Variations in dissolved and particulate organic carbon dynamics in the lower Changjiang River on time scales from seasonal to decades

Abstract

Abstract. Time series water samples were collected monthly between November 2016 and June 2019 from the Lower Changjiang (Yangtze) River at the Xuliujing station, representing the end-member freshwater exported to the ocean, 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 separate the bulk DOC into four different size-fractions to further examine changes in the molecular weight distribution of DOC. The seasonal variations of POC were characterized with higher proportions of autochthonous components in summer and lower proportions in winter on the basis of POC contents (% of total suspended particulate matter), δ13C, δ15N, and POC/PN values. The size spectra of DOC also illustrated a distinct seasonal variation pattern, although the different responses to different seasons were also shown between HMW- (high-molecular-weight) and LMW- (low-molecular-weight) DOC compounds. The strong correlation of δ13C values between HMW-DOC and POC suggested their close relationships in terms of origin and composition. Together with available literature data, DOC concentrations, POC (%), δ13C, and δ15N all showed a significant increase over the past decades, likely resulting from increase in the proportion of autochthonous organic components owing to intensified human activities and global warming in the river basin.