Beach litter footprint: indicator design and operational testing across Iberia, the Balearics, and Macaronesia

Citizens, scientists, and governmental agencies are generating vast datasets on beach macrolitter, generally reported as litter item inventories. Despite this richness of information, most assessments of coastal environmental status rely on total item counts, underutilizing data on item size and type. Here, we introduce the Beach Litter Footprint, a multidimensional indicator comprising seven source-related sub-indicators that account for both litter size and potential origins. By leveraging the full structure of beach-litter datasets, this multi-source indicator provides a more nuanced diagnostic of coastal conditions, offering more targeted guidance for management. In this manuscript, we first present the design, calculation, and interpretation of the indicator, which can be represented in the form of footprint visualizations, fine-resolution maps, or litter curves, depending on the purpose and scale of analysis. Utilizing routine beach-litter monitoring data, we demonstrate the indicator's applicability through an operational test conducted across the region extending from the Macaronesian Islands (Atlantic Ocean), the Iberian Peninsula, and the Balearic Archipelago (Mediterranean Sea). Additionally, we offer a suite of automated tools to facilitate the application of the Beach Litter Footprint to other shores worldwide.