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Examining N cycling in the northern South China Sea from N isotopic signals in nitrate and particulate phases
Author(s) -
Yang JinYu Terence,
Kao ShuhJi,
Dai Minhan,
Yan Xiuli,
Lin HuiLing
Publication year - 2017
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1002/2016jg003618
Subject(s) - sink (geography) , nitrate , biogeochemical cycle , photic zone , water column , oceanography , particulates , zooplankton , new production , environmental science , arctic , environmental chemistry , nitrogen , nitrogen cycle , plankton , phytoplankton , chemistry , nutrient , ecology , geology , biology , geography , organic chemistry , cartography
Nitrogen 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 (δ 15 N) of nitrate, zooplankton, and sinking particles. Average δ 15 N in subsurface nitrate is 4.8 ± 0.3‰, similar to that of sinking particles (δ 15 N sink 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, δ 15 N sink 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 δ 15 N 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 15 N‐depleted particles, resulting in a downward decreasing trend of δ 15 N sink . Incorporation of lighter N by bacteria and selective export of picoplankton are proposed as alternative mechanisms contributing to low δ 15 N sink in the deep waters. The significant δ 15 N sink change in the deep water column makes the SCS different from most other marginal seas; thus, caution should be made when using sedimentary δ 15 N to reconstruct paleonitrogen processes.