Ecosystem services and societal goods and benefits provided by sandy beaches are increasingly affected by human stressors operating at different spatio-temporal scales. The core paradigm of sandy beach management hinges upon the cohesion and sustainability of the littoral active zone as a cohesive unit. However, as narrow open systems with permeable boundaries, the appropriate management scale of these ecosystems extends well beyond the land-sea interface, highlighting the need to consider cross-ecosystem flows. This study emphasizes the importance of a holistic approach to managing catchment-to-coast systems. Using the DAPSI(W)R(M) cause-consequence-response framework, we examine the La Coronilla-Barra del Chuy beach in Uruguay, a multi-use and complex social-ecological system that faces significant changes. By integrating long-term quantitative data with qualitative insights from stakeholder interviews, we provide a comprehensive historical perspective on the impacts of upstream activities on downstream coastal ecosystems. The socioeconomic and ecological indicators derived from 40 years of in situ surveys of beach macrofauna and the yellow clam artisanal fishery activity, together with tourism and rice production (the main activities on the coast and in the catchment), show a marked decline in beach ecosystem services and related societal goods and benefits, highlighting interdependencies among human activity trajectories in neighbouring ecosystems. The lessons learned from this case study apply to managing similar systems worldwide, highlighting the importance of incorporating stakeholder perspectives and interdisciplinary approaches to sustain adaptive and resilient coastal environments.