By 2050 it's estimated >400 GW of energy will be gathered by offshore wind in the North Sea alone. For scale, Hinkley Point C nuclear reactor is projected to produce 3.2 GW. How will this increased anthropogenic use of our coastal seas impact already stressed marine ecosystems? And how will that same production of renewable energy offset risks of extreme climate change that, left unchecked, will increase the risk of biodiversity declines. There are many complex changes to ecosystems linked to Offshore Wind Farms (OWFs) that we need to understand now, so that the extent of increasing wind energy extraction further offshore is managed in the most sustainable way. An important effect of large wind energy extraction will be to reduce the amount of energy that would normally go into local ocean currents via surface stress, altering sea state and mixing. Conversely, there will be local increases in turbulence around turbine structures and seabed scouring near fixed foundations. Any change in ocean mixing may change the timing, distribution and diversity of phytoplankton primary production, the base of the food chain for marine ecosystems, to some degree. This has knock-on-effects on the diversity, health and locations of pelagic fish that are critical prey species of commercial fish, seabirds and marine mammals. Observed changes caused by operational OWFs in the southern North Sea include local surface temperature rise and the displacement of seabirds and fishing fleets from the OWF footprint, whereas seals often appear to be feeding near turbines. All of these changes have a linked component - , important prey fish species - , which are likely to aggregate near structures (as seen at other offshore platforms). Seabirds and fishing fleets subsequently have less space to hunt, with potentially increased competition for fish. However, if OWFs are also de facto marine protected areas and so positively affect local primary production, they may provide good habitat for fish population growth.

So, what are the cumulative effects of current OWF developments and the thousands of additional planned structures? Do the physical, biogeochemical and ecosystem changes exacerbate or mitigate those resulting from climate change? As OWFs migrate further offshore as floating structures, how can current knowledge based on shallow, coastal fixed turbines be suitably extrapolated to understand the impacts on ecosystems dependent on seasonal cycles that are typical of deeper waters?

PELAgIO will address all of these questions through an inter-disciplinary, multi-scale observation and modelling framework that spans physical mixing through to plankton production, on to the response of fish and whole ecosystems. We will collect fine-scale data using the latest multi-instrumented acoustic platforms set beside and away from OWFs, complemented by autonomous surface and submarine robots to capture continuous and coincident data from physics to fish, over multiple scales and seasons to fully understand what is 'different' inside an OWF and how big its footprint is. These new data will test the effects on seabirds and marine mammals to build an OWF ecosystem parameterization that accounts for changes to mixing and wind deficit impacts, and is scalable to next-generation OWFs. This bottom-up, comprehensive approach will enable true calibration and validation of 3D ocean-biogeochemical-sediment modelling systems, from the scale of turbine foundations up to the regional and even cross-shelf scales. Identified changes will be integrated into Bayesian ecosystem models that enable the cumulative effects of ecological, social and economic trade-offs of different policy approaches for OWFs to be quantifiably assessed for present day conditions, during extreme events and under climate change.