; CH4 通量
|其他题名: ||Studies on Carbon Exchange and Mechanism at Air-Water Interface in Freshwater Lake along The Middle and Lower Reaches of Yangtze River|
1. 对东湖固定采样点水－气界面CH4和CO2通量分别进行昼夜变化、季节动态、以及年际变化的研究。水－气界面CH4和CO2通量的昼夜变化均非常显著，一昼夜中气体的通量变化可以达到2个数量级以上。与设想不同，温度并非该地区湖泊水－气界面CH4和CO2通量昼夜变化的决定因子，在夏季两种气体的昼夜变化均受到水中浮游植物的调控。周年尺度上，东湖是大气CH4和CO2的源，季节动态特征明显不同于北方高纬度地区大部分湖泊，表现为夏季的高速CH4释放和其他季节的低速释放，CO2则是冬季的高速释放与春夏季节的显著吸收。CH4和CO2通量的时间变化与环境因子的相关分析显示：东湖CH4和CO2的一年中的时间变化主要受到水中浮游植物生命活动的调控，而营养、温度以及其他影响因子主要通过影响浮游植物的生命活动而影响水－气界面CH4和CO2通量的变化。与全球不同气候带的淡水湖泊CH4和CO2的释放通量比较，富营养化程度严重的东湖并非大的碳源，原因可能在于东湖是高度的自营养型湖泊，水体中浮游植物活跃的生命活动增加了对水体中CH4的氧化和CO2的消耗。年际变化研究显示：东湖CH4和CO2通量的年际动态远远小于月动态和季节动态，3年测定中CH4和CO2通量的变异系数分别为27.6％和9.2％，三年里由水－气界面释放到大气中的碳分别为802.5 ± 416.5、875.1 ± 435.6、696.9 ± 356.9 吨。这说明类似于东湖这种比较稳定的水体生态系统是大气比较稳定的碳源，在年际之间不会发生剧烈的变化。CH4和CO2通量的年际变化与降雨的年际变化模式一致，与温度无关，这说明目前气温的缓慢增加还不会显著的影响水－气界面的碳循环，而如果气候变化引起亚热带地区降雨总量减小则可能减弱该地区淡水湖泊作为大气碳源的功能。
4. 运用本研究所建的CH4释放模型和选定的水－气界面CO2通量模型，结合正在开展的长江中下游地区湖群比较湖沼学的研究，对长江中下游地区30多个湖泊以及三峡水库水－气界面CH4和CO2释放进行估算。结果显示：长江中下游30多个湖泊和水库均是大气CH4的源，调查中水面总面积10 045.8 km2的水体释放的CH4形态碳为4.71±2.07×1010 g C y-1。长江中下游地区大约85％的湖泊水体是大气CO2的源，有少量的湖泊从大气中吸收CO2，综合所有水体CO2交换结果显示，每年从所调查水面释放的CO2形态碳为9.88±3.73×1011 g C y-1。对所调查结果外推得得到长江中下游地区淡水水体向大气释放总碳量达2.18±0.85 ×1012 C y-1。尽管与全球湖泊和水库释放的总碳量相比，我国长江中下游地区淡水水体的碳释放只占0.11％左右，但是在区域碳收支上由水面释放的碳却是相当重要的组分，由长江水面和长江中下游淡水湖泊等水体生态系统年释放的气态碳可能抵消全国农田土壤或造林年固定碳的16-20％。|
|英文摘要: ||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.|
|Appears in Collections:||中科院水生所知识产出（2009年前）_学位论文|
|File Name/ File Size