Abstract
Coastal water quality degradation presents a global challenge that requires advanced simulations and practical approaches. This study introduces a scenario-based hotspot approach to assist water quality managers and researchers. The method enables informed development decisions in coastal watersheds through a closed-loop framework that simulates watershed conditions, locates nutrient hotspots, tests mitigation strategies, and evaluates cost-effectiveness. This approach was tested in the Peace River Watershed (PRW), a coastal watershed in Florida, where excess nutrient loading fuels harmful algal blooms (HABs) in Charlotte Harbor. The Watershed Assessment Model (WAM) was applied to simulate nutrient loading scenarios from various land uses. Six "what-if" mitigation scenarios were simulated following the base model run, representing full and targeted agricultural and urban Best Management Practices (BMPs) and septic system upgrades. Results show significant spatial variability in total nitrogen (TN) and total phosphorus (TP) loads across the PRW. Urban areas showed high nitrogen levels, phosphate mining had the highest phosphorus levels, and agriculture contributed elevated amounts of both. Agricultural BMPs reduced TN by 10% and TP by 13%; urban BMPs reduced TN by 19% and TP by 20%; Septic conversions yielded 26% TN reduction. Targeting top 5% of nitrogen hotspots achieved 4%, 3%, and 8% reductions for agricultural, urban and septic scenarios, respectively. The framework helps managers and policymakers to proactively assess and implement targeted interventions in coastal watersheds, supporting informed decision-making through scenario-based hotspot analysis for cost-effective water quality mitigation.Graphical AbstractThis study presents a scenario-based strategic framework to support water quality management in hydrologically sensitive coastal watersheds. The process begins by identifying key water quality drivers from land use and nutrient loading data. A watershed model is then employed to simulate current conditions and spatially map nutrient loading across various land use types. Estimated annual total nitrogen (TN) and phosphorus source-cell loadings range from near zero to 116 and 38 kg/ha, respectively. Nutrient hotspot areas are identified based on land use contributions. Six "what-if" scenario-based mitigation approaches are then evaluated: a global approach applying urban and agricultural Best Management Practices (BMPs) and septic upgrades across the whole watershed, and a targeted approach focusing only on the priority hotspot areas to maximize nutrient reduction and cost-effectiveness. The effectiveness of these strategies in reducing nutrient loads and improving water quality is assessed, with spatial visualization highlighting the benefits across the watershed.