夏季长江口缺氧和酸化在线监测及其耦合和非耦合的成因机制研究

Algal bloom, hypoxia and acidification in coastal areas are often concurrent, and they are also closely associated with eutrophication. However, ocean acidification in the Changjiang Estuary is poorly studied when compared with hypoxia. Our previous investigations indicated that the mixing of seawater exacerbates bottom hypoxia and acidification. Due to a lack of process study, however, little is known about the coupling and decoupling of hypoxia and acidification. We propose three hypotheses to address the scientific questions: (1) offshore subsurface water would mix onto the shelf surface, relieve the phosphate limitation of surface water, and thus fuel algal bloom. Subsequent respiration of these newly-formed organic matters will exacerbate the bottom hypoxia; (2) Due to the buffering capacity of CO2, the successive cycles of mixing, algal bloom, and respiration of the settled particulate organic carbon will likely lead to decreasing pH in bottom water; (3) Acidification and hypoxia generally couples during the early stage of hypoxia, however, the decoupling of these processes will be more pronounced at the end of the process. To test the hypotheses, we will apply buoy and mooring observation, vessel-based investigation, vessel and laboratory-based incubation experiments to understand these biogeochemical processes. Our proposal will contribute to the understanding of the coupling and decoupling of multiple ecological disasters, and lay a scientific foundation for the forecast and management of ecological disasters.

近海藻华、缺氧和海洋酸化往往是相互伴生的，且与富营养化密切相关。目前长江口缺氧已有不少成果，但海水酸化研究还很薄弱。观测资料表明，夏季台风引起的海水混合可以导致底层海水缺氧和酸化加重。然而，由于缺少针对性的过程观测，对于两者耦合和非耦合过程和机制并不清楚。本研究提出以下三个科学假说：（1）陆架次表层水混合到表层，缓解冲淡水的磷限制，从而产生更严重的藻华，有更多的有机质消耗底层氧，使底层缺氧加重；（2）由于无机碳体系较强的“记忆效应”，经混合－藻华－有机质沉降分解这几个过程之间的持续循环，可造成海水酸化加剧；（3）初夏底层缺氧形成早期，酸化和缺氧可能大体是耦合的，但缺氧发展后期两者的不耦合会更加显著。拟通过浮标锚系连续在线过程观测、船基调查和观测、船基和室内受控生态实验等手段来验证上述科学假设，以期加深我们对多种生态灾害耦合和非耦合过程的认识，为预警和防治此类灾害提供科学依据。

近海藻华－底层低氧－底层酸化往往是相互伴生的，与富营养化密切相关。目前，对于近海低氧酸化耦合与非耦合的生物地球化学成因机制尚没有很好地了解。本项目在已有在线观测系统基础上，建立了能同步观测上层富营养化与藻华和底层低氧酸化的浮标锚系观测系统，结合船基和岸基航次调查，解析夏季长江口低氧和酸化形成的主要生物地球化学过程及调控机制；查明台风和ENSO使低氧和酸化的加重的内在机制，建立低氧和酸化概念模型。本项目共开展了7个大面航次，包括针对台风后缺氧动态变化的加强观测航次，获取了大量长江口海域水体基础化学参数及缺氧酸化关键参数。建立了可以实时传输的近海缺氧和酸化监测系统，获取了长江口缺氧区表层及底层缺氧酸化参数的连续观测数据。结合船基和长江观测数据，深入研究了调控长江口低氧和酸化主要的生物地球化学机制。观测研究证实长江口缺氧区，上层海洋富营养化、藻华与底层海水缺氧、酸化耦合发生。发现冲淡水扩散可引发硅藻藻华和夜光藻赤潮伴随发生，形成大量快速沉降的有机质，为底层海水耗氧及释放CO2提供了物质基础。在项目执行期多数年份，受低盐冲淡水影响，长江口外东北海域由于层化加强和营养盐补充，均观测到大面积缺氧酸化水体。藻华和低氧发生受冲淡水营养盐动态变化的显著影响。当台风正面经过缺氧区时，在破坏底层缺氧的同时，将底层高碳、低pH缺氧水混合至上层，并释放大量二氧化碳。但没有正面经过缺氧区的台风，如“巴威”和“美莎克”，并没有破坏长江口缺氧。经过计算，藻华沉降有机质分解、水体停留时间及层化对缺氧变化的影响分别为34.5%、55.3%和5.7%。研究发现，长江口特别严重的缺氧酸化现象与ENSO等极端气候事件密切相关，长江输入营养盐，水体层化指数，冲淡水影响范围是缺氧酸化监测预测的关键因子。据此，我们提出的缺氧酸化预警监测建议已经得到自然资源部预警司和浙江省自然资源厅下属有关业务单位的采纳。