Methane flux dynamics in a submerged aquatic vegetation zone in a subtropical lake.

ABSTRACT

Submerged macrophytes are important primary producers for shallow lake systems. So far, their overall role in regulating lake methane flux is a subject of debate because the oxygen produced by their roots can promote methane oxidation in the sediment but they can also enhance methanogenesis through organic substrate production. In this study, we used the eddy covariance method to investigate the temporal dynamics of the CH4 flux in a habitat of submerged macrophytes in Lake Taihu. The results show that the nighttime CH4 flux is on average 33% higher than the daytime flux, although a clear diurnal pattern is evident only in the spring. At the daily to the seasonal time scale, the sediment temperature is the main driver of the CH4 flux variations, implying higher methane production in the sediment at higher temperatures. The annual CH4 emission (6.12 g C m-2 yr-1) is much higher than the published whole-lake mean flux (1.12 g C m-2 yr-1) and that reported previously in the eutrophic phytoplankton zone of the lake (1.35 g C m-2 yr-1), indicating that the net effect of the submerged macrophytes is to enhance methane emission. At the annual time scale, 3.5% of the carbon gained by the net ecosystem production is lost to the atmosphere in the form of CH4.