Abstract: To understand the impacts of human activities on organic carbon content in the Yangtze River mainstream, we analyzed the spatial and temporal variations of organic carbon content in the Yangtze River mainstream by collecting data from 1980 to 2019. Nonparametric analysis, variation partitioning analysis, and hierarchical partitioning were used to investigate the effects of anthropogenic disturbance and green investment. The results showed an increasing trend of dissolved organic carbon (DOC) concentration in the Yangtze River mainstream over the past 40 years, but a decreasing and then increasing trend of particulate organic carbon (POC) content. The spatial variations of DOC and POC contents over the river were significant during the Three Gorges Dam impoundment and operation periods, while the spatial variations were not significant during pre-Three Gorges Dam impoundment period. DOC transport fluxes generally increased from 2003 to 2019, while POC showed a fluctuating downward trend. The average total organic carbon flux from 2003 to 2019 was 3.66 Tg C·a^-1. Human activities and green investments contributed more organic carbon content during the wet season, which accounted for 66.33% and 44.00% of the variations in DOC and POC, respectively. Investment in industrial and domestic wastewater treatments had a greater effect on DOC and POC in the river during the dry season, whereas forestry investment contributed more on DOC in the wet season.

Key words: human activity, environmental investment, dissolved organic carbon, particulate organic carbon, spatiotemporal variation, Three Gorges Dam