Nitrogen in Atmospheric Wet Depositions Over the East Indian Ocean and West Pacific Ocean: Spatial Variability, Source Identification, and Potential Influences

Shan Jiang,Ju Chen,Zhenqiu Zhang,Jie Jin,Sumei Liu,Jianing Wang,Shuo Jiang,Lingyan Wang,Jing Zhang,Yan Chang,Ying Wu

doi:10.3389/fmars.2020.600843

Abstract

Atmospheric deposition is the dominant pathway for the loading of exogenous nitrogen (N) to open ocean. Here, rainwater samples were collected from 31 stations in the equatorial East Indian Ocean (EIO) and West Pacific Ocean (WPO) to explore the spatial variability of N species, potential sources, and related ecological influences. Among two oceans, nitrate (NO3–) and ammonium (NH4+) were the main components in the rainwater N inventory. NO3–concentrations varied from 0.19 to 100.5 μM, whereas NH4+concentrations ranged from 0.54 to 110.6 μM. Among all stations, low concentrations of NO3–and NH4+appeared in the remote ocean, whereas high concentrations were observed at the stations near the Malacca Strait and New Guinea, coupled with an enhancement of non-sea salt major ions, e.g., calcium ions (Ca2+) and sulfate (SO42–), revealing the influence from coastal human activities, such as coal and gasoline combustion. In the remote ocean, δ15N–NH4+ranged from −5.7 to −9.3‰, whereas it dropped to -15.5‰ near coasts. A logarithmic decay between δ15N–NH4+and NH4+concentrations in rainwater samples was obtained, suggesting a shift from natural source (seawater emission) in oceanic precipitation events to anthropogenic source (chemical fertilizer volatilization and vehicle exhaust) in coastal rainwaters. δ15N–NO3–in the remote ocean varied between −1.7 and 0.4‰ with low levels found in the WPO, likely related to the ascending air flow driven by the Walker Circulation. In coastal oceans, δ15N–NO3–ranged from 1.5 to 3.5‰. The linkage between δ15N–NO3–and NO3–concentrations varied in two oceans, resulting from difference in biological and fossil fuel combustion contributions. Compared with ocean surface water, N in the rainwater was markedly enriched, suggesting that N from atmospheric wet depositions could rapidly enhance the dissolved N availability in ocean surface water. However, the N redundancy according to the Redfield–Brzezinski ratio (N:Si:P = 16:16:1) in the rainwater might benefit from the growth of N-preference phytoplankton species and microbes. As the first study on N concentrations, sources, and stoichiometry balance in rainwater over the equatorial WPO and EIO, the results could be a support to the global N budget estimation and oceanic primary production modeling.

Highlights

Nitrogen (N) is essential for the biosynthesis of amino acids, phospholipid, nucleic acids, etc
Due to the positive correlation between temperature and emitted δ15N–NH4+ (Elliott et al, 2019), δ15N–NH4+ derived from oceanic emission in equatorial regions tended to be higher in this range, which fitted our observation in the remote ocean
The present study investigated the N inventory in rainwaters obtained from the EIO and WPO

Summary

Introduction

Nitrogen (N) is essential for the biosynthesis of amino acids, phospholipid, nucleic acids, etc. The impacts of N deposition to oceans are less evaluated, with sporadic measurements and limited spatial coverage archived in the past several decades (e.g., the tropical Atlantic Ocean, Baker et al, 2007; Bermuda, Altieri et al, 2013; the North Pacific Jung et al, 2013). In those unexplored oceans, our knowledge of atmospheric N deposition, with regard to concentration, species, and sources, is highly scarce. The lack of oceanic N deposition data hampers the understanding of carbon cycles because of the reaction linkage of the sequestration capability of atmospheric CO2 via primary producers (Capone, 2001)

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