The nutrient bioextraction potential of offshore macroalgae cultivation

Abstract

Due to the increasing impact of terrestrial agriculture on climate change, the attention of a myriad of industries is shifting towards the use of alternative, low-emission resources. Seaweed cultivation has presented itself as a contribution to the mitigation of the increased pressure on current resources. However, coastal and offshore marine areas are often unfit for seaweed cultivation due to increasing maritime activity. As wind farm areas are increasing, offshore seaweed aquaculture in multi-use platforms at sea (MUPS) has been proposed as one of the possibilities for smart use of ocean space. Apart from providing a multitude of benefits through the many applications of seaweed, it is also widely suggested that seaweed could offer ecosystem services during its growth by means of nutrient bioextraction of eutrophied waters. In this thesis, the critical nutrient flows of cultivation of S. latissima in MUPS at the North Sea are quantified using a dynamic mathematical nutrient model, and the impact on the marine vicinity is assessed. The assessment is performed for two scenarios: (1) a seaweed farm producing for a high-value chemicals factory, and (2) a seaweed farm producing for a fuel biorefinery. Both these scenarios are modelled over the course of one cultivation season on four offshore wind farm locations in the North Sea. Moreover, an analysis is performed on the potential role of monitoring technologies in offshore seaweed aquaculture in MUPS. The results of this study are combined to assess whether it is possible to establish ecosystem services through large-scale offshore seaweed cultivation in MUPS at the North Sea using monitoring technologies and nutrient analyses. The analysis in this study showed that offshore seaweed cultivation has a promising potential for nutrient bioextraction in the North Sea. However, nitrate depletion could occur during the last months of cultivation, when primary productivity is naturally lower. It is recommended that further research on the ecological effects of this nitrate depletion is conducted, and measures are taken to minimise the risk of detrimental effects. It is concluded that a combination of nutrient analyses and monitoring technologies could provide a more comprehensive understanding of the impact of large-scale offshore seaweed cultivation. Subsequently, this can create a solid foundation for the development of ecosystem services and the further development of the offshore seaweed sector in the near future.