The seaweed industry is growing worldwide to meet future resource needs in terms of food and fuel. In the meantime, the impact of expanding off-bottom seaweed cultivation on its environment is unclear. For example, it remains poorly understood how off-bottom seaweeds affect the local hydrodynamic environment, especially concerning turbulence that is more important for nutrient transport and availability than the mean flow velocity. Here, we carried out well-controlled flume experiments with mimic seaweed thalli, which are available, controllable, and stable, to investigate the impact of off-bottom seaweed canopies on whole-depth flow velocities in terms of both mean flow and turbulence velocity profiles. A careful comparison of behavior in the flow between natural and mimic seaweed thalli was made before these experiments. The results show that the floating seaweed thalli generate a surface boundary layer and have a profound impact on the velocity structure in the bottom boundary layer. More importantly, the generation, growth and dissipation of turbulence in the seaweed thalli area deeply affect the downstream distribution of near-bed turbulent strength and associated bed shear stress. Ignoring this turbulent variation would cause inaccurate predictions of morphological changes of the seabed. Our findings suggest that expanding the seaweed cultivation area may cause high risks of bed degradation and low diffusion in the downstream cultivation area. These findings provide novel insights into the environmental influence of off-bottom seaweed cultivation, with important implications for optimizing management strategies to promote seaweed productivity while minimizing seabed destabilization.

Seaweed cultivation attracts growing interest and sustainability assessments from various perspectives are needed. The paper presents a holistic qualitative assessment of ecosystem services affected by seaweed cultivation on the Swedish west coast. Results suggest that supporting, regulating and provisioning services are mainly positively or non-affected while some of the cultural services are likely negatively affected. The analysis opens for a discussion on the framing of seaweed cultivation - is it a way of supplying ecosystem services and/or a way of generating valuable biomass? Exploring these framings further in local contexts may be valuable for identifying trade-offs and designing appropriate policies and development strategies. Many of the found impacts are likely generalizable in their character across sites and scales of cultivation, but for some services, including most of the supporting services, the character of impacts is likely to be site-specific and not generalizable.