Abstract
This study provides the first assessment of bottom-up cumulative effects of multi-use within offshore wind farms in the Dutch North Sea, focusing on seaweed aquaculture, mussel aquaculture, and offshore floating photovoltaic (OFPV) deployments. Three scenarios were developed: a current scenario deploying multi-use within three OWFs (Borssele, Hollandse Kust Zuid, and Hollandse Kust Noord) using available aerial passports as guidance, and two tendered scenarios deploying multi-use within six Offshore Wind Farms (OWFs) from the Routekaart Windenergie op zee 21GW plan without predefined spatial allocations. These scenarios allow comparison between discrete small-scale deployments and lumped large-scale deployments.
The modelling approach used the 3D Dutch Continental Shelf Model – Flexible Mesh (DCSM-FM) with Delft3D-FLOW and Delft3D-Water Quality modules. Multi-use deployments were implemented based on previous studies of individual applications and combined for this analysis. Results show that OWFs, mussel, and seaweed aquaculture have diffuse impacts extending beyond deployment areas, while OFPV causes strong but highly localized effects. All forms of multi-use influence chlorophyll-a concentrations through different mechanisms: seaweed via nutrient competition, mussels through grazing, and OFPV by decreasing light availability. Deployment location and size are critical for aquaculture yield and ecosystem effects.
The limitations of this study include: reliance on previous modelling work, lack of calibration for ecosystem processes, simplified representation of OFPV (light effects only), and absence of field data for validation. Despite these constraints, the findings suggest that spreading and mixing multi-use types within OWFs can minimize negative impacts and optimize co-benefits. The modelling tools are valuable for identifying broad patterns, supporting high-level strategic marine spatial planning. Future work should include experimental field data collection, development of MSFD indicators and thresholds to assess impact severity, and refinement of scenarios as spatial planning data becomes available.