Large world rivers and their dissolved organic matter (DOM) fluxes could regulate ecosystem function and biogeochemical processes in coastal marine environments. Knowledge about the seasonal variations in composition and molecular size of DOM remains scarce but is important to a better understanding of the role of river fluxes. Monthly time-series (July 2018 to June 2019) surface water samples were collected from the lower Changjiang (Yangtze) River, one of the largest world rivers, for the measurements of both dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) to elucidate seasonal changes in their abundance, optical properties, and molecular size distributions among the <1 kDa, 1–3 kDa, 3–10 kDa, and 10 kDa–0.7 μm size fractions. On average, the <1 kDa low-molecular weight DOM made up about two thirds of the bulk CDOM, leaving the other one third in the 1 kDa–0.7 μm size fraction. Optical properties and molecular size of DOM transported by the Changjiang River had an evident seasonality. In general, DOM exported from the Changjiang River can be characterized as lower DOC, lower aromaticity, and lower colloidal abundance compared to other world rivers. Together with available literature data, our results show that world rivers with higher/lower DOC concentrations and higher/lower aromaticity also contain more/less colloidal (>1 kDa) DOM. Our year-long time-series study provides additional valuable baseline data for a better understanding of seasonal changes in the abundance, composition, and molecular weight distribution of DOM from the lower Changjiang River and the potential impacts on the biogeochemical cycling of DOM in the adjacent estuarine, coastal, and shelf areas.

中文翻译：

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长江下游发色溶解有机物分子大小的季节性变化

世界大河及其溶解有机质 (DOM) 通量可以调节沿海海洋环境中的生态系统功能和生物地球化学过程。关于 DOM 组成和分子大小的季节性变化的知识仍然很少，但对于更好地理解河流通量的作用很重要。每月时间序列（2018 年 7 月至 2019 年 6 月）地表水样本来自世界最大河流之一的长江下游（长江），用于测量溶解有机碳 (DOC) 和发色溶解有机质 (CDOM) ) 以阐明 <1 kDa、1–3 kDa、3–10 kDa 和 10 kDa–0.7 μm 大小部分的丰度、光学特性和分子大小分布的季节性变化。平均而言，< 1 kDa 低分子量 DOM 约占整体 CDOM 的三分之二，其余三分之一在 1 kDa–0.7 μm 尺寸部分。长江输送的DOM的光学性质和分子大小具有明显的季节性。总的来说，与世界其他河流相比，长江输出的 DOM 具有较低的 DOC、较低的芳香性和较低的胶体丰度。结合现有文献数据，我们的结果表明，具有较高/较低 DOC 浓度和较高/较低芳香度的世界河流也含有更多/更少胶体 (>1 kDa) DOM。我们长达一年的时间序列研究提供了额外的有价值的基线数据，以更好地了解丰度、成分、