Organic matter decay plays a key role in hypoxia mechanism. Studies indicate that hypoxia off the Changji-ang estuary has been deteriorating in the past a few decades, but few study has focused on the dissolved oxygen consumption rate based on in situ organic matter degradation. In this study, an organic matter degradation experiment off the Changjiang estuary was performed to shed light on the mechanism of the occurrence of hypoxia. The experiment was carried out under different dissolved oxygen (DO) levels, namely high DO (DO>95%) and low DO (DO<50%) conditions, respectively. In situ phytoplankton was collected with a phytoplankton net and was added into the near-bottom sea water. The whole experiment was carried out in dark to prevent possible photosynthesis. The collected phytoplankton and near-bottom sea water was filtered through 200 μm mesh immediately after collection to remove possible zooplankton. The experiment last for 9 days and subsamples were collected during the whole experiment in interval of 1 day. System under high DO condition showed higher pH than low DO system and under both DO conditions, pH generally decreased from 8.13 to 8.03 (DO>95%) or 8.22 to 8.16 (DO<50%). Based on flow cytometry analysis, bacteria and virus increased during the 9 days and phytoplankton decreased dramatically. Particulate organic carbon (POC) concentration showed obvious decrease in high DO condition system and the POC degradation rate was strongly influenced by system's DO level. POC decay rate was 1.5 mmol/(m3-d) under high DO condition, and 0.3 mmol/(m3d) under low DO condition, only one fifth of that under high DO condition. With respect to the phytoplankton pigments, fucoxanthin (FUCO) and chlorophyll a (Chi a) were the main pigments detected in the experiment. Both FUCO and Chi a decreased rapidly during the experiment. For example, up to 80% of fucoxanthin thoroughly degraded in the 9 days. Exponential equation fits well with both FUCO and Chi a data in the 9-days degradation experiment (FUCO: r2>0.67, Chla a: r2>0.93). Constant k for FUCO and Chi a ranged from 0.18 to 0.22 and it was slightly higher under high DO condition, indicating that pigments degrades faster under high DO condition. Based on the constant k, the half life of chlorophyll a can be calculated, which is around 3-4 days. Further, based on the reported POC/Chl a ratio, percentage of degraded phytoplankton carbon in the bulk degraded POC can be calculated. Under high DO condition, only 6%-8.5% of the bulk degraded POC was phytoplankton carbon, while under low DO condition, 20%-28% of the bulk degraded POC was phytoplankton carbon. Thus, phytoplankton tends to contribute more as an organic matter source for degradation under low DO conditions on a short time scale. According to the POC degradation rate, organic-matter-degradation-related DO consumption rate off the Changjiang estuary can then be estimated, which was 82-91 mmol/(m2d)(DO>95%)and 27-30 mmol/(m2·d)(DO<50%), respectively. The rate is much higher than reported DO flux in the deep ocean, or reported DO consumption rate in the upper 50 m layer of the South China Sea. According to the actual initial DO level of the Changjiang Estuary and adjacent area in winter, it can then further be estimated that it would take 50-150 days to develop hypoxia after stratification prevails off the Changjiang estuary. It is notable that stratification generally begins during April and May and severe hypoxia usually happens in August. Besides organic matter degradation, there is some other process that consuming oxygen as well, such as respiration. To reveal the whole mechanism of the occurrence of hypoxia off the Changjiang estuary, further study is needed.%有机质降解是低氧形成的重要原因.长江口外的低氧近几十年来日趋恶化,引起了大量学者关注,但现场有机质降解及相关的耗氧速率还鲜见报道.本研究通过人工控制体系溶氧(DO)水平,于2006年10月在曾经出现低氧的长江口外近海进行现场有机质降解培养实验,以模拟有机质在沉降进入近底层水后早期(9天)内的生物地球化学过程,估算有机碳降解速率和耗氧速率.培养实验表明,有机质在几天内即可出现明显的降解,并伴随体系pH的下降和细菌、病毒的增加.体系溶氧对有机碳(POC)降解速率影响较大,POC在DO＞95％条件下的降解速率是DO＜50％条件下的5倍,分别为1.5 mmol/(m3·d)和0.3 mmol/(m3·d).光合色素岩藻黄素(FUCO)和叶绿素a(Chla)含量在9天内呈指数下降,其中80％的FUCO在9天内发生了降解.指数方程能较好地实现对观测数据的拟合,结果表明Chl a的半衰期为3-4天.不同溶氧条件下的结果表明低溶氧条件(DO＜50％)下浮游植物更容易成为降解有机质的来源.根据POC降解速率估算得到长江口外溶氧消耗速率分别为82-91 mmol/(m2·d)(DO＞95％条件)和27-30 mmol/(m2·d)(DO＜50％条件),远高于深海氧通量和表层50 m南海的氧消耗通量;以此耗氧速率为基础,估算得到长江口外层化形成至发育出低氧的时间为50-150天.
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