Spatial And Temporal Transport Of Carbon In Changjiang Mainstream And Influences Of Three Gorges Project

As a major link of two carbon pools, the land and the sea, the river carbon cycle plays an important role in the global carbon cycle. Characteristics of the river carbon transport not only reflects the natural processes and the impacts of human activities on the rivers, the history and current status of the basin, but also has important impacts on the deposition and environment of coastal waters. So it is is very important to understand river carbon cycle correctly.Changjiang is the third longest river of the world and its huge input of terrestrial carbons have significant impects on coastal ecosystem of the East China Sea. Three Gorges Project(TGP), located on the end of the upper reach, is the the largest water control project of world. Building dams in rivers change the original material field, the energy field, the chemical field and the biological field, and may affect the biogeochemical behaviors of the rivers'biogenic elements significantly. So it is important to understand the carbon transport characteristics of Changjiang and influences of TGP correctly, not only for assessing the carbon fluxex from Changjiang to the East China Sea but also for maintaining the health of the coastal ecosystem. Based on the the sampling survey in April 2006(reservoir pool level between 135-156m), April 2008 to May(reservoir pool level between 156-175m) in Changjiang mainstream, with Wu's data of May 1997(before dam interception)and April 2003(before reservoir's impoundment 135m) and other documents data, summed up the distribution of DIC, DOC and POC in Changjiang, and discussed the influences of TGP'different impoundment stages for transport characteristics and fluxex of carbons in Changjiang. This paper also discussed the time law of Changjiang's orginic carbon transport based on the data from May 2006 to May 2007 in Datong and estimated fluxes of DIC, DOC and POC from May 2006 to May 2007 and the year 2008. Research findings are as follows:(1) DIC concentrations in Changjiang decreased from upstream to downstream while the high-values appeared in the headwaters and Jinshajiang. Dilution water from Dongting Lake and Poyang Lake had significant impacts and made the distribution of DIC show three steps along the mainstream. DIC concentrations in Changjiang decreased on the number of negative with the increasing of discharge. Not only the flow sizes of Jinshajiang and major tributaries but also the rainfall discharges affected DIC concentrations of Changjiang and led to the differences between voyages.(2) DOC concentrations in Changjiang were affected by man-made pollution obviously, and there was no significant correlation between DOC and discharge. Low-values of concentrations appeared above Yibin and high-values between Chongqing and the river mouth were due to anthropogenic emissions. POC and TSM had similar distribution along the mainstream and POC% decreased with the increase of TSM. The extra-low TSM and POC and extra-high POC% in the Three Gorges Reservoir after water storage were caused by the sediment interception in the reservoir.(3) Concentrations of DIC, DOC and POC showed obvious seasonal changes. The high DIC appeared not only in the beginning of June, which was the period of water, but also in winter which had the lowest discharge.The low valeus of DIC appeared in summer which had abundant rain and high discharge. The high DOC appeared in fall and the relationship between DOC and flow was complex due to the runoff's contain and dilution to the pollutions. Changes of POC and TSM showed good consistency and had higher values in flood season than in other seasons, reflecting the changes of basin mechanical erosion intensity with the seasons.(4) Fluxes of DIC, DOC and POC in Datong were respectively 12.9×106tC, 1.17×106tC,1.88×106tC from Jun 2006 to May 2007 and 15.0-16.6×106tC,1.31-1.62×106tC,4.1-6.9×106tC during the year 2008.(5)Three Gorges Reservoir's water storage of 135m and 156m in 2003 and 2006 had significant impacts on the Changjiang's runoff into the sea. Dam's interception to the high DIC water from upper reach caused the reduction of DIC flux in Datong. Run way of the reservoir changed the distributions of runoff and DIC during a year. DIC concentrations of middle and lower reaches were higher than the nature state from May to June which was the water period and lower in October which was the storage period. After water storage, a large number of sediment deposited in the reservoir and concentrations of TSM and POC reduced obviously. The reduction of surface suspended matter size and the increase of phytoplankton-POC caused by the rise of water penetrability led to the rapid increase of POC%. Dam's interception to the sediment resulted the reduce of sediment discharge and surface TSM content in middle and lower reaches, so that POC% was higher and POC flux was lower significantly than pre-impoundment. There was not abvious increasing trend in DOC based on the data of four voyages btween the year 1997 and 2008, which had some connection with the short running time of Three Gorges Reservoir.