Abstract:

Surface water samples were collected from seven cascade reservoirs across the mainstream Wujiang River and Hongfeng Reservoir on the tributary of Maotiao River, to measure the concentration of total suspended matter (TSM). Samples were analysed for elemental and stable carbon isotopic ($\delta ^{13}$C) composition and lignin phenols to investigate the variability in the spatial and seasonal distribution of particulate organic carbon (POC). The TSM in surface water varied greatly across the seasons. It was higher in the spring and summer than in autumn and winter. POC and lignin content ($\Sigma $8 and $\Lambda $8, respectively) in surface water showed high variability that were greatly influenced by season. POC in upstream waters was higher than in downstream waters in the autumn and winter, reflecting dam interception. $\Sigma $8 and $\Lambda $8 in the TSM samples exhibited a significant linear relationship, stronger in autumn and winter than in spring and summer. The $\delta ^{13}$C of TSM was significantly negative. This was potentially due to the more depleted $\delta ^{13}$C of terrestrial plant debris and freshwater plankton, that was likely to exist in surface water. Angiosperms were the dominant vegetation species reflected by lignin. There was a high diversity in the plant sources of terrestrial organic matter in surface particulate matter samples. The extent of degradation of TSM samples was relatively low, and aggravated degradation was observed in July and October, likely related to vigorous microbial activity.

Key words: Wujiang River, particulate organic carbon (POC), reservoir, spatial and seasonal distribution, lignin, cascade-damming