Physical controls on CH4 emissions from a newly flooded subtropical freshwater hydroelectric reservoir : Nam Theun 2.
Résumé
In the present study, we measured CH4 ebullition and diffusion with funnels and floating chambers in the footprint of an eddy-covariance system measuring CH4 emissions at high frequency (30 mn) in the Nam Theun 2 Reservoir, a recently impounded (in 2008) subtropical hydroelectric reservoir located in Lao PDR, southeast Asia. The EC fluxes were very consistent with the sum of the two terms measured independently (diffusive fluxes + ebullition = EC fluxes), indicating that the EC system picked-up both diffusive fluxes and ebullition from the reservoir. The EC system permitted to evidence a diurnal bimodal pattern of CH4 emissions anti-correlated with atmospheric pressure. During daytime, a large atmospheric pressure drop triggers CH4 ebullition (up to 100 mmol m-2 d-1) whereas at night, a more moderate peak of CH4 emission was recorded. As a consequence, fluxes during daytime were twice higher than during nighttime. A total of 4811 measurements of CH4 ebullition with submerged funnels at a weekly/fortnightly frequency were performed. The data set covers a water depth ranging from 0.4 to 16 m, and all types of flooded ecosystems. This dataset allowed to determine that ebullition depends mostly on water level change among many other variables tested. On average, ebullition was 8.5 ± 10.5 mmol m-2 d-1 (10-90 percentile range: 0.03-21.5 mmol m-2 d-1) and ranged from 0-201.7 mmol m-2 d-1. An artificial neural network model could explain up to 45% of variability of ebullition using total static pressure (sum of hydrostatic and atmospheric pressure), variations in the water level and atmospheric pressure, and bottom temperature as inputs. This model allowed extrapolation of CH4 ebullition at the reservoir scale and performing gap-filling over four years. Our results clearly showed a very high seasonality: 50% of the yearly CH4 ebullition occurs within four months of the warm dry season. Overall, ebullition contributed 60-80% of total emissions from the surface of the reservoir (disregarding downstream emissions) suggesting that ebullition is a major pathway in young hydroelectric reservoirs in the tropics.