A bottom-up approach to model water quality changes along the Apulia region coastline: a Bayesian Network application to seagrass meadows under a changing climate.

Water quality (WQ) deterioration in marine-coastal areas (MCA) is among the main threats affecting socio-economic systems and ecosystem functioning, calling for urgent actions to preserve ecosystems' resilience. Nature-based Solutions (NBS) improve ecosystem resilience and biodiversity, transforming nature management while providing environmental and societal benefits. Yet, little is known on NBS capacity in reducing WQ deterioration. Understanding this nexus requires establishing functional relationships between marine ecosystems status and climatic and human pressures. The relationship between CC impacts and marine-coastal ecosystems is investigated through a spatio-temporal Bayesian Network model, which allows estimating the adverse effects of human-induced and climate pressures on seagrass meadows (Posidonia oceanica) along the Apulia coast (Italy). To this aim, both anthropogenic (e.g., land use, MPAs) and environmental data (e.g., nutrients, temperature, transparency) were integrated in the model, jointly combined at the coastal water bodies scale, and elicited by expert knowledge. Baseline environmental conditions were compared against 'what-if' scenarios, representing different climate conditions (RCP4.5 and 8.5), and three ecosystem services (ES - i.e., carbon sequestration, bioremediation and food provisioning) provided by seagrass meadows were assessed. Key results emphasize the main variables affecting the status of seagrass meadows both on land and sea, primarily depth, water transparency, and the presence/absence of protection actions along MCA. On the other hand, results from scenario analysis highlight that under RCP4.5 the environmental conditions remain more suitable for seagrass habitat survival and growth, compared to RCP8.5 in both 2050 and 2100. Moreover, the evaluation of ES was instrumental to quantify their multiple benefits for coastal communities and to provide representative figures for stakeholders and policymakers. This paves the way for integrating management actions, primarily linked to land-use changes and widening of MPAs, benefitting WQ conditions for P.oceanica status, while contributing to achieve the SDGs, and the Good Environmental Status as required by relevant EU legislation.