Examining N cycling in the northern South China Sea from N isotopic signals in nitrate and particulate phases

Abstract

AbstractNitrogen sources and dynamics, one of the key issues in marine biogeochemical cycles, remain poorly constrained in marginal seas. Here we examine the nitrogen cycle in the northern South China Sea (SCS) by combining data from previous reports with a new data set of N isotopic compositions (δ15N) of nitrate, zooplankton, and sinking particles. Average δ15N in subsurface nitrate is 4.8 ± 0.3‰, similar to that of sinking particles (δ15Nsink of 4.4‰) through the euphotic zone (EZ) collected by floating traps and to documented mean (4.6‰) for long‐term mooring traps at 200 m. This along with oft‐observed shallow nitracline (<100 m) suggests that subsurface nitrate is the primary source of new N to support export production. Moreover, δ15Nsink at the bottom of the EZ resembles those of suspended particles (4.2 ± 1.0‰) and zooplankton (5.4 ± 1.0‰) inside the EZ. High similarity in δ15N among various types of particles including zooplankton in different size fractions in the EZ implies rapid N turnover in the ecosystem. In deep waters at 2000–3000 m, however, additional particulate N fluxes are found due to lateral transport, which contain 15N‐depleted particles, resulting in a downward decreasing trend of δ15Nsink. Incorporation of lighter N by bacteria and selective export of picoplankton are proposed as alternative mechanisms contributing to low δ15Nsink in the deep waters. The significant δ15Nsink change in the deep water column makes the SCS different from most other marginal seas; thus, caution should be made when using sedimentary δ15N to reconstruct paleonitrogen processes.