<p>Identifying nitrogen dynamics in coastal ecosystems remains a key challenge for environmental monitoring and management. In this study, nitrogen distribution patterns were examined in Matsushima Bay, a semi-enclosed coastal system, and its adjacent areas. The results revealed the utility of the natural abundance of ¹⁵N in all dissolved nitrogen (δ¹⁵N-TDN) as a qualitative indicator of coastal nutrient sources and status. Quantitative analyses showed substantial nitrogen loading from the Sunaoshi and Naruse rivers, but limited downstream detectability, likely because of rapid attenuation and mixing. Notably, δ¹⁵N-TDN measurements indicated persistent isotopic differences between the mouth of Matsushima Bay and Sendai Bay, underscoring their enduring disparities despite their geographic proximity. Furthermore, SUVA₂₅₄ (specific UV absorbance at 254&#xa0;nm), a parameter derived from dissolved organic carbon (DOC), outperformed bulk DOC concentration data by more effectively capturing terrestrial organic matter influences. These findings highlight the value of δ¹⁵N-TDN and SUVA₂₅₄ as refined qualitative indicators for delineating nuanced variations in nutrient sources and status, particularly in areas where treated wastewater contributes substantially. They reveal that terrestrial nitrogen inputs, while substantial near river mouths, attenuate rapidly within the bay, leaving detectable isotopic and compositional signals in downstream waters. Furthermore, the findings emphasize the value of integrated qualitative and quantitative approaches for better nutrient status assessment and for more effective environmental management strategies.</p><p></p>