台风后长江口水柱POC来源甄别及其对缺氧的贡献

Hypoxia has great impact on the ecosystem and environment of the Changjiang Estuary, and the condition became worse. Mooring observations revealed that typhoons destroy hypoxia firstly, but then rebuild and exacerbate hypoxia condition. Such phenomenon was probably related to tremendous input of biological particulate organic carbon , terrestrial POC and POC from sediment resuspension. However, the POC flux and sources after typhoon, as well as their contributions to hypoxia in the Changjiang Estuary were unclear. In our proposal, we will use floating sediment trap to achieve POC flux data. End-member mixing model based on C/N isotope and biomarker will be used for POC source evolutions before and 1-2 weeks after typhoon in the Changjiang Estuary. Furthermore, mooring observations will be incorporated to reveal the bottom oxygen variations, and to explore the contributions of three POC sources to hypoxia in the Changjiang Estuary after typhoon. Our proposal will contribute to the understanding of coastal hypoxia variations during extreme weather, and lay a scientific foundation for the forecast of hypoxia.

底层缺氧问题严重影响长江口生态环境，并且已呈日益加重的趋势。全水柱锚系观测结果表明：台风等大风事件既可以破坏底层缺氧，也可以重建并形成比原先更严重的缺氧。该现象可能与台风后生源颗粒有机碳（POC）、陆源POC以及再悬浮POC沉降的变化有关，但目前对于台风后长江口POC通量和来源的变化，及其是否驱动台风后的缺氧现象并不十分清楚。本项目拟以时间序列漂流式沉积物捕获器为主要观测手段，分析计算台风前后1-3周长江口的POC沉降通量；通过颗粒有机碳氮同位素、生物标志物分析，构建颗粒物端元混合模型估算不同来源POC对沉降通量的贡献，甄别台风前后1-3周长江口缺氧区水柱POC主要来源及其连续变化；结合缺氧锚系连续观测，了解台风后长江口底层溶解氧连续变化，进而评估台风前后生源及陆源POC对长江口缺氧不同发展阶段的贡献，以期加深对极端天气条件下近海缺氧消失和重建过程的认识，为生态灾害的预警提供科学依据。

长江口缺氧问题严重影响长江口生态环境问题，而台风对长江口缺氧动态变化有重要影响，本项目旨在研究台风前后POC来源变化及其对缺氧的影响。在项目资助期内共参加了6个长江口大面航次及3个夏季长江口锚系观测，获取了2020年台风“巴威”和2021年台风“奥麦斯”前后POC样品及底层溶解氧的时空变化，深入研究了台风后长江口颗粒物来源及缺氧动态，航次观测结果表明2020年台风“巴威”后缺氧核心区面积从约5000平方公里增加至约13000平方公里，航次期间观测到海水溶解氧最小值仅为0.65mg/L。锚系观测结果与大面航次结果一致，台风后三天底层溶解氧开始快速降低，从3mg/L左右降低至1.5mg/L。航次期间观测到缺氧水体上层溶解氧过饱和、叶绿素a超过15μg/L的现象，POC的碳同位素结果表明台风“巴威”前缺氧区表层和底层POC主要为海源POC，上述结果表明新鲜的海源有机质分解可能是台风后底层缺氧加重的重要原因。另外，台风在破坏底层缺氧的同时，将底层高碳缺氧水混合至上层，可向大气释放大量二氧化碳，强烈影响缺氧区海-气碳源汇。项目执行期间，共发表文章5篇，其中第一作者SCI文章2篇，国内核心期刊3篇，合著国际专著一章，授权专利1个，合作培养硕士研究生两名，完成了预期目标。