Impacts of freshwater scarcity on fish biodiversity in an Indo-Burma Ramsar wetland: A case study of U Minh Thuong National Park, Vietnam

U Minh Thuong National Park (UMTNP), a Ramsar site characterized by Melaleuca forests and peatlands, is vital for biodiversity conservation in the Mekong Delta. However, seasonal fluctuations, particularly during the dry season, threaten freshwater availability and the stability of ecosystems. To evaluate key conservation threats, we investigated how seasonal water scarcity and water quality impact fish diversity in UMTNP. From 2020 to 2021, we conducted field surveys across five habitat types: canals, flat-forms with and without aquatic plants, Melaleuca forests, and buffer zones; over four seasonal sampling events. We analyzed fish diversity and water quality using the Shannon-Wiener, Simpson, and Bray-Curtis indices, alongside nine physicochemical parameters. Our surveys recorded a total of 1,438 fish individuals, representing 32 species, 19 families, and 8 orders. Fish diversity was found to be highest in canals and flat-forms without aquatic plants, while it was lowest in buffer zones and Melaleuca forests. Statistical analyses and principal component analysis (PCA) revealed significant seasonal and spatial differences, particularly in biological oxygen demand (BOD), hydrogen sulfide (H₂S), and total ammonia nitrogen (TAN). The buffer zones showed persistent pollution, and dry-season water scarcity further diminished fish diversity. These findings provide essential data to inform targeted conservation strategies and enhance the resilience and sustainability of aquatic ecosystems in UMTNP. Such insights underscore the urgent need for integrated water and fisheries management across Indo-Burma Ramsar sites facing similar challenges.