黑潮东海分支硝化过程对东海低氧区的影响

Dissolved oxygen (DO) in seawater is the crucial factor for marine lives, while the hypoxia appeared in the East China Sea (ECS) seriously threaten the marine production. Traditionally, the eutrophication resulted from human activities is believed to take the main responsibility for the hypoxia. However, according to recent research, the Kuroshio, a current originates from the Western Pacific Ocean, has a branch intruding into the ECS continental shelf. The spatial and temporal locations of Kuroshio Branch Current (KBC) and the ECS hypoxia were highly coincident. The nitrification process was observed in the KBC, which can consume oxygen and reduce DO. It was likely to be attributed for the formation of ECS hypoxia. Therefore, the proposal of this project aims to further confirm such hypothesis through the qualitative and quantified investigation. Based on prior research, the spatial and temporal variations of nitrification in KBC would be described by the in situ investigations. The rates and controlling factors of nitrification would be studied by the field incubation experiments. Through comparing with the influence of human activities, contributions of nitrification in KBC were to be explained to the ECS hypoxia. This project could broaden our sight on how ocean intrusion affects the coastal environment through the key process of nitrogen cycles, and promote current understanding on the mechanism of ECS hypoxia.

海水中的溶解氧（DO）是海洋生命的基础，而我国东海出现的低氧现象（Hypoxia）则对海洋生产活动造成极大威胁。传统观点认为，人类活动引起的近海富营养化可能是造成低氧现象的原因。然而近年研究发现，来自西太平洋的黑潮水存在入侵东海陆架的分支，与东海低氧区的时空位置一致，且该水团内部存在持续的硝化过程，消耗氧气造成DO浓度降低，可能是影响东海低氧区形成的重要原因之一。为了证实这一假设，本项目拟从定性和定量的角度研究黑潮分支硝化过程对东海低氧的影响。在前期研究的基础上，通过现场调查系统阐述黑潮分支水团内硝化过程的时空变化特征，探寻硝化过程对低氧区产生的直接证据，并借助现场培养实验量化该过程对DO的消耗及其关键影响因素。通过与人类活动的影响做比较，阐述黑潮分支硝化过程对东海低氧区的贡献。从氮循环关键过程的角度，揭示大洋入侵对近海生态环境的影响，为探索我国东海低氧区的形成机制提供科学依据。

黑潮次表层水可入侵我国东海陆架，为东海低氧区的形成提供盐度跃层，然而其内部化学过程对低氧区的影响尚不清楚。本项目从氮循环的角度出发，通过多个现场航次调查、培养实验等研究方法，借助硝酸盐氮、氧稳定同位素等先进技术手段，较为系统地研究了黑潮次表层水团在东海陆架的分布特点，辨析了水团内部硝化、矿化等关键氮循环过程的时空特征与溶氧（DO）消耗作用，追踪了引起该过程的物质来源并量化了对溶氧消耗的贡献，阐明了其对近海低氧的影响。. 研究结果显示，当北赤道流在西太平洋转为黑潮时，次表层水团并未发生明显的耗氧过程，而是以南海水团对硝酸盐的物理补充过程为主。当该水团入侵至东海陆架后，其内部发生持续的有机质矿化耗氧过程，为近岸低氧区提供低溶解氧背景。在春季东海南部区域，主要反应为内源有机质再矿化过程；而在北部近岸海域，内源有机质的再矿化和外源有机质的沉降分解形成了近乎等效的溶氧消耗。在此基础上，受黑潮影响的长江口南部低氧区浮游植物沉降速率明显高于北部低氧区，同时底部沉积物的耗氧强于其他区域。上述过程进一步消耗溶解氧，最终导致低氧区的产生。研究形成了开阔大洋-陆架近海-河口近岸的完整证据链条，扩展了近海低氧区形成机制的认知，为近海生态环境安全研究提供了科学依据。