Deetman, B.

Since 2011, unprecedented beaching events of Sargassum seaweed have caused major environmental, health and economic problems in the Caribbean, Gulf of Mexico, northern Brazil and the western coast of Africa. Not only are Sargassum influxes threatening already fragile and often endangered coastal ecosystems, such as coral reefs, mangroves and seagrass beds, they also disrupt the livelihoods of communities, especially those associated with the tourism and fishing sectors. The aim of this study is to develop a plan to turn these “brown tides” into opportunities for sustainable, scalable and efficient harvesting and valorisation approaches that will deliver environmental and socioeconomic benefits to the Dutch Caribbean and other end-users in the region. In this report we have reviewed the state-of-the-art of research on recent Sargassum blooms and influxes in the Caribbean with respect to biology, ecology, origin, distribution, socio-ecological impact and management options, in addition to existing valorisation chains and uses of Sargassum biomass, and environmental and socio-economic impacts of Sargassum valorisation. We conclude that value chains based on valorisation of nearshore Sargassum biomass into biofuel and agricultural products (i.e. fertilizer, animal feed supplement) seem the most promising for the Dutch Caribbean islands, since they will contribute to sustainable energy and food security, while reducing environmental impact of the energy and agricultural sector. Some of the identified management and valorisation strategies could also be applied to other areas that are affected by massive Sargassum influxes, such as the Gulf of Mexico. During 2020 pelagic Sargassum samples have been collected in different locations in the Caribbean region (Bonaire, Mexico, St Maarten) and Florida. These samples have been analysed for major components (including sugars and ash) and for content in iodine and heavy metals. The values obtained in these analysis were compared to values reported in the literature. The results of these analysis are of importance for the definition of possible applications of the Sargassum biomass or its products as component in the food and feed value chains Based on our literature review, a number of knowledge gaps have been identified that are related to the availability of pelagic Sargassum biomass, the environmental impacts of the harvesting, technological challenges, effects of Sargassum leachates to the environment (in case of agricultural uses) and on socio-economic impacts of the current and predicted actions (Chapter 7). These knowledge gaps need to be addressed before (commercial) harvesting and valorisation actions concerning pelagic Sargassum are taken. Therefore, we defined an implementation plan (Section 7.5 and Fig. 1), which addresses these knowledge gaps to set the first steps in the short term towards efficient and sustainable management and valorisation of pelagic Sargassum in the Dutch Caribbean. In the potential value chains defined, prevention of landings of Sargassum on the coasts is preferred to harvesting onshore. This is due to the negative ecological impact of harvesting onshore and the lower quality of beached Sargassum, that decays very rapidly.