Sources and distribution of carbon within the Yangtze River system

Dissolved, particulate, soil and plant samples were collected from the Yangtze River (Changjiang) system in May 1997 and May 2003 to determine the sources and distribution of organic and inorganic matter within the river system. Average dissolved organic carbon (DOC) concentrations within the main stream were 105 mM C in 1997 and 108 mM C in 2003. Particulate organic carbon (POC) ranged from 0.5% to 2.5% of total suspended matter (TSM). Both dissolved inorganic carbon (DIC) and particulate inorganic carbon (PIC) concentrations decreased from upper to lower reaches of the river, within the ranges 1.2e2.7 mM and 0.08e4.3% of TSM, respectively. d13C and d15N values for tributaries and the main stream varied from 26.8& to 25.1& and 2.8& to 6.0&, respectively. A large spatial variation in particulate organic matter (POM) is recorded along the main stream, probably due to the contributions of TSM from major tributaries and POM input from local vegetation sources. The dominance of C-3 plants throughout the entire basin is indicated by d13C and d15N values, which range from 28.8& to 24.3& and from 0.9& to 5.5&, respectively. The d13C and d15N values of organic matter within surface soil from alongside tributaries and the main stream vary from 28.9& to 24.3& and 2.7&to 4.5&, respectively. Although these differences are subtle, there is a slight enrichment of 15N in soils along the main stream. Various approaches, such as C/N and stable isotopes, were used to trace the sources of organic matter within the river. Riverine POM is mostly derived from soil; the contribution from phytoplankton is minor and difficult to trace via the composition of particles. POC flux has decreased from >5 106 t yr 1 during the period 1960e1980 to about 2 106 t yr 1 in 1997. This trend can be explained by decreasing sediment load within the Yangtze River. The export of TOC from the Yangtze River at the end of the 20th Century is approximately equivalent to that of the Zaire River, less than that of the Amazon River, and higher than that of other large rivers such as the Mississippi. Large amounts of DOC and POC were transported to coastal areas of the East China Sea over a short period during 1998 flood events, containing large amounts of nutrients and pollutants. Such an event could be an important trigger for coastal environmental problems and changes to the health of ecosystems.