Methane Emissions from Surface of Mangrove River on Hainan Island, China

The surfaces of rivers are considered important sources of atmospheric methane (CH4), however research on this topic is still constrained, especially in freshwater rivers and with the consideration of spatial heterogeneity. Three regions (upper reaches, midstream and downstream) were selected to examine the CH4 fluxes from a freshwater river surface in a mangrove forest wetland from 2012 to 2013, using floating chambers. Results showed that the CH4 fluxes varied significantly among the three regions, with the lowest fluxes at downstream (0.50 ± 0.20 mg m−2 h−1), and highest at upper reaches (1.19 ± 0.36 mg m−2 h−1). The average emission rate at midstream was 0.95 ± 0.37 mg m−2 h−1. The methane flux also varied with seasons, with higher flux in rain-abundant seasons. On average, the CH4 flux in our research river was 0.88 ± 0.31 mg m−2 h−1, which was less than other tropical rivers. In addition, we found that the CH4 flux was significantly correlated with the water characteristics of temperature and atmospheric pressure. Thereby, this study quantified the methane emission from a freshwater river surface in a tropical mangrove forest, enriching the existing knowledge of river surface CH4 flux.