The estuary comprises various organic matter sources originating from both marine and terrestrial inputs. The East Asian Monsoon enhances precipitation and river discharge, making the complex estuarine ecosystem. The presence of estuarine dikes can extend water residence time and potentially reduce the transport of terrestrial organic matter. This makes ecological differences from natural estuaries. There are case studies that focus on the dike or dam effect on the ecosystem but comparative study on dike estuaries and natural areas is still restricted. For effective estuarine ecosystem management, this study was conducted on three dike estuaries (Geum/GE, Yeongsan/YSE, Nakdong/NDE estuary) for 2023, 2024, 2025 and one natural estuary (Seomjin estuary/SJE) for all three years. Fish, invertebrates, and basal sources (algae, terrestrial plants, seagrass) were captured in each site and season (pre-monsoon and post-monsoon). The integrated approach of amino acid carbon and nitrogen isotope analysis has been applied to characterize multiple basal sources.

In 2023, the isotopic niche area of SJE showed a wider niche area than GE, indicating seasonal changes. In 2024 and 2025, the isotopic niche area of SJE was similar to or narrower than that of the dike estuary (YSE, NDE) with no seasonal change. According to basal source contribution, GE and YSE show a similar contribution trend between the two seasons. Algal contribution occupied most of their basal sources in YSE; this might be due to the dike hindering the transport of terrestrial organic matter to the estuary. In the case of SJE, omnivore species assimilate the essential amino acids derived from terrestrial plant and seagrass near or over 20%, especially in post-monsoon. In the NDE, terrestrial plants occupy over 10%, similar diet contribution for omnivore fish (K. punctatus) to those in SJE in 2023 and 2025. In the SJE of 2024, omnivore species show the seagrass contribution (10% to 20%) rather than terrestrial plant sources as their diets. Carnivore fish species, P. indicus have higher bacteria/detritus contribution than other species through assimilating the bacterial rework source. The present results suggest that trophic positions of individual estuarine heterotrophs should be determined by the βMix trophic position equations, considering the significant contributions of terrestrial and seagrass as some of their diets in both dike estuaries and natural estuary.