海洋生态与环境科学重点实验室知识库

 河流—河口—近海连续体是陆源营养物质向海洋输送的重要纽带，也是陆—海界面碳循环的关键场所，例如我国的长江—河口系统。长江每年向河口地区输送大量的淡水及泥沙为东海（the East China Sea，ECS）有机质提供了重要的来源。但是，目前从长江流域到河口连续系统中关于有机质时空变化的综合研究却较为少见。尤其是近几十年来，人类活动对该生态系统的影响不断加强，三峡大坝（the Three Gorges Dam，TGD）的建设对河口生态系统碳循环也已产生一定的影响。基于此目的，本文对2009年来长江干流、长江口及其邻近海域共计13个航次的综合调查数据进行分析，测定了水体颗粒有机碳（POC）、颗粒有机氮（PON）、溶解有机碳（DOC）和沉积物总有机碳（TOC）、总氮（TN）及其碳、氮稳定同位素（δ¹³C、δ¹⁵N）和温度（T）、盐度（S）、悬浮体（TSM）、叶绿素a（Chl a）和沉积物粒度（Mz）等理化环境要素。研究长江—河口系统中有机质的时空分布特征，探讨影响该系统有机质时空变化的因素及其来源，解析水体有机质与环境要素之间的关系，估算近年来沉积物中有机质的沉降通量。获得主要结论如下：

1．2014年春季和秋季长江—河口系统中，POC和PON在干流具有相似的空间变化特征，均呈现由上游攀枝花到宜昌含量逐渐降低、而中下游含量均表现出上升的趋势，二者的季节变化趋势均不明显。在河口区域，POC和PON也都表现出明显的空间变化，春、秋季POC均呈由河口向外海、随离岸距离的增加而降低的趋势；PON则不同，还存在显著的季节变化特征，春季PON在外海区的含量高于近岸区，原因是春季外海区域的初级生产水平较近岸高，秋季PON和POC变化趋势一致。DOC在干流的空间变化不明显，秋季河流和近岸区域的DOC含量均显著高于春季，这可能与河流流量增加和水温升高有关。δ¹³C、δ¹⁵N三端元混合模型结果显示：春、秋季，干流中陆地来源的有机质均超过70%。河口区域陆源输入的比例仅占57%。春季，河口地区海洋自生来源所占比例高于人为输入来源，秋季这两种来源恰好相反。这一结果反映了长江口海域春季浮游植物的大量繁殖、初级生产力旺盛对有机质贡献较大。与世界其他主要大河相比，长江—河口系统中下游和河口的DOC、POC含量特别高。近年来，人类活动（修建TGD、富营养化）的增加是引起这种变化的重要驱动因素。

2. 2015年秋季长江口及其邻近海域的综合调查发现，表、底层水中POC和DOC的平面分布趋势相似，均表现为近岸海域含量高、随向海延伸含量逐渐降低的分布趋势。在垂直方向上，二者的分布却存在显著差异，POC表现为表层含量低、底层含量高；DOC则呈现表层含量高、底层含量低的变化趋势。通过有机质与水环境要素的相关性分析和线性拟合发现，TSM的吸附、解析及再悬浮等作用是影响POC空间分布的重要因素，S和COD均对POC和DOC有较大贡献，主要表现在海水对长江径流输送的陆源物质有稀释作用、这促进了POC向DOC的转化，同时，沿岸工厂和农业的排放的污废输入也影响了有机质的含量及分布。与春季不同的是，秋季浮游生物的初级生产作用较弱，对水体中有机质的贡献也较弱。

3. 2014年春、秋季长江口及其邻近海域的表层沉积物中：TOC和TN在这两个季节均表现出显著的空间差异性，都显示出近岸处含量高、随着离岸距离的增加而向逐渐向海降低的分布趋势，整个海域秋季TOC和TN的平均含量明显高于春季。δ¹³C二端元混合模型估算得到两个季节陆源有机碳所占的平均比例均超过50%；陆源输入有机质的比例也呈现由河口向外海降低的趋势。沉积物粒度、底层水体温度和Chl a的含量以及水动力作用是影响该海域表层沉积物中有机质空间变化的重要因素。

4. 2009～2019年春季长江口及其邻近海域表层沉积物中TOC的平均含量在2009年最高，为0.50±0.39%，近年来有逐渐下降的趋势；TN的平均含量变化不大，在0.04%与0.05%之间浮动。陆源输入有机质所占比例大都超过50%（仅2017年春季除外），且陆源输入有机质的分布也随离岸距离的增加而降低，这主要与长江冲淡水入海携带的大量颗粒物质有关；本研究中有机质的沉积通量呈先下降后缓慢增加而后又降低的趋势。近40年来，TOC含量经历了TGD蓄水前及蓄水初期较为稳定的阶段（在0.50%上下浮动）和蓄水10年后TOC含量降低的阶段（平均含量为0.33%）；TN的含量较蓄水前也有降低的趋势。由于TGD的修建，造成了近十几年来长江径流量和输沙量的下降、输送到河口的POC含量也随悬浮体浓度的降低而下降，POC的通量与沉积物中TOC的通量也大幅降低。

综上所述，本论文通过分析长江—河口系统水体及表层沉积物中颗粒态、溶解态和沉积态有机质的时空分布、影响因素及其来源变化，定量估算不同来源的有机质及长时间序列里总有机碳在河口的沉积通量。相关结果为研究流域—河口生态系统水体与沉积物之间的碳循环奠定了基础。 

The River-ocean continuum is an important link to the transformations of terrestrial input nutrients and a key place for the carbon cycle at the land-sea interface, such as the Yangtze River-Estuary continuum (YREC) in our country. The Yangtze River transports large amounts of freshwaters and sediments to the estuary every year, providing an important source of organic matter in the East China Sea (ECS). However, there are few comprehensive studies on the spatiotemporal variations of organic matter in the continuums from the Yangtze River basin to the estuary. Especially in recent decades, the impact of human activities on the ecosystem has been increasing, and the construction of the Three Gorges Dam (TGD) has also had some impacts on the carbon cycle of the estuary. To account for this, we analyzed the comprehensive survey data of 13 cruises in the Yangtze River mainstream and the Yangtze River estuary and its adjacent waters (YRE) in the past 10 years. We measured the particulate organic carbon (POC), particulate organic nitrogen (PON), and dissolved organic carbon (DOC), and surface sedimentary total organic carbon (TOC), total nitrogen (TN), and carbon, nitrogen stable isotopes (δ¹³C, δ¹⁵N), and temperature (T), salinity (S), Total suspended matter (TSM), chlorophyll a (Chl a), and sediment grain size (Mz), and other environmental physical and chemical factors. We studied spatial-temporal and distribution characteristics of organic matter in the YREC and explored the factors and sources that affect the spatiotemporal variations of organic matter in this system. We also analyzed the relationship between water organic matter and environmental factors and estimated the organic matter deposition flux in sediments in recent years. The main conclusions obtained are as follows:

1. In the YREC in spring and autumn of 2014, POC and PON have similar spatial variations in the mainstream. Both of them showed a gradual decrease in content from the upper reaches of Panzhihua to Yichang, while the POC and PON content in the middle and lower reaches showed an increasing trend. The seasonal changes were not obvious. In the YRE, POC and PON showed obvious spatial variations. POC showed a trend of decreasing from the estuary to the offshores as the distance increased in both spring and autumn; PON was different, and there were significant seasonal variations. In spring, PON contents in offshores were higher than that in nearshores. The reason is that the primary production in the offshores in spring was higher than that in the nearshores. The POC and PON variations in autumn were the same. The spatial variations of DOC in the mainstream were not obvious. DOC contents in the Yangtze River and nearshores in autumn were significantly higher than that in spring, which may be related to the increase in water discharge and surface water temperature. The results of the δ¹³C and δ¹⁵N three-terminal mixed model showed that in spring and autumn, the terrestrial input organic matter in the mainstream exceeded 70%. The proportion of allochthonous sources in the YRE only accounted for 57%. In spring, the proportion of autochthonous sources in the YRE was higher than that of human input (anthropogenic) sources. While in autumn, the two sources were exactly the opposite. This result reflected that plenty of phytoplankton reproduction and strong primary productivity in the YRE in spring had made a greater contribution to organic matter. Compared with other major large rivers in the world, DOC and POC contents in the lower rivers and estuary of the YREC were particularly high. Human activities (dams, eutrophication) causing variations in terrestrial inputs were likely an important factor causing this.

  2. In the Yangtze River estuary and its adjacent waters in autumn of 2015, POC and DOC had similar horizontal distributions on the surface and bottom waters, showing a trend of high values in nearshores and low values on offshores. While in the vertical, POC and DOC distributions were significantly different. POC on the surface water was low, and POC in the bottom water was high, while the DOC presents a trend of high values on the surface and low values on the bottom. Through the Pearson Correlation Analysis (PCA) of organic matter and water environment factors, it was found that the TSM adsorption, analysis, and resuspension were very important factors affecting POC spatial distributions. Both S and COD had a greater contribution to POC and DOC. The main manifestation is that seawater has a diluting effect on the terrestrial input matters transported by the Yangtze River, which promotes the conversion of POC to DOC. At the same time, the coastal factories and agricultural wastewater inputting also affected the content and distribution of organic matter. Different from spring, the phytoplankton primary production in autumn was weaker, and the contribution to the organic matter in water was also weaker.

3. In the Yangtze River estuary and its adjacent waters in spring and autumn of 2014, TOC and TN in the surface sediments showed significant spatial differences. Both of them showed a trend of gradually decreasing toward the sea. The average content of TOC and TN in autumn was obviously higher than that in spring. δ¹³C two-terminal mixed model was used to estimate the average proportion of terrestrial organic carbon in two seasons. Terrestrial organic carbon in both seasons was account for more than 50%; the proportion of terrestrial organic matter input also showed a trend of decreasing from the estuary to the offshores. Sediment grain size, temperature, and Chl a in the bottom, and hydrodynamic effects were important factors that affected the spatial-temporal variations of organic matter in the surface sediments of the Yangtze River estuary and its adjacent waters.

4. The average TOC contents in surface sediments of the Yangtze River estuary and its adjacent waters in spring from 2009 to 2019 was the highest in 2009, which was 0.50±0.39%. In recent years, TOC has been a gradual downward trend. TN average content had not changed much, fluctuating between 0.04% and 0.05%. The proportion of terrestrial input organic matter was more than 50% (except in spring 2017). And the terrestrial input organic matter distributions also decreased with the increase in the distance from the inshore, which was mainly related to the large amount of particulate matter carried by the Changjiang Diluted Water (CDW). The organic matter deposition flux showed a trend of first decreasing, then slowly increasing, and then decreasing. In the past 40 years, TOC contents had gone through a relatively stable stage before and at the initial stage of the Three Gorges Dam (around 0.50%), and then the TOC contents decreased after 10 years of water storage (average: 0.33%). TN contents also showed decreasing. Due to the construction of the Three Gorges Dam, the Yangtze River water discharge and sediment transport have decreased in the past ten years. POC contents transported to the estuary had also decreased with the decreasing of the suspended matter. The POC and TOC flux has decreased.

In conclusion, we analyzed the water and sedimentary organic matter spatial-temporal distributions, influencing factors, and sources in the YREC. We quantitatively estimated the organic matter from different sources and the deposition flux of TOC in the estuary in a long time period. Our results established the foundation for the study of the carbon cycle between waters and sediments in the YREC.