Carbon Dioxide Exchange Flux In The Water-air Interface And Its Mechanism In The Changjiang River Basin

The large-scale(both regional and global)estimates of carbon dioxide(CO2)exchange fluxes in the water-air interface of freshwater systems was a weak link in the research of carbon cycle.The Changjiang River is an important river in the world and one of the rivers most affected by human activities.This research combined the historical datas nearly 30 years,relevant literature reports,and field measurement datas(2013-2014,2015-2016)to conduct a comprehensive and systematic study on the CO2 exchange in the water-air interface of the Changjiang River Basin.The research results had a reference and promotion significance for discussing the CO2 exchange in the water-air interface of the similar rivers in the mid and low latitudes.The main results are as follows:(1)The pH of rivers kept relatively stable overall during the two historical periods 1960s-1985 and 1991-2003.From the upstream to the downstream,the pH of the tributaries and mainstream of the Changjiang River gradually decreased.There was acidification in the downstream of the Changjiang River.The pH at the Datong Hydrometric Station showed a decreasing trend from 1964 to 2011.(2)CO2 was oversaturationin the mainstream and tributaries of the Changjiang River,and the mainstream and tributaries were the source of C02 in the atmosphere.pCO2 of the tributaries was higher than that in the mainstream.pCO2 of the mainstream showed an increasing trend overall from the upstream to the downstream and the closer to the estuary,the higher.In addition,pCO2 showed a decreasing trend with the increasing of river series(Strahler).Small streams with larger turbulence tended to release large amounts of CO2.Controlled by the decomposition of dissolved organic carbon,pCO2 of the heterotrophic rivers was generally higher.On the contrary,thermal stratification of the lakes(natural lakes and reservoirs)can control the concentration of CO2 and the metabolism of plankton in the water.During the seasons when was suitable for the phytoplankton growth,CO2 was absorbed through photosynthesis.Thus thepCO2of the lakes appeared to be at a low level.(3)The Changjiang River was similar to other major rivers in the world and it was an important source of CO2 in the atmosphere.According to the CO2 releasing flux estimation model of the inland waters(river,reservoir,and natural lake)that established in this paper,the C02 released rates were 107.8 mol/m2-yrof the rivers,5.7 mol/m2·yr of the reservoirs,and 6.1 mol/m2-yr of the natural lakes in the Changjiang River basin.The CO2 releasing flux of rivers in each tributary system was greater than the amount of carbon that transported to the mainstream,indicating that CO2 exchange in the water-air interface of rivers played an important role in the river carbon cycling.(4)In the Changjiang River basin,the CO2 released flux of the rivers was 77.7Tg·C/yr,accounting for about 4.3%of the CO2 flux that released from the global rivers.The CO2 released flux of the reservoirs was 0.7 Tg·C/yr,accounting for about 1.5%of the CO2flux that released from the globalreservoirs.The CO2 released flux of the lakes was 1.3 Tg·C/yr,accounting for 0.2%of the CO2 flux that released from the global lakes.Although the CO2 released flux from the lakes and reservoirs in this basin only accounted for about 0.35%of the CO2 release flux from the world's lakes and reservoirs,the CO2 released from the surface water of lakes is an important component in estimating regional carbon release.The CO2 released from the Changjiang River was 3 times of the carbon flux(24.8Tg·C/yr)that transported to the ocean,accounting for about 3.4%of the CO2 released fluxes of the global inland waters(rivers,natural lakes and reservoirs).