渤海西部季节性耗氧酸化过程对溶解态甲烷源汇过程和海-气交换通量时空演变新特征的影响机制

From the year of 2011, a new marine environmental phenomenon named oxygen depleted & acidification occurred in the bottom water of the west Bohai sea, and displayed a deterioration trend, and with an abnormal high concentration of dissolved methane. The preliminary research showed that the content of organic matter in the sediments could be elevated by the mariculture and red tide or algal blooms. Influenced by the oxygen depleted & acidification process, the methane source in the sediments could be strengthened obviously. Then a great amount of methane would be accumulated in the bottom water for the restriction of the stratification, and the air-sea flux would weaken. After the recession of the stratification and the oxygen depleted & acidification, a great amount of dissolved methane in the bottom water would diffusion into the surface water and then into the atmosphere, resulting in a short-term enhanced air-sea methane flux. Based on the special-purpose observation and simulation experiments, the relationship between the oxygen depleted & acidification and the source strengthen in the sediments and the new distribution of dissolved methane in the water column will be researched. Then, the main source of the dissolved methane in the bottom water would be determined, and the influence mechanism of the oxygen depleted & acidification process on the source-sink process of methane in the sediments, water column and atmosphere will be analyzed. Furthermore, the regulation of the oxygen depleted & acidification on the new normal of the temporal evolution of air-sea methane flux would be calculated and assessed.

2011年以来，渤海西部底层水体出现季节性耗氧酸化这一新环境现象，并有逐年加剧的趋势，还伴随溶解态甲烷(CH4)浓度的异常偏高。初步分析推测，海水养殖和赤潮及藻华爆发等过程可能使该海域沉积物有机质含量大大增加，在夏季底层水体耗氧酸化影响下，可能导致沉积物产CH4源增强效应。同时水体层化使大量CH4蓄积于底层水体，降低了海-气CH4交换通量。待秋季水体层化和耗氧酸化消退，底层水蓄积的大量溶解态CH4将迅速进入表层水体和海表大气，导致秋季海-气CH4交换通量短期增强。本项目将通过针对性的现场观测和模拟实验，研究水体层化背景下，渤海西部底层水体耗氧酸化过程与沉积物产CH4效率和水体中CH4垂直分布新特征之间的关系，确定底层水体溶解态CH4的主要来源。进而分析耗氧酸化过程对沉积物-水柱剖面-海表大气间CH4源汇过程的影响机制，并定性和定量估算其对海-气CH4交换通量季节演变新常态的调控作用。

近几十年，受自然过程和人为活动等过程影响，渤海中西部底层水体呈逐年恶化的季节性耗氧现象，影响面积仅次于长江口低氧区，可能导致甲烷（CH4）源汇过程和浓度分布及海-气交换通量的时空演变新特征。基于本项目支持，通过搭载9次基金委共享航次和依托单位业务化监测航次，采用国内外广泛认可的光腔衰荡光谱分析法，顶空平衡-气相色谱法和Wanninkhof (2014) 改进的海-气交换通量估算方法，对上述科学问题开展了针对性观测研究。结果显示，观测期间，沉积物环境应是渤海耗氧海域水体中溶解态CH4的主要来源，海-气交换过程是溶解态CH4主要的汇。夏季渤海底层海水中溶解态CH4浓度异常上升和耗氧过程并无直接的因果关系，事实上，两者均应为人为活动和自然过程导致沉积物有机质含量逐年快速上升的结果，但耗氧过程可能在一定程度上减弱底层海水中溶解态CH4的有氧氧化消耗速率，进而导致夏季渤海耗氧海域溶解态CH4“源增强而汇减弱”的新特征。渤海耗氧海域表层海水中溶解态CH4浓度整体呈初秋（9月上旬）季节最高，冬季最低，其他季节居中的季节演变特征，但水柱剖面观测数据显示，溶解态CH4浓度垂直分布呈夏季差异最大，秋末和冬季垂直分布均匀的分布特征。上述空间分布特征主要调控因素包括季节性CH4 源汇过程，季节性水体层化和水温变化，海底地形及涡流导致的水动力环境等。其中，渤海耗氧海域底层海水中溶解态CH4的夏季蓄积和秋初快速释放现象应为一直存在的自然过程，但受人为活动对溶解态CH4源汇过程的影响，其蓄积和释放的时空演变过程可能表现出持续加剧的特征。本研究有助于深化认识人为活动影响下，渤海耗氧海域溶解态CH4源汇过程和浓度及海-气交换通量时空分布演变特征，并为优化渤海海洋开发利用政策提供基础信息支持。