Abstract: Changes in carbon content, nitrogen content and δ¹⁵N value of macroalgae tissues are of essential importance for the assessment and indication of nitrogen nutrient input. Ulva pertusa, Chondrus ocellatus,Sargassum thunbergii were sampled monthly on the intertidal zone of Xiaochangshan Island from July 2020 to June 2021 in order to explore the characteristics of the changes of carbon content, nitrogen content and δ¹⁵N value in macroalgae tissues. Furthermore, the physicochemical factors of the seawater of macroalgae habitats were measured. The results of the study revealed a significant correlation between the nitrogen content of U. pertusa tissue and the concentrations of NO₃-N and DIN in seawater. Similarly, a significant correlation was observed between the nitrogen content of C. ocellatus tissue and the concentration of NH₃-N in seawater. However, no significant correlation was found between the carbon content and C/N ratio of macroalgae tissue and the concentrations of carbon and nitrogen nutrients in seawater. Besides, the δ¹⁵N values of U. pertusa tissue also exhibited a significant correlation with the concentrations of NO₃-N and DIN in seawater. These findings suggest that U. pertusa can serve as an indicator species for identifying the sources of NO₃-N and DIN. Based on these findings, it can be inferred that the intertidal seawater around Xiaochangshan Island provides sufficient nutrients for the growth of macroalgae, mostly influenced by domestic sewage and aquaculture activties.