Beachcast or beach wrack, washed-up algae and seaweed, used to be a highly sought-after agricultural resource (fertiliser and soil conditioner) in coastal communities around the world before being replaced by chemical fertilisers, but considering the talks of a circular bio-economy – can we reintroduce it? 

Since the abandonment of beachcast in agriculture, the amount of beachcast has significantly increased as an effect of eutrophication caused by the use of chemical fertiliser, and the material is commonly considered waste. Meanwhile, soils have degraded and need the multiple positive effects on agroecosystems that beachcast could provide. So, while harvesting the biomass serves to remedy the adverse ecosystem effects on marine ecosystems (from excessive organic material, i.e. macroalgae blooms), beachcast could potentially contribute to more sustainable food production.

Sadly, it is not simple. This potentially symbiotic process infers uncertainties regarding soil cadmium accumulation, coastal ecosystem effects, and other challenges - and despite multiple studies and attempts at a waste-to-resource conversion of beachcast, the challenges remain unsolved, and a novel take on the topic seems necessary.

On Gotland, Sweden, with a historical tradition of using beachcast in agriculture, a modern governance system for management has emerged from a national policy subsidising beachcast harvesting, which has partly come to connect the marine and agricultural domains, among other actors. In this case, beachcast harvesting is subsidised as a measure to curb eutrophication and reduce the detrimental effects of excessive beachcast, a phenomenon present in the Baltic Sea and many other coastal regions worldwide that are caused by an accelerating nutrient accumulation at sea and sediment stocks. So, by further applying beachcast as a fertiliser in agriculture and horticulture, a practical example of a regional land-marine nutrient loop could be created to make beachcast management more sustainable. However, multiple challenges need to be addressed. 

This thesis and case study of Gotland, Sweden, presents an illustrative example of contemporary beachcast governance and management that provides knowledge and insights to support policy development and more sustainable beachcast management practices. insights for policy that can support the development of more sustainable management practices. This is done by using semi-structured interviews to understand the beachcast governance and management practices based on stakeholder perceptions (Paper I), performing a cost-benefit analysis of beach-cast harvest to assess the economics of closing land-marine nutrient loops in the Baltic Sea region (Paper II), conducting a cultivation experiment to predict future trends in soil cadmium concentration from applying beachcast as fertiliser (Paper III), and mapping regional variations in chemical composition of fresh and composted beachcast to consider future treatments (Paper IV). 

The results show that to overcome the challenges and achieve more sustainable practices, the following measures and considerations are essential: further supporting continued experimentation and monitoring of marine ecosystems to handle the harvest-hesitation caused by the uncertainties associated with marine ecosystem effects; sampling of composted material and careful agricultural use to remedy the waste-resource ambiguity associated with Cd contamination; and a continued subsidy and cross-sectoral collaborations to abide managerial and financial responsibility for beachcast activity.   

In this in-depth and transdisciplinary case study with a mixed-methods approach, the findings point to a theoretical and practical incongruity In seeking a waste-to-resource conversion of beachcast, which indicates that beachcast cannot compete with commercially viable products in today's intensive agricultural regime. Instead, agroecology, an alternative pathway according to FAO, appears to be a more promising context for beachcast to be acknowledged and receive the necessary investment to resolve its ambiguous resource status. 

Harvesting beachcast from coastal zones to use the biomass in agriculture or horticulture could mitigate eutrophication while contributing to resource substitution of fossil-based inputs in food production. As such, beachcast holds great resource potential in a bio-based circular economy, but its chemical properties prove challenging, and more research is required to develop treatment techniques that will allow the realisation of such a system. We compiled results from chemical analyses of fresh beachcast from a database within the marine policy scheme, LOVA, in Gotland, Sweden, to study local and seasonal variations in macronutrients, C:N ratio, and Cd content. This data complemented with analyses of fresh and composted beachcast (passive pile treatment), for which the contents of macronutrient, ammonium, nitrate, and Cd, were measured, calculating C:N ratios and maturity indices (NH4+-N/NO3—N). The results confirm that regional variations in the above-mentioned properties require investments in treatment techniques and strategies to make beachcast usable. 

The ecological consequences of using beachcast compost as an agricultural resource input merit study. Using beachcast compost as a biofertiliser has multiple positive effects on agroecosystems, while also serving to remedy the negative effects on marine ecosystems caused by excessive beachcast production due to eutrophication. This process thus presents an opportunity to contribute to circular nutrient management and the development of sustainable agricultural, but it may also result in accumulation Cd (Cd) in the soil. In Gotland, Sweden, an example of cross-scale interaction between marine and agricultural domains has emerged from a national policy subsidising beachcast harvesting, which may help reintroduce the historical tradition of using beachcast in agriculture. To estimate potential risks, a field experiment and Cd mass balance were conducted to predict the rate of Cd accumulation, changes in soil Cd fractions, and potential beachcast application methods that avoid Cd soil accumulation. In the scenario where the maximum Cd input from beachcast compost is set to the same threshold as the level established for sewage sludge—0.75 g ha–1 year–1—beachcast compost with a Cd content of 1.5 mg kg–1 dw (the EU threshold for biofertilisers) could be applied in an amount of approximately 2000 kg ha–1 per year (one-tenth of the amount applied in this study). Therefore, the long-term effects of Cd soil accumulation resulting from continuous application of beachcast as fertiliser on agroecosystems cannot be disregarded and are of global relevance. 

Harvesting beach-cast can help mitigate marine eutrophication by closing land-marine nutrient loops and provide a blue biomass raw material for the bioeconomy. Cost–benefit analysis was applied to harvest activities during 2009–2018 on the island of Gotland in the Baltic Sea, highlighting benefits such as nutrient removal from the marine system and improved recreational opportunities as well as costs of using inputs necessary for harvest. The results indicate that the activities entailed a net gain to society, lending substance to continued funding for harvests on Gotland and assessments of upscaling of harvest activities to other areas in Sweden and elsewhere. The lessons learnt from the considerable harvest experience on Gotland should be utilized for developing concrete guidelines for carrying out sustainable harvest practice, paying due attention to local conditions but also to what can be generalized to a wider national and international context.