9 / 2024-07-29 17:17:53

In-situ On-line Monitoring of Greenhouse Gases Emitted from Seaweed Bed in the Lagoon of Dongsha Atoll, South China Sea and Their Seasonal and Diurnal Variation Trends

Dongsha Atoll, seaweeds in lagoon, greenhouse gases (GHGs), in-situ monitoring, seasonal and diurnal variation, global warming potential

This study developed an on-line greenhouse gases (GHGs) sampling/monitoring system to investigate the seasonal and diurnal aviation of GHGs emitted from seaweeds at the Dongsha Atoll in South China Sea (CSC). The system was designed on the basis of non-dispersive infrared (NDIR) absorption to continuously measure GHGs emissions. The emissions of three GHG species (CO₂, CH₄, and N₂O) were simultaneously monitored over the lagoon surface at the Dongsha Atoll in the summer and winter of 2021-2022, which were then converted to the vertical emission/uptake fluxes of GHGs. During the monitoring periods, the emissions of CO₂, CH₄, and N₂O were in the ranges of 297.9-493.4, 2.7-5.7, and 1.3-9.5 ppm, respectively. In terms of seasonal variation, the GHG emissions in summer were higher than those in winter. Additionally, a significant diurnal variation of CO₂ emissions was observed with lower daytime CO₂ emissions than nighttime. High solar radiation in the daytime enhanced the uptake of CO₂ by seaweeds via photosynthesis, and also accelerated the microbial activities in sea water and sea bed. Both effects caused the decrease of CO₂ emissions. An opposite diurnal trend was observed for CH₄ and N₂O emissions. The production and emissions of CH₄ and N₂O correlated highly with the temperatures of seawater.  High temperature in the daytime facilitates microbial decomposition of organic matter in seawater and on sea bed. Biomass debris on the sea bed stimulated methanogenic bacteria to generate CH₄. The production of N₂O were attributed from nitrification and denitrification processes. Although the concentrations of GHG emissions from seaweeds were ordered as CO₂>CH₄>N₂O, N₂O has the highest impact on global warming potential.