Seagrass and macroalgae on shores: unraveling the decomposition of beach wrack on Danish coasts

Blue Carbon Ecosystems (BCEs), characterized by highly productive habitats such as seagrass meadows and macroalgae beds, play a crucial role in the marine carbon cycle. Drifting macrophytes, exported from these ecosystems, accumulate as beach wrack onshore. Although some aspects of the fate of beach wrack, such as mineralization and offshore re-deposition, have been explored, comprehensive field-based studies are necessary to understand the dynamics of decomposition in natural settings. Hence, this study conducted an in-situ decomposition experiment of beach wrack on a Danish coast to explore the decomposition process of beach wrack in natural environments. Seagrass Zostera marina and macroalgae Fucus vesiculosus were selected as experimental materials. Two deposition scenarios (deposited above ground (AG) and buried underground (BU)) were set up for each wrack species. After around two months of decomposition, the mass loss and change in phosphorus (P) content were both significantly affected by beach wrack species and deposition scenarios. Specifically, the mass loss of F. vesiculosus was considerably greater than Z. marina, and 78 % of the mass was decomposed at AG over two months. The loss of P was largest in Z. marina at AG, showing a decrease of 73 %. Contrastly, carbon (C) and nitrogen (N), as well as isotopic signatures δ¹³C and δ¹⁵N, showed no significant change. The C/N ratio showed an increase in both beach wrack species after two months of decomposition, although without significant changes due to the large variation in results. This study provides insights into the decomposition dynamics of beach wrack in natural environments, contributing to a better understanding of their role in carbon and nutrient cycling within BCEs.