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 delta N-15(PN), yet, the mean in the upper 100 m (2.0 to 5.3 parts per thousand) is consistently smaller than that in the lower 100 m, implying the occurrence of vertical biological fractionation and/or an addition of N-15-depleted N from the atmosphere. The delta N-15(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 delta N-15(PN), and high N* values. N fixation is likely the cause for the intermonsoon delta N-15(PN) lows. The delta N-15(PN) values of trapped material at 374 m and 447 m range from 3.3 to 7.3 parts per thousand with a flux-weighted mean of 5.6 parts per thousand resembling the delta(NO3)-N-15 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 similar to 20 +/- 26 mmol N m(-2) yr(-1). This value accounts for only similar to 5-10% of the new production on an annual basis.