潮汐影响下长江口盐沼水体溶解有机质的来源及降解转化机理

Estuarine salt marshes are an important component in the coastal ecosystems and bury enormous quantities of carbon，therefore are among one of the most significant contributors to the global “blue carbon” pool. Due to the unique biogeographical and hydrological settings, very little is known about the processes controlling the source, fate as well as the transport and degradation of the dissolved organic matter (DOM) within the marsh-estuarine system. This information is highly valuable to evaluating the potential of carbon budget and therefore requires systematic research and quantitative evaluation. To this end, this project seeks to understand the biogeochemical processes of DOM in the aquatic phase in the salt marshes, using the marshes adjacent to the Yangtze River Estuary as a test bed. A comprehensive array of methods that combines field observations, laboratory incubations, various spectroscopy methods including optical and mass spectroscopy, will be used to investigate the dynamics of the sources and fate of DOM under the background of seasonality and different marsh vegetations during the tidal exchange. The ultimate goal is to better understand what regulates sources and fates of DOM as it is transported by the tides to the adjacent estuaries. Results from this work will advance our understanding of the salt marshes biogeochemical processes across a range of spatial and temporal scales and improve predictions of the coastal carbon cycle models.

作为海岸带生态系统碳循环一个重要组成部分，河口盐沼湿地拥有巨大的碳储量，是全球“蓝碳”碳汇的主要贡献者。由于独特的生物地理背景以及水文条件，水体溶解有机质在盐沼-河口界面的来源、迁移和转化过程十分复杂，仍缺乏系统的研究和量化的评估，因此成为制约碳收支研究的关键因素之一。本项目拟以长江口盐沼湿地为例，从盐沼水体溶解态有机质的生物地球化学过程入手，开展野外观测和实验室培养实验，利用分子光谱与高分辨率质谱相结合的研究手段，系统的描绘一个潮汐周期内盐沼水体溶解有机质在不同季节及盐沼植物群落背景下的来源与构成的动态变化特征，并以此为基础深入剖析潮汐作用下盐沼水体有机质的降解转化机制及控制因素，回答“滨海盐沼湿地水体溶解有机质的来源和最终归宿”这一科学问题。项目预期成果将有助于厘清潮汐影响下盐沼水体有机质的时空变化格局和影响机制，为完善海岸带碳循环模型提供科学支撑。

由于河口独特的水文条件，河口湿地水体中溶解态有机质在湿地-河口界面的来源、迁移和转化过程十分复杂，亟需系统的研究和量化的评估。本研究聚焦长江河口，以上海崇明西沙淡水湿地和东滩盐沼为研究对象，通过野外现场观测和实验室测试分析，刻画了潮汐影响下两处湿地水体溶解态有机质来源和组成的高动态变化过程。结果表明，在一个潮汐周期内，两处河口湿地均输出溶解态有机质，与涨潮时相比，落潮时水体具有更高的溶解态有机碳浓度、更强的紫外吸收系数、更复杂的分子结构，以及更高的荧光腐殖质含量。降解培养实验结果表明，河口湿地输出的有机质在向河口迁移的过程中，光化学降解占主导，且光化学降解可以促进水体微生物对湿地溶解态有机质的利用，而单独的微生物对溶解态有机质的降解作用甚微。在崇明东滩盐沼附近河口区域开展了水体生物-光学性质的观测，结合Landsat-8/9、Sentinel-2A/B等卫星遥感数据，结果表明在大河背景下，由于受大河径流稀释作用的强烈影响，河口湿地输出的有机质在湿地-河口界面的梯度变化并不显著，暗示了盐沼伴随潮汐过程侧向输出的有机质对于河口碳循环的影响有待进一步评估。