Introduction: The international mangrove wetland of Khurkhoran on the southern coast of Iran, plays an important role in absorbing and stabilizing the retention of nutrients such as nitrogen and phosphorus, which reduces pollution of water resources and improves water quality. The increase in the use of chemical fertilizers in agriculture, the entry of urban and industrial wastewater, and changes in land use have put great pressure on this ecosystem and have increased the nutrient load in the waters entering the wetland. Therefore, the quantification and economic valuation of this ecosystem service is essential for the planning and sustainable management of wetlands. This study aims to: (1) Quantitatively assess the retention of nitrogen and phosphorus by the Khorkhoran International Wetland; (2) Map the spatial distribution of nutrient export and retention; and, (3) Estimate the economic value of this ecosystem service using both the avoided cost and benefit transfer approaches.
Materials and Methods: In this study, the NDR model from the InVEST software package was used to simulate the export and retention of nitrogen and phosphorus in the study area. The key input data included a land use map, a precipitation and surface runoff map, DEM, and nutrient loads from agricultural and urban activities. The input data were processed in the ArcGIS software environment and the model was run. Subsequently, the model outputs included maps of extracted nutrient export and retention and key areas of this service were identified. Then, using the avoided cost and the benefit transfer methods, the economic value of the nutrient retention service was calculated based on the replacement cost of treating these materials in wastewater treatment engineering systems.
Results: The modeling results showed that the Khorkhoran mangrove wetland annually retains 243.75 and 834.18 tons of phosphorus and nitrogen respectively, which prevents these substances from entering water resources and causing problems such as algal blooms. Also, areas covered by mangrove forests and coastal marshes played the greatest role in retention these elements, while agricultural and urban areas showed the highest nutrient export. The annual phosphorus and nitrogen export from the study area was estimated to be 21.48 and 90.52 tons respectively, indicating the negative effects of human uses on the nutrient load entering the wetland. The economic value of the nutrient retention service was calculated. Accordingly, the cost of removing each kilogram of nitrogen and phosphorus in wastewater treatment systems was estimated to be 2.7 and 14.3 US$ in 2023. Considering the amount of nutrient retention in the wetland, the total value of this service in the study area was estimated to be 2639503 million Iran’s rials (Equal to US$ 5738050).
Discution: The results of this study emphasize that the Khor Khoran mangrove wetland is an effective natural filter for reducing water resource pollution. Preservation and management of this wetland can be a suitable economic alternative to costly wastewater treatment methods. As a result, in environmental management policies, paying attention to ecosystem services such as nutrient retention can significantly contribute to improving water quality and reducing pollution. In addition, it is suggested that future research should use field data and more accurate modeling methods to better assess these services. To improve the accuracy of predictions and assessments of ecosystem function, it is also necessary to simulate various climate conditions and examine the results under different scenarios.