Various nitrogenous nutrients—such as nitrate (NO₃⁻), nitrite (NO₂⁻), ammonium (NH₄⁺), and urea—are discharged through estuaries, stimulating phytoplankton growth in coastal environments. Estuaries act as a crucial modulator by facilitating phytoplankton uptake of nitrogen (N) species, thereby influencing the relative output of N species into downstream coastal zones. However, the factors driving this modulation, particularly in tropical regions with numerous small estuaries connecting to offshore coral reefs, remain poorly understood. To address this, we employed ¹⁵N isotope tracer incubation techniques to investigate phytoplankton uptake rates of NO₃⁻, NO₂⁻, NH₄⁺, and urea during cold and warm seasons in the Dongzhai Harbor (DZH) estuary. Our results revealed that although NO₃⁻ was the predominant component of the reactive N pool in both seasons, NH₄⁺ was the most preferred N source during the warm season. Contrary to traditional expectations, urea did not exhibit temperature stimulation pattern; instead, it was the most preferred N source for phytoplankton during the cold season, likely due to the shift in phytoplankton community. While uptake rates of inorganic and organic N differed in response to temperature and phytoplankton community changes, increased inorganic N uptake in the warm season, driven by both temperature and community composition, led to a reduction in inorganic N output. Conversely, enhanced organic N uptake in the cold season, facilitated by advantageous phytoplankton species, resulted in a higher retention of inorganic N, which then flowed into coastal zones.

urea uptake, ammonium uptake, nitrate uptake, nitrite uptake, phytoplankton community, temperature