N₂ fixation is a major source of new N in the oligotrophic subtropical sea areas, fueling the upper ocean ecosytem and regulating the oceanic sequestration of atmospheric CO₂. On the other hand, zooplankton plays a non-negeligible role in the ecosystem dynamics. Nevertheless, the potential link between N₂ fixation and zooplankton in the ocean has rarely been examined and yet clear to date. Here we will present preliminary results focusing on the potential relationship based on nitrogen isotope (¹⁵N) in the northern South China Sea. We did not find direct evidence that zooplankton (copepod) acts as a loci for N₂ fixation, although more evidence is in needed covering a wider spatial range. Interestingly, our results also show that diazotrophic derived nitrogen (DDN) may be somewhat effectively transferred to zooplankton, implying that zooplankton do play an important role in ecosystem N dynamics.