Surrounding pressure controlled by water table alters CO 2 and CH 4 fluxes in the littoral zone of a brackish-water lake

Abstract

We studied the effect of water table on CO 2 and CH 4 fluxes at different time scales in the littoral zone of Lake Obuchi, a brackish lake in northern Japan. The vegetation formed three distinct zones along the water table gradient, two dominated by emergent aquatic macrophytes (the Phragmites australis-dominated zone and the Juncus yokoscensis-dominated zone) and one dominated by terrestrial macrophytes ( Miscanthus sinensis and Cirsium inundatum-dominated zone). To clarify the impact of variations in water table on monthly and yearly summed CO 2 and CH 4 fluxes, we examined the relationship between water table and the ratio of observed flux to calculated flux, whereby the calculated flux was based solely on the exponential relationship between flux and soil temperature for each gas. This study revealed that the impact of variations in water table on monthly and yearly summed CO 2 and CH 4 fluxes differed markedly between the vegetation zones. By taking the temporal change in water table into account in the estimation of both the CO 2 and CH 4 fluxes, the monthly summed CO 2 and CH 4 fluxes in the Phragmites-zone were markedly greater in every month of the year compared to estimation based on temperature alone. In the Juncus-zone, the effect of water table on monthly summed CO 2 and CH 4 fluxes differed between months. In addition, the magnitude of water-table effects controlling monthly summed CO 2 and CH 4 fluxes differed with atmospheric conditions, i.e., between the pressure-falling and low-pressure phase on the one hand and other pressure phases on the other hand. After weighting all the impacts of temporal changes in water table on fluxes, the yearly summed CO 2 and CH 4 fluxes showed a 1.26–6.64-fold increase compared with not taking water table effects into account, and the increase differed among the three vegetation zones.