Sargassum horneri, a prominent species in the “gold tide” phenomenon in the Yangtze River Estuary, exhibits two distinct ecotypes due to the life history: stationary and floating types. There is currently a gap in knowledge regarding the absorption kinetics and growth responses of different ecotypes of S. horneri to various nitrogen sources. To address this gap, the study was conducted on the stationary and floating ecotypes of S. horneri in the Shengsi Sea area of the Changjiang Estuary to investigate the impact of nitrogen enrichment on their rapid proliferation. The study compared the absorption and growth responses of the two ecotypes to different nitrogen forms (NaNO3, NH4Cl, Urea-N) under controlled indoor culture conditions. The findings suggested that the absorption of different dissolved nitrogen sources by the two ecotypes of S. horneri was consistent with the characteristics of saturated absorption kinetics. The relationship between the absorption rates and the nitrogen concentrations in water could be described by the Michaelis-Menten equation. There were significant differences in the maximum absorption rate (Vmax) for NH4-N and the semi-saturation constant (Km) between the stationary and floating types , with the stationary type showing higher values (39.86 ± 4.15 μmol L-1 h-1 for Vmax and 34.25 ± 3.99 μmol/L for Km, respectively), whereas the Vmax for Urea-N was significantly higher in the floating type compared to that in the stationary type. The results demonstrated that the stationary type exhibited a higher absorption rate of NH4-N but a lower affinity, whereas the floating type showed a greater absorption potential for Urea-N. When Urea-N was used as the nitrogen source, the floating type achieved a higher relative growth rate, and the total nitrogen content in the algal tissues was significantly elevated. Conversely, when NO3-N was utilized as the nitrogen source, the floating type did not exhibit decay during the culture period, although the relative growth rate was lower. The present study indicated that Urea-N could enhance the growth of the floating type of S. horneri. In contrast, NO3-N could be stored in the body fluid vesicles of the algal, serving as a sustainable nitrogen source under conditions of nitrogen deficiency, thereby supporting the prolonged survival of the floating type. As a whole, the floating type of S. horneri exhibited greater efficiency in utilizing both NO3-N and Urea-N compared to the stationary type. The accumulation of NO3-N and Urea-N in the seawater of the Yangtze River Estuary, along with their synergistic effects, could represent a significant factor contributing to the persistent occurrence of “gold tide”.