南海北部最小含氧带水下滑翔机观测结果初步分析*

最小含氧带(Oxygen Minimum Zone, OMZ)是指海洋中层水体处存在的稳定的溶解氧(Dissolved Oxygen, DO)极小值层, OMZ的分布与变化对南海生态系统和生地化循环具有重要意义。本文利用2019年7—9月“海翼”号水下滑翔机(Sea-wing Glider)在南海北部陆坡区的组网观测数据, 对南海北部陆坡区OMZ的空间分布特征进行了分析。结果显示, 在垂向上, 研究区域内DO极小值层出现在深度约700~900m处, 其浓度约为80~100μmol·L^-1, 且700~900m深度范围内DO浓度变化不大, 形成了厚度约为200m的OMZ。在水平方向上, OMZ自陆坡西南部起向东北方向延伸, 厚度由西南至东北逐渐变薄, 整体呈楔形分布, 并在靠近吕宋海峡处逐渐消失。此外, 本研究还选取了两台水下滑翔机7—8月连续两周内的观测数据, 经计算显示, OMZ区域内的DO浓度在跨陆坡方向上的平均变化速率为0.023μmol·L^-1·d^-1(增加), 在沿陆坡方向上为-0.034μmol·L^-1·d^-1(减少)。沿吕宋海峡入侵南海的高氧水能够解释OMZ东北部DO浓度局部升高的现象, OMZ的分布特征和形成原因与海水的平流运动、水团分布和水体层化等物理过程, 以及生物呼吸、有机物分解和还原性物质的氧化等多种影响因素有关。

Underwater glider observation of oxygen minimum zone in the northern South China Sea

The Oxygen Minimum Zone (OMZ) is a stable dissolved oxygen (DO) minimum layer that forms in the mid-ocean water (200~1000 m). High-resolution surveys of DO in the water columns near the continental slope of the northern South China Sea (NSCS) were performed during July-September 2019 using an observation network including seven Sea-Wing Gliders. Our results revealed the existence of a stable basin-scale OMZ in the middle layer of the NSCS. Our data indicated that the depth of the OMZ was about 700 m with minimal DO content of about 80-100 μmol·L^-1. Besides, the DO content remains relatively stable within the depth range of 700-900 m, with an averaged OMZ thickness of about 200 m. The OMZ generally shows a wedge-shaped distribution in the horizontal direction with a gradual decrease of OMZ thickness and intensity from the southwest to the northeast along the continental slope of the NSCS. The OMZ eventually disappears near the Luzon Strait. In addition, we used data of two sections near the Xisha regions with repeated glider surveys to estimate the temporal change of OMZ over time. Our results suggested that the DO concentration within the OMZ core increased by ~0.023 μmol·L^-1 per day in the cross-slope direction but decreased by ~0.034 μmol·L^-1 per day in the along-slope direction. The increase of DO content in the northeastern OMZ can be explained by the intrusion of high-oxygen water through the Luzon Strait. Based on the above observation results, we believe that the distribution and formation of the OMZ be affected by physical processes such as advection, water mass distribution and stratification; it is also related to biological respiratory, organic decomposition, oxidation of reducing substances, and other factors.