Mémoire

Abstract

Interest in macroalgae cultivation is growing in Europe, driven by its economic and ecological benefits. \textit{Saccharina latissima} (sugar kelp) is among the most studied species in the North Sea and is drawing interest in several cultivation and modeling projects. This study assesses the potential of its cultivation into offshore wind farms located in the Belgian coastal area of the North Sea. It first examines the productivity of these sites by comparing their yields with those observed at other locations (Ostend and the Scheldt estuary). Secondly, the ecological impact of these farms is analyzed by taking the influence of mussels already present on offshore wind structures into account. To do this, the three-dimensional hydrodynamic model ROMS was used, coupled with a biogeochemical module (Fennel) and biological modules relating to the growth of macroalgae and the filtration of blue mussels (\textit{Mytilus edulis}). The simulations allowed to analyze the influence of geographical location on dry biomass per individual. They also assessed the environmental impact of mussels present in the offshore wind farms, as well as the impact of deploying seaweed farms under different scenarios. These analyses focused in particular on nutrient dynamics, primary production, and fecal production of mussels. The results show that the nutrient-rich Scheldt estuary site generates the highest biomass (11.8 g/ind), while the yield at the wind farm site (9.7 g/ind) remains higher than that at the Ostend coastal site (7.6 g/ind). The presence of mussels in wind farms leads to a decrease in primary production (up to 2.5\%) and a slight reduction in nitrate concentration (1.3\%). Macroalgae farming, on the other hand, causes a marked depletion of nitrate (up to 18\% locally) in the most intensive scenario, as well as a reduction in phytoplankton primary production (up to 7.6\%), with effects extending to the largest farming scales. Mussel fecal pellet production, on the other hand, is only moderately affected (2.2\% locally). These results provide essential information for guiding site selection, farm size, and the assessment of ecological impacts necessary for the sustainable co-location of macroalgae cultivation in offshore wind farms.