Methane and carbon dioxide fluxes from a shallow hypereutrophic subtropical Lake in China

Abstract

Up to now, there have been few studies in the annual fluxes of greenhouse gases in lakes of subtropical regions. The fluxes of methane (CH₄) and carbon dioxide (CO₂) across air–water interface were measured in a shallow, hypereutrophic, subtropical Lake Donghu (China) over a year cycle, using a static chamber technique. During the year, Lake Donghu emitted CH₄ and CO_2; the average flux of CH₄ and CO₂ was 23.3±18.6 and 332.3±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, but exhibited a large emission in winter. Annual carbon (C) budget across air–water interface in Lake Donghu was estimated to be 7.52±4.07×10⁸ g. CH₄ emission was correlated positively with net primary production (NPP) and temperature, whereas CO₂ flux correlated negatively with NPP and temperature; however, there were no significant relationships between the fluxes of CH₄ and CO₂ and dissolved organic carbon, a significant difference from boreal lakes, indicating that phytoplankton rather than allochthonous matter regulated C dynamics across air–water interface of subtropical lake enriched nutrient content.

Introduction

Increasing atmospheric concentrations of the radiatively active gases CH₄ and CO₂ have stimulated research to determine their emission from both terrestrial and aquatic environments (e.g., Cole et al., 1994; Conrad, 1996; Segers, 1998). Up to now, limited studies concerning CH₄ and CO₂ exchange in lakes have focused on boreal lakes (e.g., Miller and Oremland, 1988; Jonsson et al., 2003). There are a few estimates of CH₄ flux from some tropical lakes in the Amazonian floodplain (e.g., Bartlett et al., 1990; Smith and Lewis, 1992). Some studies suggested that CH₄ emission rates were significantly higher in subtropical and tropical wetlands than in boreal wetlands due to high temperature (Purvaja and Ramesh, 2000; Verma et al., 2002), and thus subtropical lakes were assumed to be larger C sources. However, it is unclear whether subtropical lakes are C source or not due to quite limited data. On the other hand, most of subtropical lakes are shallow and eutrophic, and are different from northern, cold, dimictic lakes which have distinct mixing and stratification periods (Riera et al., 1999). Thus, it is unclear whether the dominant factors in boreal lakes would also control C exchange across air–water interface of subtropical lakes. To obtain a better estimate and understanding of the C cycle of lakes, more and integrated data are needed from lakes situated in different geographical regions (Huttunen et al., 2003). In China, there are numerous lakes in the middle and lower reaches of the Yangtze River, accounting for 23.3% of the total lake area (91, 019.6 km²) in China (Wang and Dou, 1998). Assessing the role of the subtropical lakes in the global exchange of greenhouse gases and exploring the dynamic mechanisms responsible for emissions are academically necessary and valuable.

In addition, previous sampling programs in boreal and arctic lakes were generally carried out during ice-free periods or during dry and wet seasons, lacking some important interim periods (Moore and Knowles, 1989; Michmerhuizen et al., 1996). To quantify the annual CH₄ and CO₂ exchanges in subtropical lakes which have no distinct ice period and significant wet–dry seasons and to explore the underlying mechanism, a sampling program needs to run long enough to cover the entire exchange season (Whalen and Reeburgh, 1988; Moore and Knowles, 1989).

The purposes of this study are (i) to determine the fluxes of CH₄ and CO₂ across the air–water interface of Lake Donghu, a shallow, hypereutrophic, subtropical lake, (ii) to describe the temporal variations of CH₄ and CO₂ fluxes in a year cycle, and (iii) to estimate the C source/sink function of the air–water interface, with discussion on the possible dominant factors in shaping these patterns.