| Summary: | Accumulation of beach cast biomass on coastal zones around Gotland is an increasing problem that reduce the recreational value of beaches and cause environmental degradation of coastal environments. Beach cast was once regularly harvested on Gotland, as it was considered a valued biofertilizer, but as it seized to be used in combination with the eutrophication of the Baltic sea, beach cast accumulation has increased in scale. The use of beach cast as a resource for bioenergy and agriculture does not only provide important services that can replace the use of greenhouse gases and finite resources but can also mitigate eutrophication and increase the quality of coastal zones as nutrients are retrieved from the water. Macroalgae and seaweeds are hyperaccumulators of heavy metals which pose a challenge for the utilization of beach cast as a fertilizer. Especially cadmium, a heavy metal that is harmful for human consumption, is strictly regulated and limits the amount of beach cast that can be used for agriculture. This thesis has analysed the potential benefit of beach cast harvesting and three potential utilization strategies of beach cast; fertilizer to food crops, fertilizer to fast growing energy forest (Salix) and biogas production with digestate utilization. The result indicate that all strategies are possible and viable utilization strategies under certain conditions. Because of the complex and site specific factors that affect beach cast utilization, adapting strategies depending on the conditions and needs of the local coastal zones is required. Cadmium uptake is affected by a range of factors that can be controlled so that beach cast can provide nutrients and improve soil structure without contaminating the soil. Especially promising is the prospect of fertilizing Salix with beach cast as the cadmium absorption and growth rate of Salix enables large amounts of beach cast to be spread without risk for it accumulating in the soil. The cadmium contained within the Salix can later be removed from the environment entirely as it is collected from the ash when Salix is converted to bioenergy. Recent studies also indicate that food crops can be safely fertilized with beach cast without cadmium being transferred if specific crops are chosen. Cadmium uptake to crops can also be limited if specific beach cast with low cadmium content are used and if the beach cast is pre-composted with other substrates. Both for Salix and food crops its instead legal restraints on cadmium spreading that limits the use of beach cast. When abiding by the set cadmium restrictions, beach cast can only marginally supply the macronutrient requirement of the average food crop on Gotland. Biogas production and digestate utilization from beach cast provides many environmental benefits as clean renewable energy is generated that can replace fossil fuels and the nutrients contained in the digestate can be spread on arable land. From the conducted energy balance of the system on Gotland it was found that beach cast has theoretically good conditions to ferment beach cast. However, beach cast isn’t practically viable on Gotland because the substrate can’t compete financially with other available substrates and it requires costly pre-treatments. Regardless of which beach cast utilization strategy chosen, harvesting is concluded to have a positive effect on mitigating coastal eutrophication and beach and water quality. Although it cannot on its own mitigate a net nutrient loading to coastal zones in Gotland, harvesting easily available beach cast can reduce the nutrient loading to coastal zones on Gotland with up to 27 % and 4,5 % phosphorus and nitrogen respectively.
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