| 引用本文: | 朱卓毅,张 经,吴 莹,张莹莹,林 晶,季 倩.长江口外颗粒有机碳及光合色素在不同溶氧水平下早期降解速率研究.海洋与湖沼,2013,44(1):1-8. |
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| 长江口外颗粒有机碳及光合色素在不同溶氧水平下早期降解速率研究 |
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朱卓毅, 张 经, 吴 莹, 张莹莹, 林 晶, 季 倩
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华东师范大学 河口海岸学国家重点实验室
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| 摘要: |
| 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(Chl a)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。 |
| 关键词: 低氧, 颗粒有机碳, 光和色素, 长江口, 降解实验, 耗氧速率 |
| DOI：10.11693/hyhz201301001001 |
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| 基金项目:国家重点基础研究发展计划(973)项目, 2006CB400601 号; 上海市科委重大基础研究项目, 07DJ14003 号 |
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| EARLY DEGRADATION RATE PARTICULATE ORGANIC CARBON AND PHYTOPLANKTON PIGMENTS UNDER DIFFERENT DISSOLVED OXYGEN LEVEL OFF THE CHANGJIANG (YANGTZE) RIVER ESTUARY |
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ZHU Zhuo-Yi, ZHANG Jing, WU Ying, ZHANG Ying-Ying, LIN Jing, JI Qian
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State Key Laboratory of Estuarine and Coastal Research, East China Normal University
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| Abstract: |
| Organic matter decay plays a key role in hypoxia mechanism. Studies indicate that hypoxia off the Changjiang 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/(m3·d) under low DO condition, only one fifth of that under high DO condition. With respect to the phytoplankton pigments, fucoxanthin (FUCO) and chlorophyll a(Chl a) were the main pigments detected in the experiment. Both FUCO and Chl 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 Chl a data in the 9-days degradation experiment (FUCO:r2>0.67, Chla a: r2>0.93). Constant k for FUCO and Chl 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/(m2·d)(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. |
| Key words: Hypoxia, POC, Pigment, The Changjiang estuary, Degradation experiment, Oxygen consumption rate |
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