Diurnal exchanges of CO2 and CH4 across the water-atmosphere interface in a water chestnut meadow (Trapa natans L.)

CH4 and CO2 fluxes across the water-atmosphere interface were measured over a 24 h day-night cycle in a shallow oxbow lake colonized by the water chestnut (Trapa natans L.) (Lanca di Po, Northern Italy). Only exchanges mediated by macrophytes were measured, whilst gas ebullition was not considered in this study. Measurements were performed from 29 to 30 July 2005 with short incubations, when T. natans stands covered the whole basin surface with a mean dry biomass of 504 ± 91 g m−2. Overall, the oxbow lake resulted net heterotrophic with plant and microbial respiration largely exceeding carbon fixation by photosynthesis. The water chestnut stand was a net sink of CO2 during the day-light period (−60.5 ± 8.5 mmol m−2 d−1) but it was a net source at night (207.6 ± 6.1 mmol m−2 d−1), when the greatest CO2 efflux rate was measured across the water surface (28.2 ± 2.4 mmol m−2 h−1). The highest CH4 effluxes (6.6 ± 1.8 mmol m−2 h−1) were determined in the T. natans stand during day-time, whilst CH4 emissions across the plant-free water surface were greatest at night (6.8 ± 2.1 mmol m−2 h−1). Therefore, we assumed that the water chestnut enhanced methane delivery to the atmosphere. On a daily basis, the oxbow lake was a net source to the atmosphere of both CO2 (147.1 ± 10.8 mmol m−2 d−1) and CH4 (116.3 ± 8.0 mmol m−2 d−1).