Impact of extreme drought and the Three Gorges Dam on transport of particulate terrestrial organic carbon in the Changjiang (Yangtze) River

To investigate the impact of extreme drought and the Three Gorges Dam on organic carbon (OC) exported by the Changjiang (Yangtze) River, suspended particulate matter samples collected in 2003 (normal year) and 2006 (drought year) from the middle and lower reaches of the mainstream of the river were analyzed for elemental carbon, stable carbon isotope, and lignin-phenol yields. In comparison with suspended particulate matter samples collected in 2003, the 2006 samples showed elevated organic carbon content [OC%: (1.91 ± 0.43)% and (0.95 ± 0.07)% respectively in 2006 and 2003], higher autochthonous OC [Chl-a/OC: (20.68 ± 9.03)mg/g OC and (7.33 ± 2.97)mg/g OC in 2006 and 2003], decreased lignin yields [δ8: (0.90 ± 0.41)mg/100mg OC and (1.91 ± 0.33)mg/100mg OC in 2006 and 2003], and higher degradation state [(Ad/Al)v: 0.44 ± 0.21 and 0.27 ± 0.028 in 2006 and 2003]. These differences may be explained by changes in grain composition and concentration of the suspended particulate matter, which were caused by the lower water discharge and sediment load in 2006. Extreme drought and impoundment of water by the Three Gorges Dam not only affected the signatures and fluxes of terrestrial OC, but hid the seasonal signal of riverine OC. The impact of the Three Gorges Dam was more significant in the middle reach than in the lower reach. Lakes and tributaries in the lower Changjiang River supplied great amounts of terrestrial OC to the mainstream. In the drought year, the supplement of water provided by the lower lakes and tributaries was more significant. The absolute magnitude of reduction of terrestrial OC in the Changjiang River during drought and Three Gorges Dam influence (2006) would have severely affected the local carbon cycle and would have influenced the global carbon cycle to some extent.