Atmospheric wet and dry deposition of dissolved inorganic nitrogen to the South China Sea

Abstract

At the global scale, atmospheric inputs of nitrogen are an important source of the new nitrogen that supports new marine production, especially in oligotrophic open oceans and marginal seas. This study reports quantities of atmospheric deposition of dissolved inorganic nitrogen (DIN) to the largest marginal sea in the North Pacific (the oligotrophic South China Sea, SCS) based primarily on rainwater sampling in the open northwestern region (Yongxing Island) from 2013 to 2015, and aerosol sampling from the SCS basin in June 2017. Atmospheric wet and dry deposition of DIN and their potential contributions to productivity were estimated. The volume-weighted mean rainwater concentrations during the wet and dry seasons were 4.9 and 18.1 μmol L−1 for N+N ( NO 3 − + NO 2 − ), and 5.7 and 4.0 μmol L−1 for NH 4 + , respectively. Rainwater concentrations of DIN were lower in the marginal seas than in the open ocean. The aerosol NO 3 − concentration was 1.15 ± 1.18 μg m−3 during the wet season, which is slightly lower than reported for the East China Sea and East Sea, but higher than in the Arabian Sea. Monthly wet and dry deposition rates ranged from 0.4–3.9 and 0.4–1.2 mmol m−2 mon−1 for NO 3 − , and 0.2–1.3 and 0.01–0.02 mmol m−2 mon−1 for NH 4 + , respectively. The annual wet and dry deposition fluxes of DIN were estimated to be 16.8 and 10.1 mmol m−2 yr−1, respectively. Compared to other marginal seas, the SCS receives less atmospheric NO 3 − inputs than the Yellow Sea, East China Sea, East Sea, and northeastern Mediterranean Sea. The total atmospheric DIN deposition may account for 1.8–11.1% of the nitrogen supporting new production and 0.7–1.8% of the nitrogen supporting primary production.