長江大洪水事件增強東海表水二氧化碳分壓之吸收力

Abstract

現有的認知對於大河異常洪水影響沿岸海洋之二氧化碳吸收為何還不明確，尤其是以河川佔主導地位的邊緣海域。本研究主要是探討2010年7月長江大洪水對於東海表水二氧化碳分壓與其海氣交換通量之影響。藉著2010年7月6日至17日利用海研一號932航次，於東海進行現場偵測大氣與海水中之二氧化碳分壓（fCO2）和水文參數。結果顯示，表水二氧化碳分壓濃度範圍介於97.3 ~ 467.4 uatm，平均值為291.7 ± 63.3 uatm (n = 1599)，大氣二氧化碳分壓濃度為380.2 ± 1.2 uatm (n = 686)。因為大洪水將大量的水和陸源物質以及營養鹽帶至東海，使得東海表水二氧化碳分壓存在一特殊的空間分佈。強烈生物作用主要發生在羽區內 (plume area) (即是長江沖淡水範圍內，Changjiang Diluted Water，CDW，鹽度小於31)，造成低二氧化碳分壓濃度 (249.7 ± 26.0 uatm)，約佔了整個航次總面積的50％；而具有高溫度、高鹽度及低營養與高二氧化碳分壓濃度之特性的水團 (即是台灣暖流水和黑潮水) 面積(13％)相較於來得少。整體而言，2010年7月大洪水事件促使東海成為強烈二氧化碳的匯 (~ -4.0 molC m-2 yr-1)。此外，本研究亦分析了過往14年 (1998-2011年) 的航次資料，得到了相同的結論，當CDW為主導水團時，因為高營養鹽和大量生物作用，加強了大氣二氧化碳的吸收。整體來說，長江大洪水明顯地增強東海二氧化碳吸收。進一步分析的結果表示，過去十幾年來，東海夏季7月大洪水之碳吸收量比起非洪水時期之碳吸收量多出約10倍。隨著自然環境的變遷使得大洪水事件發生的頻率可能會漸趨頻繁，為了解大洪水事件對於沿岸海洋二氧化碳吸收的貢獻度，所以我們需要長期地觀測並研究。

The understanding of how anomalously large river floods affect the CO2 uptake in the coastal oceans, especially for the river-dominated marginal seas, is unclear. This study is to investigate the effect of the Changjiang flood in July 2010 on the surface distribution and air-sea exchange flux of CO2 in the East China Sea (ECS). We had performed the underway measurements of the air and sea surface fCO2 with hydrographic variables from July 7th to 17th 2010 during the flood event in the field survey of OR1-932. The results showed that fCO2w ranged from 97.3 to 467.4 uatm, with an average of 291.7 ± 63.3 (n=1599), which was under-saturated relative to atmospheric CO2 (380.2 ± 1.2 uatm, n=686). The fCO2 distribution exhibited a typical pattern spatially, varying from the normal summer. The serious flood brought huge amounts of runoff waters with terrigenous materials, including nutrients for phytoplankton, to the ECS. The flood-induced biological production largely took place in the plume area (i.e., Changjiang diluted water, CDW, Salinity <31‰) along with low fCO2 (249.7 ± 26.0 uatm), which occupied the 1/2 of total survey area. The high fCO2 waters associated with saline, warm oligotrophic waters (e.g., Taiwan Warm Current and Kuroshio waters) become less (13％ of total area) relative to the normal summer levels (58％). Overall, the flood in July 2010 induced a net strong CO2 sink (~-4.0 molC m-2 yr-1). Moreover, similar consequences of the floods in the ECS were observed from a 14-year observation (1998-2011) that the CDW, as dominated water mass, spread over the ECS along with high nutrient and high biomass growth and then strengthen the atmospheric CO2 uptake. Overall, the Changjiang floods significantly enhance the CO2 uptake in the ECS. The results further show the CO2 uptake in the ECS in July 1998 and 2010 about 10 times the amount during non-flooding periods in the past decades. As the frequencies of floods increase world wide as a result of climate change, the contribution and magnitude of the flood-enhanced CO2 uptake in the coastal ocean shall be further examined via a long-term observation.