Supporting strategic decisions for coastal watersheds in crisis: A system dynamics model for socio-ecological sustainability

Sustainable development of coastal lagoons facing complex socio-ecological crises requires quantification of impacts of solutions on all sectors involved. We present a System Dynamics Model (SDM) representing interactions between agriculture, livestock, tourism, and nutrient stocks (aquifer, soils, wastewaters) of the Mar Menor socio-ecological system (SE-Spain), following a participatory process. We applied the SDM to evaluate 22 individual solutions and 3 adaptation scenarios based on contrasting Shared Socioeconomic Pathways (SSP1, SSP2, SSP5) and stakeholder inputs. Results show that agriculture drives degradation while recycling nutrients from livestock and urban wastewater, and that legacy Nitrogen and Phosphorus in soils and groundwater limits lagoon recovery. Business-as-usual (SSP2) and agricultural intensification (SSP5) scenarios reduce water security due to climate change and groundwater contamination. End-of-pipe interventions do not provide long-term solutions, whereas source-oriented interventions lower nutrient inputs, providing socioeconomic benefits. A sustainability-focused scenario (SSP1) results in moderate economic growth, environmental improvements, and highest Total Economic Value.