Reef effect of offshore wind farms: Impacts on primary carbon production (CARBOREEF)

The English Channel, a macrotidal epicontinental sea, is influenced by climate variations and subject to various anthropogenic pressures. The Normandy coastline is expected to be France’s main producer of wind energy by 2030. Offshore wind farms have both positive and negative socio-environmental impacts. Wind farm structures act as artificial reefs, creating new habitats that can increase local productivity, but also lead to problems such as the introduction of non-native species, habitat degradation, and changes in carbon flows. The lack of knowledge about these impacts is highlighted, and the precise influence of the nature of the structures on the reef effect has not yet been quantified in the English Channel, although the increasing complexity of these artificial ecosystems appears to be beneficial. Projects using marine eco-engineering have shown positive results, limiting negative effects and improving the ecological status of artificial areas. The Interreg MARINEFF project (2019-2023) tested the contribution of eco-engineering to the biodiversity and productivity of artificial ecosystems. The teams developed innovative materials, designs, and methodologies, evaluating eco-designed structures in comparison with traditional structures. The experiments conducted by the MERSEA team focused on algae colonization and associated primary production, showing different types of reef effects, ranging from so-called “primary producer” reefs rich in macroalgae to so-called “primary consumers” whose functioning relies on the use of phytoplankton. In this context, an eco-design approach aims to evaluate the impact of different types of reef effects on primary carbon production and sequestration in ecosystems.

This thesis project seeks to understand and evaluate carbon flows linked to primary producers in relation to the artificialization of the ecosystem due to the development of wind farms. To this end, a dual approach will be used, consisting of both in situ observations and experiments conducted in mesocosms at the CREC-Station Marine. For the in situ approach, two pilot sites will be used to represent different areas and depths of the Courseulles-sur-Mer wind farm. The study will focus on the “macroalgae,” “microphytobenthos,” and “phytoplankton” compartments to understand the changes in processes and their impact on primary production and potential carbon sequestration. The in situ approach will assess primary productivity and carbon sequestration according to different types of reef effects. The eco-designed prototypes, provided by Builders – Ecole d’Ingénieurs, will be evaluated in comparison with traditional structures. Environmental monitoring of submerged structures will include the use of non-destructive methods such as photography and scraping samples. Community metabolism will be characterized using innovative benthic bells and photosynthesis measurements based on variable fluorescence techniques. Carbon biomass will be measured based on carbon lability to estimate sequestration. At the same time, experimental studies in mesocosms will be conducted on prototypes placed in mesocosms where conditions will be controlled. Three types of reef effects will be obtained, and carbon fluxes between biological compartments will be studied using 13C enrichment. Biological diversity and carbon pools will also be characterized. The complementary in situ and mesocosm approach will provide insight into the dynamics of colonization and associated carbon fluxes during the installation of artificial structures in the English Channel, contributing to the advancement of carbon flux modeling in this region.