三峡水库建设对长江下游颗粒有机碳通量及碳同位素组成的影响

Influence of the impoundment of the Three Gorges Reservoir on the flux and isotopic composition of particulate organic carbon in the lower Changjiang mainstream

2008年4月至2009年4月,在南通长江干流每周采集一次表层悬浮物样品,共51个,分析了其粒度、颗粒有机碳(POC)、颗粒氮(PN)含量以及有机碳同位素(δ13Corg)组成,研究了下游干流颗粒有机碳组成的季节性变化特征。悬浮物的平均粒径在一年内变化不明显,而POC和PN含量呈现洪季缓慢减小,枯季增加的趋势;δ13Corg值的变化正好相反,介于–24.1‰与–25.6‰之间,具有明显的季节性变化规律。POC/PN比值的季节性变化不显著。平均粒径与POC、PN及δ13Corg值均无显著相关性。颗粒有机碳的总通量及陆源有机碳的贡献具有明显的季节性变化,陆源有机质所占的比例呈现洪季高、枯季低的特点,同径流量和含沙量关系密切。与前人在长江南通段的研究结果比较揭示,三峡水库建设后长江下游干流颗粒有机质组成与入海陆源有机碳通量发生明显变化,其对河口及边缘海的生物地球化学循环的潜在影响值得关注。

A total of 51 water samples were collected weekly from the lower mainstream of the Changjiang River near Nantong City from April 2008 to April 2009. The grain size （Mz）, particulate organic carbon （POC）, particulate total nitrogen （PN） and organic carbon isotope ratios （δ^13Corg） in the suspended particulate matters （SPM） were measured. The mean grain size of the SPM exhibits small variation during the sampling period, the concentrations of POC and PN, however, changed considerably, showing lower values in flood season （May to October） and higher in dry season （November to next April）. The δ^13 shows a clear seasonal variation opposite to those of POC and PN, ranging from 24.1‰ to 25.6‰. The POC/PN ratio has no significant seasonal variability. Weak correlations exist between Mz and POC, PN and δ^13Corg. By using a simple two-end member model, we observed obvious seasonal changes in the flux of POC and the contribution of terrestrial organic carbon, which are higher in flood season and closely related to the water discharge and sediment loads. Compared with previous data obtained from the Changjiang River at Nantong, the compositions of total and terrestrial particulate organic carbon into the sea have significantly changed since the impoundment of the Three Gorges Reservoir. The potential influence of the Three Gorges Reservoir on ecological environment and biogeochemical cycle in the estuary and marginal seas deserves more research attentions.