The biogenic elements retention in reservoirs of the Yangtze River basin and effect on the nutrient flux into the sea

Abstract

The Yangtze River catchment is among the most human-impacted basins globally, with a population of approximately 590 million and over 60,000 dams constructed by 2020. While the effect of reservoir dams on runoff and sediment has been extensively studied, assessing the impact of biogenic element retention in reservoirs on nutrient flux into the sea remains a challenge. This study analyzed the total organic carbon (TOC), total organic nitrogen (TN), biogenic silica (BSi), grain size, and 210Pb chronology of sediment cores collected from the Pengshui, Wuqiangxi, Three Gorges, and Danjiangkou reservoirs. The analysis was combined with the database of annual reservoir area and nutrient flux into the sea of the Yangtze River. The results showed that the average annual sedimentary retention of TOC, TN, and BSi in the four reservoirs increased by 16%, 18%, and 26% after dam construction and rapid eutrophication. The total water area of large reservoirs increased from 30 to 132,000 km2 between 1960 and 2016, resulting in total accumulated retention increments of TOC, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphate (DIP), and dissolved silicon (DSi) in reservoirs reaching 27, 3.5, 0.1, and 27.2 million tons, respectively. The retention increments accounted for 20.6%, 7.0%, 8.8%, and 12.1% of the total flux of TOC, DIN, DIP, and DSi into the sea during the same period, respectively. The retention of TOC and DSi in reservoirs led to a decrease in their annual flux into the sea by 67.2% and 61.4% in 2016 compared to 1960. However, the retention of DIN and DIP increased their annual flux by 6.4-fold and 4.2-fold in 2016 compared to 1960, which was attributed to the substantial emission of fertilizer and sewage in the downstream catchment area of dams. These findings provide valuable insights into biogenic element retention in reservoir catchments and their impact on nutrient flux into the sea.