|Other Abstract||Increasing atmospheric concentrations of the radiatively active gases CH4 and CO2 have stimulated extensive studies on carbon budget and greenhouse gas cycling from both terrestrial and aquatic environments. The carbon budget of freshwater ecosystems along the middle and lower reaches of the Yangtze River is an important part of carbon budget in the whole terrestrial ecosystems of China. Freshwater lakes in this region are characteristic of shallow water depth, high water temperature and high eutrophic level, and knowledge about carbon cycling in such systems is therefore an indispensable important part for understanding the mechanisms of carbon cycling in global aquatic ecosystems. A comprehensive study was conducted in freshwater lakes in the middle and lower reaches of the Yangte River and the main purposes were 1) to study the exchange and fluxes of CH4 and CO2 across air-water interface and the diel, seasonal and interannual variations of CH4 and CO2 fluxes with underlying mechanism controlling the variations, 2) to study the change of gaseous carbon budget during retrogressive succession of lakes, 3) to develop semi-empirical model of CH4 fluxes and 4) to estimate carbon release across air-water interface in these lakes by the developed models. The main contents and conclusions are as follows.
1. The static chamber and gas chromatography technique were used to study the fluxes of CH4 and CO2 across air-water interface in Lake Donghu. The diel studies of gas fluxes showed that there were great diel variations of CH4 and CO2 fluxes in all seasons, and the gas fluxes changed with two orders of magnitude in a diel cycle, and there were no significant relationships between gas fluxes and parameters in most studies. Only in summer, significant relationships between gas fluxes and biomass of phytoplankton could be observed, indicating that phytoplankton mainly regulated diel variation of carbon exchange across air-water interface in summer. In a year cycle, Lake Donghu was sources of CH4 and CO2 with respect to the atmosphere and the average fluxes of CH4 and CO2 were 23.37±18.6 and 332.37±160.1 mg m-2 d-1, respectively. The fluxes of CH4 and CO2 showed strong seasonal dynamics: CH4 emission rate was highest in summer, remaining low in other seasons, whereas CO2 was adsorbed from the atmosphere in spring and summer, exhibiting a large emission in winter. CH4 emission were correlated positively with net primary production (NPP) and temperature, while CO2 flux were correlated negatively with NPP and temperature; however, there were no significant relationships between the fluxes of CH4 and CO2 and dissolved organic carbon (DOC), which is significantly different from boreal lakes. This indicates that phytoplankton rather than allochthonous organic matter regulated C dynamics across air- water interface in subtropical autotrophic lakes enriched with nutrients. The amplitudes of interannual variations in fluxes of CH4 and CO2 were significant lower than those of seasonal and diel variations. During the period of three years, the carbon losses from surface water of Lake Donghu were 802.5 ± 416.5、875.1 ± 435.6、696.9 ± 356.9 tons, respectively, suggesting that Lake Donghu is a relative stable and consistent carbon source with respect to the atmosphere.
2. Due to serious eutrophication, overstocking of grass carp and many other human disturbances, large-scale declines of aquatic macrophytes and retrogressive succession were observed in many shallow lakes of China. A case study in Lake Biandantang was undertaken to investigate the change of carbon fluxes during such a regime shift. Dissolved carbon and gaseous carbon across air–water interface were compared in three sites with different vegetation covers. Dissolved carbon had no significant change with the progress of retrogressive succession in short time, but the gaseous carbon release showed great changes. Correlation analysis showed that macrophytes stimulated CH4 emission but suppressed CO2 release. On the whole, a larger carbon loss from surface water was observed in phytoplankton-dominated zone, indicating that with the decline of macrophytes and overgrowth of phytoplankton, the lakes will be larger carbon sources to the atmosphere.
3. To estimate carbon fluxes and budget in a regional scale，predictive models for regional ecosystems need to de developed. Based on the mechanisms of CH4 emission in lakes and a developed model for semi-freshwater ecosystem, a model of CH4 flux was developed for freshwater ecosystems along the middle and lower reaches of the Yangtze River. Comparison between simulated CH4 flux and measured values showed that the model can provide reliable prediction of CH4 emission from freshwater ecosystem. Similarly, the comparison between simulated CO2 flux and measured values showed that the selected CO2 flux model can effectively simulate CO2 exchange of freshwater ecosystems.
4. Based on limnological parameters of lakes along the middle and lower reaches of Yangtze River, the exchanges of CH4 and CO2 across the air-water interface of freshwater ecosystems were estimated using the flux models. The results showed that all the surveyed lakes and reservoir in this region were CH4 source with respect to the atmosphere, and the total CH4-C emission from the surveyed waters (10 045.8 km2) was estimated to be 4.71±2.07×1010 g C y-1. Except for a few lakes incorporating atmospheric CO2 into water, approximately 85% waters were supersaturated with respect to atmospheric pressures of CO2. The annual total CO2-C from the surveyed lakes and reservoir reached 9.88 ± 3.73×1011 g C y-1. If extrapolating the estimated results to the whole lake area (21 171.6 km2) along the middle and lower reaches of the Yangtze River, a total carbon release was estimated to be 2.18±0.85 ×1012 C y-1. Compared to the reported total carbon release from global lakes (0.14×1015 g y-1), the carbon release from freshwater ecosystems along the middle and lower reaches of Yangtze River only accounted for 0.11%. On the other hand, compared to other terrestrial ecosystems, carbon release from freshwater ecosystems in the region can offset about 16-20％of the annual carbon fixation by agriculture soil and forestation in China. Hence, carbon emission from surface waters is an important loss factor in both regional and global carbon budgets. For a better understanding of the global carbon cycling and budget, it is necessary to integrate carbon cycling and budget of aquatic ecosystems into the whole carbon budget study.|