Over the coming decades, changes in the North Sea are expected due to climate change and planned human activities, including offshore aquaculture , and oyster restoration efforts . One of the goals of the EU-funded FutureMARES research project was to examine the se effects and their impact on each other . To achieve this goal, we used a state-of-the-art North Sea model to simulate marine conditions under optimistic as well as pessimistic climate change scenarios. Dynamics of seaweed, mussels and oysters were explicitly computed, as well as their feedbacks on the environment. To investigate effects on the environment , we compared simulations without aquaculture/oyster restoration, to simulations with large-scale seaweed cultivation only, large-scale mussel cultivation only, co-cultivation of seaweed and mussels, and aquaculture and oyster restoration combined (e.g. Figure 1). In the model, seaweed and mussel farms were located in future Du tch offshore wind farms suitable for cultivated species. Oyster bed locations were chosen based on areas with a high habitat suitability and where fishing efforts are expected to be excluded.
For all scenarios, t he model results estimate that seaweed aquaculture will reduce the local winter nutrients and thereby also the surface chlorophyll-a concentrations, as seaweed builds up its nutrient reserves before the spring bloom . This is visible not only in the vicinity of the farms but also far downstream. Mussel aquaculture will reduce chlorophyll-a concentrations in the vicinity of the farms, since they directly feed on phytoplankton . It will also likely increase local nutrients due to their excretion/increased re-mineralization. When restored oyster beds were confined to more offshore locations, where habitat suitability is the highest , they have a small effect on the local nutrients and do not interact much with the aquaculture, which is likely to occur closer to the coast. However, when oyster beds are also restored in more moderately suitable areas, closer to the coast , they have a larger effect on the marine environment and aquaculture in neighbouring farms (especially mussel yields) .
Restoring oysters would be a slow process as the most suitable, offshore areas have limited primary production and therefore food availability for the oysters, as well as heavy stratification, leading to very low growth rates . Model results suggest that the most optimal location to start with the oyster restoration process would be in the Frisian front .