Spatiotemporal response of water quality in fragmented mangroves to anthropogenic activities and recommendations for restoration

Mangroves have received substantial attention for their pivotal role as ecological barriers between land and sea, owing to their capacity to effectively capture considerable quantities of terrestrial pollutants. Mangrove fragmentation has been a widespread global trend. There is limited information on the water quality status of these small scattered mangrove patches in coastal sub-developed areas, coupled with a paucity of efficient and intuitive assessment methodologies. To address this gap, the Water Quality Index (WQI) was introduced to evaluate the spatiotemporal characteristics of mangrove water quality. The major sources of pollution and anthropogenic activities that affect mangrove water quality were identified. The results revealed an average WQI value of 44.1 ± 13.3 for mangrove patches, consistently indicating a "low" water quality classification throughout all seasons. Both the size and natural conditions impact the water quality of mangroves. The large artificial patch (WQI: 56.4 ± 7.61) and the natural patch (WQI: 46.6 ± 13.6) exhibited relatively superior water quality, while the WQI value of a size-equivalent artificial patch compared with the natural patch is 38.6 ± 11.8. Aquaculture was the primary human activity that adversely affected the water quality of mangroves, and the potential sources of pollution were rainfall runoff and river discharge. These findings elucidate the unfavorable water quality characteristics and dominant pollution of fragmented mangroves, and validate the applicability of the WQI method for long-term evaluation of the water quality in mangrove patches. This study provides a basis for decision-making in water quality assessment and management of coastal wetlands and marine ecosystems. Scientific guidance to the management for mangrove protection and restoration was offered, such as regulating aquaculture activities, controlling non-point source pollution, implementing mangrove reforestation by using native species in historical mangrove sites.