Dissolved Nitrous Oxide and Methane in the Taiwan Strait: Distribution, Seasonal Variation, and Emission

Abstract

The global ocean plays an important role in the overall budgets of nitrous oxide (N₂O) and methane (CH₄), especially in continental estuaries and shelf areas. Four cruises were conducted between 2021 and 2022, covering the spring, summer, and fall seasons, to study the spatial and seasonal characteristics of N₂O and CH₄ distributions and emissions in the Taiwan Strait (TWS). The surface N₂O and CH₄ concentrations gradually decreased from the coast to the open sea, with maximum values (14.3 and 15.6 nmol L⁻¹) occurring near the Jiulong and Minjiang estuaries, respectively. The mean surface N₂O concentration (8.2 nmol L⁻¹) was highest in the spring and approximately the same in the summer as in the fall. The mean surface concentrations of CH₄ (8.8 nmol L⁻¹) were greater in summer than in spring and fall, probably because of the high freshwater input in summer. Except for several stations in fall, surface waters were oversaturated with N₂O and CH₄ relative to the atmosphere in other seasons, and the TWS was a net source of atmospheric N₂O and CH₄ in the spring, summer, and fall. In situ production is the main source of N₂O and CH₄ in the TWS, with nitrification being the dominant mechanism of N₂O production in the TWS. In contrast, physical influences (riverine inputs and water mass mixing) reshape the distributions of N₂O and CH₄. The annual emissions of N₂O and CH₄ from the TWS were estimated to be 0.9 × 10⁻³ ± 2.9 × 10⁻³ Tg yr⁻¹ and 1.6 × 10⁻³ ± 3.0 × 10⁻³ Tg yr⁻¹, respectively. Taken together, the TWS accounts for 0.025% of the surface area of the world's oceans and 0.022% ± 0.07% and 0.017% ± 0.033% of global oceanic N₂O and CH₄ emissions, respectively.