Methane emissions during the tide cycle of a yangtze estuary salt marsh

Methane (CH₄) emissions from estuarine wetlands were proved to be influenced by tide movement and inundation conditions notably in many previous studies. Although there have been several researches focusing on the seasonal or annual CH₄ emissions, the short-term CH₄ emissions during the tide cycles were rarely studied up to now in this area. In order to investigate the CH₄ emission pattern during a tide cycle in Yangtze Estuary salt marshes, frequent fixed-point observations of methane flux were carried out using the in-situ static closed chamber technique. The results indicated that the daily average CH₄ fluxes varied from 0.68 mgCH₄m^-2h^-1 to 4.22 mgCH₄m^-2h^-1 with the average flux reaching 1.78 mgCH₄m^-2h^-1 from small tide to spring tide in summer. CH₄ fluxes did not show consistent variation with both tide levels and inundation time but increased steadily during almost the whole research period. By Pearson correlation analysis, CH₄ fluxes were not correlated with both tide levels (R = -0.014, p = 0.979) and solar radiation (R = 0.024, p = 0.865), but significantly correlated with ambient temperature. It is temperature rather than the tide level mainly controlling CH₄ emissions during the tide cycles. Besides, CH₄ fluxes also showed no significant correlation with the underground pore-water CH₄ concentrations, indicating that plant-mediated transport played a more important role in CH₄ fluxes compared with its production and consumption.