temporal (12 h) and spatial (> 1 km) scales, significant changes in Chl-a were identified throughout the vertical water column and during the prevailing south-west and north-west winds, which generated a persistent upwelling/downwelling dipole across the simulated time series. The spatial variations of potential energy anomaly (PEA) and Chl-a were the most informative variables as they displayed distinct values and spatial distributions linked to the upwelling/downwelling dipoles. The downwelling cell was characterised by fresher and warmer waters, especially at the surface, with areas displaying an increase in stratification with a resulting decrease of Chl-a in seasonally stratified waters. On the other hand, upwelling regions were characterised by saltier and cooler waters with increasing PEA in permanently mixed and intermittently stratified waters showing consistently increased Chl-a production. The changes to levels of stratification are ecologically important as they change the vertical characteristics of the water column differently over a seasonal cycle. The analyses confirmed that it is critical to identify the temporal and spatial scales at which important changes to the physics and Chl-a production occur as they will play a role in assessing the range of impacts of OWFs.