利用稳定同位素研究珠江口溶解氧的源汇过程和季节性缺氧的形成机制

Dissolved oxygen (DO) is probably the most important parameter related to water quality and biological habitat in aquatic environments. Thus, it is critical to understand how the oxygen sources and sinks operate in aquatic systems. The stable isotopes of dissolved oxygen (δ18O) offer a unique perspective on changes in the oxygen cycle in aquatic systems. This tracer does not readily undergo isotopic exchange with water or other gases, and so isotopic fractionation should mainly result from the dissolution of atmospheric gases, respiration, photosynthesis, or oxidation-reduction processes. Low oxygen bottom waters and severe hypoxic events have been reported in the Pearl River Estuary and adjacent coastal waters (PRE) during summer. Previous studies have shown that hypoxia appeared in several specific areas of the PRE from year to year,and large areas of hypoxia in the whole estuary are lack,despite the nutrients and organic matter sustained at a high level. The reason why this is so is not fully understand, although it may closely related to the oxygen cycles and biogeochemical processes. To address this problem, we measured the concentration and the stable isotope ratios of dissolved oxygen in the water column of the PRE to identify the dominant factors controlling the fractionation factor for respiration, to determine the relative importantce of water-column versus benthic respiration to the depletion of oxygen in bottom waters, to assess the importance of oxygen sources and sinks, and to understand the formation mechanisms of the summer hypoxia and the lack of large areas of hypoxia in the Pearl River Estuary.

溶解氧（DO）的稳定同位素组成（δ18O）是水生系统DO生物地球化学循环研究的有效示踪剂。最近研究证实溶解氧同位素能够很好地反映海洋环境中更为详细的溶解氧周转速率等信息。然而，通过溶解氧同位素研究我国河口或近海海域DO的循环过程特别是氧的亏损过程还未见报道。珠江口及邻近海域存在着季节性的海水缺氧现象。珠江口缺氧现象在不同年份通常出现在若干特定海域，并且整个河口范围的缺氧通常难以出现。然而，关于这一现象的原因尚不十分清楚，可能与河口湾的溶解氧循环机制和过程密切相关。本项目拟利用溶解氧稳定同位素示踪技术，并且通过海上现场调查和培养实验，研究水体耗氧过程中18O同位素效应的关键调控因子，弄清水柱耗氧和沉积物耗氧在河口季节性底层缺氧形成过程中的相对贡献；并且结合水团来源和生源要素分析，研究阻止珠江口缺氧现象大面积发展的主要机制；期望能够初步阐明珠江口溶解氧的源汇过程和季节性缺氧区的现状及控制机制。

本研究以珠江口为主要研究区域，主要研究溶解氧及有关生源要素的稳定同位素（C、N、O）组成，探讨水体耗氧过程的关键调控因子，探讨水柱耗氧和沉积物耗氧在河口季节性底层缺氧形成过程中的相对贡献，以及阻止大范围缺氧发生的主要机制。项目执行期间，组织实施了4次珠江口采样航次，获取大量的水体及颗粒物和现场培养实验数据。取得以下重要成果：（1）由温盐垂向梯度（由珠江径流主导）引起的水体分层作用和颗粒有机物降解是引起珠江口水体溶解氧亏损的关键过程；（2）水柱耗氧是虎门上游和河口外部氧消耗的主要过程，沉积物耗氧的相对贡献有限；在河口中游，沉积物耗氧主导着底层水体溶解氧消耗，其贡献比例在60%以上；（3）结合水文和生源要素及稳定同位素分析，发现有限的水体分层和易降解颗粒有机质溶解氧消耗限制是珠江口缺氧无法大范围形成的主要原因。