Effects of Atmospheric Dry Deposition on External Nitrogen Supply and New Production in the Northern South China Sea

The South China Sea (SCS) is one of the world’s largest oligotrophic marginal seas. Increases in biomass and primary production in the surface layer of the northern SCS are affected by anthropogenic aerosol use among north Asian peoples. The seasonal variation of dry deposition and its contribution to new production in the ocean are vital to determining the effect that such dry deposition has on the biogeochemical cycle of the SCS. This study collected 240 samples of total suspended particles at Dongsha Island in the northern SCS from April 2007 to March 2009; the major ions and water-soluble nitrogen species in the samples were analyzed. The analysis results indicated that the concentration distributions of major water-soluble ions and nitrogen species in total suspended particles exhibited significant seasonal (source) variation. The north-east monsoon seasons (autumn to spring) brought relatively high concentrations because most air masses during this period arrived from the northern continental region. We found that the concentration of nitrogen species shows a latitude distribution, gradually decreasing from north to south. In addition, this study also discovered that the ratio of organic nitrogen to total dissolved or water-soluble nitrogen also varies in a similar manner, resulting in a concentration of <20% for locations north of 30° N and >30% for those south of 30° N. Aerosols at Dongsha Island mainly comprised sea salt; however, significant chloride depletion was observed during the north-east monsoon season. The molar ratio of NH₄⁺ to non–sea salt (NSS) sulfate (nss-SO₄²⁻) was 0.8, indicating that the amount of artificially produced NH₄⁺ in the region was insufficient for reaction with nss-SO₄²⁻. Therefore, NH₄⁺ was mainly present in the form of NH₄HSO₄. The fluxes of water-soluble inorganic nitrogen (WSIN) and water-soluble organic nitrogen (WSON) within the region were 23 ± 13 and 27 ± 15 mmol m⁻² y⁻¹, respectively. The new production converted from atmospheric water-soluble nitrogen species in the northern SCS was estimated to be 0.52–0.81 mmol C m⁻² d⁻¹. This flux made about 5.6–8.7% (the global average was about 3.5%) contribution to the primary production (9.24 mmol C m⁻² d⁻¹) of the SCS surface water. This result indicates that the ocean’s external nitrogen supply, provided by anthropogenic aerosols, is vital for the biogeochemical cycle in Asian marginal seas, particularly the northern SCS.