Dissolved and particulate organic carbon dynamics in the lower Changjiang River on timescales from seasonal to decades: Response to climate and human impacts

Knowledge about current status and decadal evolution of organic matter regimes in large river systems, under the combined effects of intensive anthropogenic activities and global climate change, is of fundamental importance. To address these issues, time-series water samples were collected between November 2016 and June 2019 from the lower Changjiang (Yangtze) River at Xuliujing station (representing the end-member river water), in addition to monthly samples from November 2019 to January 2020 during an extreme drought event. Concentrations of dissolved (DOC) and particulate organic carbon (POC) as well as the stable isotopic composition of POC and PN (particulate nitrogen) were measured to elucidate their seasonal variations. Ultrafiltration was also conducted for selected samples to fractionate the bulk DOC into four different size-fractions to examine changes in the mo-lecular weight distribution of DOC. The seasonal variations of POC were characterized with higher proportions of autochthonous components in summer but lower in winter, as manifested in POC contents (in %), delta 13C, delta 15N, and POC/PN values. The size distribution of DOC also illustrated a distinct seasonal variation pattern, showing different responses between the >1 kDa HMW-(high-molecular-weight) and LMW-(low-molecular-weight) DOC fractions. Together with the available literature data, DOC concentrations, POC (%), delta 13C, and delta 15N all showed a significant increase over the past decades, likely resulting from the fact that the Changjiang River has changed from a "primitive" (with more allochthonous contribution from river basin) to a "civilized" river system (with more autochthonous contribution from in situ biological processes), as a response to intensive changes, such as damming, land use, and warming in the river basin. These new decadal evolution trends of POC and DOC regimes in the lower Changjiang River would not have been revealed without decadal scale observations.