Impacts of an offshore wind farm on biogeochemical fluxes in a seasonally stratified shelf sea

The significant expansion of offshore wind farms (OWFs) is a core element of the UK’s decarbonisation strategy. However, in the urgency to meet Net Zero, care must be exercised to avoid exchanging one environmental crisis for another. The PELAgIO (Physics to Ecosystem Level Assessment of Impacts of Offshore Wind) project proposes to inform current and emerging marine policy and management to deliver Net Gain and help achieve Good Environmental Status (GES) using an integrated and co-developed approach to measure, model and understand critical drivers and dependencies of marine ecosystems within a region containing existing and future planned OWFs. Part of the PELAgIO project uses state of the art numerical models combined with observations at the SSE Seagreen wind farm site in a seasonally stratified area of the North Sea (Firth of Forth) to assess the impact of OWFs on water column stratification, mixing and fluxes of nutrients, oxygen and chlorophyll. Observations consisted of a ship-base survey during spring-summer 2023, as well as two gliders which were equipped with sensors to measure temperature, conductivity, turbulence microstructure, dissolved oxygen and chlorophyll fluorescence. Preliminary analysis of the results from the PELAgIO field campaign indicated that the OWF area plays host to significant internal wave activity, evidenced by considerable vertical movement of the thermocline. Additionally the gliders captured an important extreme event in the June 2023 marine heatwave where surface layer temperatures in the Firth of Forth exceeded 16°C, highlighting the need for models to help elucidate natural variability, climate-driven and OWF-driven changes to stratification from one another. Here we show preliminary model and observation results of stratification, nutrient and oxygen fluxes from the Seagreen OWF in the PELAgIO project, and discuss this in terms of impacts to the subsurface chlorophyll maximum. We also outline spatial and temporal gaps in our observations we need to overcome to investigate impacts of OWFs effectively, and discuss how we will attempt to resolve these in the PELAgIO 2024 spring-summer field campaign.