THE MASS BALANCE AND ISOTOPE CHANGE OF CARBON IN A SMALL LAKE IN CARBONATE REGION OF SOUTHWEST CHINA

The mass-balance of DIC and POC in Lake Hongfeng, a stratified, mesotrophic-eutrophic lake, in southwest china is here modeled. It is discovered that dissolved CO2 in lake originates from CO2 in the watershed, which is transferred as DIC after dissolving carbonate. About 9-10°Á108 mol CO2 is annually absorbed in the weathering of carbonate and silicate rocks in the watershed. It is also revealed that 2.29°Á109 mol C of DIC is annually brought into the lake through the river inflow from the watershed, comparatively, 1.25°Á109 mol C of DIC is produced by the mineralization of organic carbon within the lake. The efflux of CO2 is approximately 1.7°Á108 mol a-1, equivalent to 7 percent of the amount of DIC input via river inflow, and 20 percent of the amount of CO2 absorbed in the watershed. The carbon retained in the sediments is about 13 percent of the total DIC from the watershed. The seasonal change of C isotope of DIC(¦Ä13CDIC) is not obvious. However, two types can be distinguished in the relationship of¦Ä13CDIC and ¦²CO2. One is a negative correlation. It implies that photosynthesis and respiration dominatingly control the processes of carbon cycle within the lake. The other is the positive correlation between the¦Ä13CDIC and ¦²CO2. Also, C isotope of POC (¦Ä13CPOC) varies intensively with its sources and sampling season, on basis of which POC in Lake Hongfeng is though to be autochthonous as production of photosynthesis within the lake.¦Ä13CPOC is positively correlated with ¦Ä13CDIC in summer when photosynthesis exceeds respiration, and¦Ä13CPOC is negatively correlated with ¦Ä13CDIC in autumn and winter as respiration exceeds photosynthesis. Although POC in lake is produced from DIC via photosynthesis, the amplitude of seasonal change of ¦Ä13CPOC is much larger than those of DIC. Obviously the change of ¦Ä13CPOC is mainly decided by the isotopic fractionation of photosynthesis. The partial pressure of CO2 (pCO2) and temperature are linearly correlative with ¦Å¦Ä13CCO2-POC, which imply pCO2 and temperature, as the main factors affecting isotopic fractionation, can be reflected by the C isotope of organic matter in sediments.