Martin Johnson ^1,2* Quentin Jutard ^3,4 Maël Jaouen ^3,4 Nikolai Maltsev ^3,4 Margaux Boyer ⁵ Chloe Guillerme ^5,6 Deirdre McElligott ¹ Simona Paolacci ^1,7 Julie Maguire ¹ Antoine Mangin ³ Philippe Bryère ^3,4

• ¹ Bantry Marine Research Station, Bantry, Ireland
• ² Ecodiversity Ltd, Schull, Ireland
• ³ ACRI-ST, Sophia Antipolis, France
• ⁴ Argans, Brest, France
• ⁵ Cofrepeche, Paris, France
• ⁶ Région Hauts-de-France, Lille, France
• ⁷ AquaBioTech (Malta), Mosta, Malta

Large-scale seaweed and shellfish aquaculture are increasingly being considered by policymakers as a source of food, animal feed and bioproducts for Europe. These aquacultures are generally thought to be low impact or even beneficial for marine ecosystems as they are 'extractive'i.e., growing passively on foodstuff already available in seawater, and with potential habitat provision, fisheries, climate mitigation and eutrophication mitigation benefits. At some scale however, over-extraction of nutrients or chlorophyll could potentially have a negative effect on natural systems. Understanding the likely impacts of aquaculture production at scale is important to identify when safe limits are being approached. Taking seaweed aquaculture as the primary focus, this work uses operational oceanographic model outputs to drive prognostic growth models to predict the likely optimal distribution of seaweed farms across European waters to meet different production scenarios. A novel nutrient transport scheme is then used to model the interacting 'footprints' of nutrient drawdown from aquaculture facilities to demonstrate the likely spatial impact of large-scale aquaculture. Evaluation of both seaweed and shellfish contributions to CO2 balance under large scale production, and the potential impact on fisheries are also considered. The study finds that the impact of intensive seaweed aquaculture on nutrient availability could be significant where many farms are placed close together; but at the regional / basin scale even the highest level of production considered does not significantly impact total nutrient budgets. Seaweed aquaculture has the potential to extract large amounts of carbon dioxide, but the impact on carbon budgets depends on the end-use of the extracted seaweed. Shellfish aquaculture is a net source of CO2 due to the impact of calcification of shells on the carbonate system (i.e., alkalinity removal). However, gram-for-gram the CO2 impact of shellfish production is likely to be less than the impact of land-based meat production. Whilst operational oceanographic models are useful for taking a 'broad brush' approach to likely placement and impacts of aquaculture, reliable yield predictions for individual locations across European waters would require models integrating more physical and biogeochemical factors (wave environment, local currents, riverine inputs) at a finer scale than currently achievable.