Isotope constraints on particulate nitrogen source and dynamics in the upper water column of the oligotrophic South China Sea

Particulate nitrogen (PN) dynamics in the oligotrophic northern South China Sea (around the SouthEast Asian Time‐series Study (SEATS) station) was explored by examining the isotopic compositions of suspended PN in the top 200 m over 3 years and sinking PN collected by sediment traps. The PN inventory (IPN) in the upper 100 m is larger than in the lower 100 m, exhibiting stronger seasonality. Both layers reveal significant seasonality in mean δ 15 N PN , yet, the mean in the upper 100 m (2.0 to 5.3‰) is consistently smaller than that in the lower 100 m, implying the occurrence of vertical biological fractionation and/or an addition of 15 N‐depleted N from the atmosphere. The δ 15 N PN surges in winter, when the mixed layer is deeper, indicate an intensified nitrate supply from thermocline, during which relatively stronger downward transfer efficiency was inferred by a small IPN gradient. The largest vertical gradient in IPN appeared during intermonsoon periods, corresponding with weak vertical mixing, low δ 15 N PN , and high N* values. N fixation is likely the cause for the intermonsoon δ 15 N PN lows. The δ 15 N PN values of trapped material at 374 m and 447 m range from 3.3 to 7.3‰ with a flux‐weighted mean of 5.6‰ resembling the δ 15 NO 3 of upwelled sources. By using a mass‐isotope balance model under the assumption of no atmospheric N deposition, we obtained an N fixation input of ∼20 ± 26 mmol N m −2 yr −1 . This value accounts for only ∼5–10% of the new production on an annual basis.