Coastal and lagoon environments are dynamic systems where terrestrial, marine and atmospheric processes interact to create unique geochemical conditions. Sediment composition here is influenced by natural and anthropogenic factors, including climate change, pollution, land-use change, and the accumulation of marine debris and microplastics. These changes affect not only sediment chemistry but also geochemical cycles and overall ecosystem health. In particular, marine litter and microplastics can disrupt geochemical processes, thereby affecting coastal and lagoon biota and their adaptive responses. This session invites geochemists, geomorphologists, and environmental scientists to explore how sediment geochemistry drives ecosystem development in these areas. The session will present case studies that explore the link between sediment geochemistry and ecological outcomes, with a focus on the role of microplastics in coastal lagoons. Innovative methodologies will be presented, as well as predictive models linking these changes to future environmental trends. By combining insights from geochemistry, marine debris research and environmental sciences, this session highlights the value of interdisciplinary collaboration and advanced technologies. These approaches are crucial for understanding the complex impacts of natural and human activities on coastal ecosystems, supporting better environmental risk assessment, and promoting resilience in coastal lagoons.