Nitric oxide (NO) in the Bohai Sea and the Yellow Sea

Ye Tian,Chun-Ying Liu,Wei-Hua Feng,Chao Xue,Hermann W Bange,Pei-Feng Li,Gui-Peng Yang

doi:10.5194/bg-16-4485-2019

Abstract

Abstract. Nitric oxide (NO) is a short-lived compound of the marine nitrogen cycle; however, our knowledge about its oceanic distribution and turnover is rudimentary. Here we present the measurements of dissolved NO in the surface and bottom layers at 75 stations in the Bohai Sea (BS) and the Yellow Sea (YS) in June 2011. Moreover, NO photoproduction rates were determined at 27 stations in both seas. The NO concentrations in the surface and bottom layers were highly variable and ranged from below the limit of detection (i.e., 32 pmol L−1) to 616 pmol L−1 in the surface layer and 482 pmol L−1 in the bottom layer. There was no significant difference (p>0.05) between the mean NO concentrations in the surface (186±108 pmol L−1) and bottom (174±123 pmol L−1) layers. A decreasing trend of NO in bottom-layer concentrations with salinity indicates a NO input by submarine groundwater discharge. NO in the surface layer was supersaturated at all stations during both day and night and therefore the BS and YS were a persistent source of NO to the atmosphere at the time of our measurements. The average flux was about 4.5×10-16 mol cm−2 s−1 and the flux showed significant positive relationship with the wind speed. The accumulation of NO during daytime was a result of photochemical production, and photoproduction rates were correlated to illuminance. The persistent nighttime NO supersaturation pointed to an unidentified NO dark production. NO sea-to-air flux densities were much lower than the NO photoproduction rates. Therefore, we conclude that the bulk of the NO produced in the mixed layer was rapidly consumed before its release to the atmosphere.

Highlights

Nitric oxide (NO) is a short-lived intermediate of the oceanic nitrogen cycle (Bange, 2008)
NO in the surface layer was supersaturated at all stations during both day and night and the Bohai Sea (BS) and Yellow Sea (YS) were a persistent source of NO to the atmosphere at the time of our measurements
The maximum NO surface and bottom concentrations were measured at stations B21 and B28 in the northern YS (NYS), respectively (Fig. 2b), whereas the lowest NO surface and bottom concentrations were measured in the center of the southern Yellow Sea (SYS) (Table 1)

Summary

Introduction

Nitric oxide (NO) is a short-lived intermediate of the oceanic nitrogen cycle (Bange, 2008). It received limited attention so far because its determination in seawater is challenging (Zafiriou and McFarland, 1980; Lutterbeck and Bange, 2015; Liu et al, 2017). NO in surface seawater can be produced via the photolysis of nitrite (NO−2 ) (Zafiriou and McFarland, 1981; Olasehinde et al, 2009, 2010; Liu et al, 2017): NO−2 + H2 O hν −→ NO OH−. NO is a messenger molecule in marine organisms: phytoplankton responds to exogenous NO (Zhang et al, 2005) and produces NO dur-

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