| The investigations were deployed in Lake Donghu, a shallow eutrophic lake located in Wuhan and Lake Biandantang, a shallow mesoeutrophic Lake Biandantang located in Ezhou. Methane and carbon dioxide fluxes across water-air interface in different stations in the two lakes, with different trophic level, in the two lakes, were studied from Apr 2003 to Mar 2004 by static chambers and gas chromatography. Based on the weekly investigations and several diel observations, the main influence factors shaping greenhouse gas flux change were discussed in the paper and at the same time by means of the comparison of gas fluxes between different trophic level and type lakes, the main dominate factors in different type lakes were discussed.The chief results and conclusions are showed as follows:1 .As a whole, with respect to atmosphere, methane in Lake Donghu was always supersaturated and the methane emission rates in station I IK III were 0.97, 1.071 and 1.136 mg m-2 h-1, respectively. The trophic level of water has no significantly influence on methane emission, while it play a significant role in the restriction on maximum emission. Temperatures had a significant positive influence on methane emissions and the higher temperature, the greater methane emissions, so the main emission time in a year was found in summer. Precipitation and wind speed had also significant positive influence on methane emission.2.The change of Carbon dioxide flux across water-air interface in Lake Donghu was drastic from Apr 2003 to Mar 2004 and the flux of carbon dioxide displayed a significant change between emission and uptake. However, with the respect to atmosphere, annual average of carbon dioxide was supersaturated, and the emission rates of carbon dioxide in station K IK III were 9.87, 13.33 and 26.06 mg m-2 h-1, respectively. With the decrease of nutrient level in the three stations, the fluxes of carbon dioxide significantly increased, and this suggested that the eutrophicationenhanced lake productivity and the depletion to carbon dioxide from atmosphere, and weakened the carbon source of lake. Temperature precipitation and wind speed had no significa: : influence on carbon dioxide flux.3.Diel variation investigations of methane flux in different seasons showed that methane across water-air interface of Lake Donghu in daytime and nighttime was always supersaturated and there was no significant oxidation in Lake Donghu. The diel variations in spring and summer were much greater and the maximal fluxes in a day were hundreds times greater than the minima, while the diel variations in autumn and winter were weak and the emission values were obvious lower than those in spring and summer. According to the correlation analysis, sediment temperature played a more moderate role in methane rate than that of water temperature. It suggested that the methane flux across water-air interface in Lake Donghu was more controlled by the transport mechanism rather than by production mechanism.4. Diel variation investigations of carbon dioxide flux in different seasons displayed that carbon dioxide fluxes across water-air interface of Lake Donghu in daytime and nighttime had a drastic change, and showed a significant turn from uptake to emission. The patterns of diel variation of carbon dioxide in spring and summer were similar and the variations were most drastic in all investigations, while the pattern in autumn and winter were similar and there were no obvious uptake in daytime. The diel variations of carbon dioxide flux were mainly controlled by radiance of the sun, and the radiance acted on the activity of phytoplankton, and then controlled the exchange direction and the magnitude of carbon dioxide across interface. In short, the great diel variations of methane and carbon dioxide showed that it was necessary to calibrate the discrepancy of routine sample time in order to have a more accurate estimationcarbon flux across water-air interface.5.The investigation to Lake Biandantang. which dominated by aquatic macrophyte. showed that methane of th...
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