太湖湖滨带秋、冬季CH_4排放特征及其影响因素初步研究

CH_4 Emissions in Autumn and Winter from Littoral Zone of the Taihu Lake

2003年9月至2004年2月期间，在太湖北部的梅梁湾湖区，采用原位静态暗箱方法。沿水体至陆地方向对两种典型湖滨带进行了CH4的近地（水）表面浓度体积分数和CH4排放通量研究。研究结果表明，观测期间的富营养化湖泊湖滨带是CH4的排放源，其近地（水）表面的CH4体积分数变化范围为1．889×10^-6 ～14．151×10^-6，高于大气背景的CH4体积分数（1．745×10^-6）。研究区中，有植被的水向辐射区CH4体积分数最高，为（13．208±1．333）×10-^6。观测期间，研究区的CH4排放通量变化在-179～83344μg／（m^2·h）之间，秋、冬季CH4排放通量的平均值分别为（10530±22030）μg／（m^2·h）和（106±354）μg／（m^2·h）。在有植被的湖滨带，CH4排放通量具有明显的空问梯度变化，CH4排放通量从水体向陆地方向先升高，至水向辐射区达到最高，然后随地表土壤层水分含量的降低而降低，并且有植被的水向辐射区与其它各区的CH4排放通量存在显著性差异，有植被的水向辐射区是湖滨带的CH4高排放区，因此在进行水体CH4排放评估时必须单独考虑有植被的水向辐射区。

Methane, as an important radiative trace gas, account for about 15 % of the greenhouse effect. Wetland is one of the important natural CH4sources, which occupies 40% the global methane flux to the atmosphere. Littoral zone between aquatic and terrestrial environments can be recognized as biogeochemical active wetlands that support high CH4 release. China has more than 2 300 lakes greater than 1 km^2 with a total area about 71 000 km^2 and currently most of them are in a eutrophic state. These lakes; including their littoral zones, may have been the potential significant sources of CH4, however, such researches are still very few in China, the neglect of eutrophic freshwater bodies, especially their littoral zones, raises serious uncertainties about estimates of the regional CH4 emissions. In this pilot study, Two two representive types of littoral zone in hyper - eutrophic Meiliang Bay, the Taihu Lake Taihu, were selected, one is a macrophyte -covered littoral zone （section A） , and another is eroded by wave and is a bare （ non - macrophyte） littoral zone （ section B ）. Ten sampling sites （ A0 - A9 ） in section A and three sites （ B1 - B3 ） in section B were established along the gradient from the open water to the land. Section A includes four subzones, i.e. pelagic zone （ A0）, non - macrophyte - covered infralittoral zone （ A1 - A2 ）, reed - covered infralittoral zone （ A3 - A4 ）, eulittoral zone （ A5 -A7 ） and supralittoral zone （ A8 - A9 ）. There is only a non - macrophyte - covered infralittoral zone （ B1 - B3 ） in section B. CH4 concentration near the land/water surface and its flux in the littoral zone were measured by the in - situ closed - chamber technique approximately once or twice a month from September 2003 to February 2004. Duplicate chambers were used in sites of sections A and B, except that triplicate chambers were used at site A0. At the same time, the environmental variables related to CH4 fluxes were also investigated. The results showed that littoral zones of eutrophic lake are important CH4 sources in the atmosphere. The CH4 concentrations near land - or water - surface gradually decreased as temperature came down at all sites, ranging from 1. 889 × 10^-6 to 14. 151 × 10^-6 The concentration of CH4 was higher than the atmospheric CH4 background （ 1. 745 × 10^-6 ） during our investigation, and the highest value was in the reed - covered infralittoral zone （ 13. 028 ± 1. 333 ） × 10^-6. The CH4 fluxes ranged from - 179μg/（m^2·h） to 83 344 μg/（m^2·h）,with means （10 530 ±22 030 ）μg/（m^2·h） and （106 ± 354 ）μg/（m^2·h） in autumn and winter, respectively. CH4 emission had distinctly spatial variation in the macrophyte - covered littoral zone.From the open water to the land, firstly, it increased, reaching its peak value decreased gradually as water contents came down in the non -inundated area. in the infralittoral zone, and then Analysis of variance showed that the infralittoral zone in the macrophyte - covered littoral zone had significant variation against other zones. In the bare littoral zone, there were no significant differences in CH4 flux between infralittoral zone and pelagic zone. All results indicated that reed - covered infralittoral zone was the key area of CH4 emission, where CH4 flux was about 5 -20 times higher than that from other zones, . and must be calculated separately in freshwater body for CH4 emission inventories at region scale. Although the littoral area is relatively smaller than the open water body, our results indicated that the fluxes of CH4 in the littoral zone played a disproportionately large role and it must be considered in regional estimation of CH4 flux from water systems. Moreover, in the little lakes, ponds and reservoirs with area less than 1 km^2, the cover of littoral zone will increase, then the contribution of littoral zone in CH4 flux increases and even controis the whole quantity of CH4 flux from the lake system. Drift algae accumulated naturally near the downwind shore in eutrophic water systems, and have become a common phenomenon along the shorelines. Fresh organic C derived from algae and other suspended paricles would be enriched nearshore, especially in the macrophyte - covered area. The lack of available data precludes an accurate estimation of total CH4 emission from littoral regions of Chinese lakes. There is a strong need for more data on littoral CH4 fluxes given the importance of these areas as hotspots of biogeochemistry in the landscape.