講演情報
[ACG67-P04]Strong methane release from mangrove system in the southern Vietnam
*亀山 宗彦1、ド クーン2、ブイ オアン2、チャン ユン2、ヴォ ルオン フック2、カオ ユイ2、ト ヒエン2 (1.北海道大学、2.ベトナム国家大学ホーチミン市校自然科学大学)
キーワード：
メタン、フラックス、海洋観測、沿岸海洋、ベトナム
Methane is the second most important greenhouse gas contributing to global warming after carbon dioxide, and the ocean acts as a source of methane to the atmosphere. Marine methane emissions are geographically concentrated in coastal regions, primarily driven by the decomposition of terrestrial organic matter transported from land to coastal areas. Oceanic methane observations have been conducted worldwide, and global databases such as MEMENTO (https://memento.geomar.de/index.html) have been established. Based on these databases, global ocean-to-atmosphere methane fluxes have been estimated (e.g., Weber et al., 2019). However, spatial and temporal data coverage remains highly uneven due to the limited frequency of in situ observations. Although previous estimates (Weber et al., 2019) identified the East Sea/South China Sea coastal region as a significant methane source, the underlying data were derived from only a single research cruise, resulting in extremely limited spatial and temporal coverage and substantial uncertainty in the flux estimates.
In particular, the tropical monsoon region in the southern part of this area is characterized by extensive mangrove ecosystems with high productivity and is expected to supply large amounts of organic matter—the ultimate source of methane—to surrounding aquatic environments. This study aimed to estimate methane distributions and emission rates in rivers and coastal areas, including mangrove regions, in the tropical monsoon region of southern Vietnam. Because strong seasonal differences in biogeochemical conditions are expected, observations were conducted during both the wet season (August 2024) and the dry season (January 2025).
Methane concentrations ranged from 21.6 to 80.7 nM in the wet season and from 19.7 to 111.1 nM in the dry season. Although the overall concentration ranges were comparable between seasons, mean methane concentrations were slightly higher during the wet season. Lower nutrient and dissolved oxygen concentrations during the wet season likely enhanced organic matter decomposition, leading to increased methane production. Comparisons among rivers (within mangrove system) and coastal waters showed that methane concentrations were significantly higher in mangrove regions during both seasons. This indicates that methane produced within mangrove systems is transported downstream via rivers to coastal waters. Such transport increases the air–water contact area of methane-rich waters, thereby promoting efficient methane emission to the atmosphere. These results indicate that mangrove ecosystems in tropical monsoon regions of Southeast Asia are significant sources of atmospheric methane and suggest that previous methane flux estimates may have been underestimated due to insufficient observational coverage.
In particular, the tropical monsoon region in the southern part of this area is characterized by extensive mangrove ecosystems with high productivity and is expected to supply large amounts of organic matter—the ultimate source of methane—to surrounding aquatic environments. This study aimed to estimate methane distributions and emission rates in rivers and coastal areas, including mangrove regions, in the tropical monsoon region of southern Vietnam. Because strong seasonal differences in biogeochemical conditions are expected, observations were conducted during both the wet season (August 2024) and the dry season (January 2025).
Methane concentrations ranged from 21.6 to 80.7 nM in the wet season and from 19.7 to 111.1 nM in the dry season. Although the overall concentration ranges were comparable between seasons, mean methane concentrations were slightly higher during the wet season. Lower nutrient and dissolved oxygen concentrations during the wet season likely enhanced organic matter decomposition, leading to increased methane production. Comparisons among rivers (within mangrove system) and coastal waters showed that methane concentrations were significantly higher in mangrove regions during both seasons. This indicates that methane produced within mangrove systems is transported downstream via rivers to coastal waters. Such transport increases the air–water contact area of methane-rich waters, thereby promoting efficient methane emission to the atmosphere. These results indicate that mangrove ecosystems in tropical monsoon regions of Southeast Asia are significant sources of atmospheric methane and suggest that previous methane flux estimates may have been underestimated due to insufficient observational coverage.