Abstract
Water bodies are major areas for methane release. Eutrophic water bodies may promote methane flux. The sediment-water interface is the major location for methane release, and studies on sediment-water interactions are necessary to regulate methane release in water. However, relevant studies on methane flux at the sediment-water interface are limited due to methodological difficulties. Using an innovative gas trapping device, this study investigated the seasonal characteristics and diel variation in summer methane flux from eutrophic water bodies and analyzed the correlations between temperature, dissolved oxygen (DO), different forms of nitrogen and the methane flux at different time scales. The results showed that methane flux in the eutrophic pond was high and had distinct seasonal variations and diel variations: the average value was 2.81 ± 0.19 mmol m−2 h−1 in summer, which was significantly greater than that in spring (0.62 ± 0.14 mmol m−2 h−1), autumn (0.63 ± 0.10 mmol m−2 h−1) (p < 0.01) and winter (approached zero). The diel characteristics of methane flux in summer exhibited a unimodal pattern of increase at night and decrease during the day. The correlation analysis showed that the sediment-water methane flux rate of the water body was significantly positively correlated with the temperature and NH4+ concentration and significantly negatively correlated with DO, NO3− and NO2− concentration. Meanwhile, among different time scales, the correlations between NO3−/NH4+ concentration and methane flux were the highest at the diel scale in summer (R2 = 0.68, 0.87 respectively) when the temperature and DO vibration was low and the relationship between temperature/DO and methane flux was poor (R2 = 0.45, 0.87 respectively). This study considered that higher NH4+-N and lower NO3−-N/NO2−-N content in eutrophic water could have an effect on the high methane flux in summer as well as the low dissolved oxygen content.
Highlights
Methane is an important greenhouse gas [1]
Diel characteristics of the methane flux under stable hydrologic conditions in summer demonstrated a unimodal pattern as a whole: the CH4 flux rate increased after sunrise
The CH4 flux from the eutrophic pond peaked at 7.33 ± 2.09 mmol m−2 h−1, and more than 30% of all monitoring data were higher than 2 mmol m−2 h−1, which was much higher than the methane release rate of low nutrient rivers [30], general reservoirs [18,31,32,33], a lake [32,34], some eutrophic lakes [29] and even a paddy field [35]
Summary
Methane is an important greenhouse gas [1]. The atmospheric volume fraction of methane in air has been increasing since the Industrial Revolution [2,3]. Wetlands are important sources of methane release and have been extensively studied [4,5]. Especially in China, eutrophication in water has taken place along with the development of urbanization and agricultural production [6,7]. The eutrophication of a water body, containing higher nutrients and more dissolved organic carbon than oligotrophic areas, could accelerate the production and release of methane [8,9,10]. A study of the characteristics of methane flux in eutrophic water bodies is necessary to identify ways to reduce methane release
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