Spatial and temporal patterns of methane and its influencing factors in the Jiulong River estuary, southeastern China

Abstract

Estuaries are important sources of methane (CH4) to the atmosphere. However, the lack of adequate field studies for understanding the sources/sinks of CH4 in estuaries hampers global atmospheric budgeting. Therefore, more studies are needed to bridge this gap and improve our understanding of CH4 dynamics. This study investigated the impacts of tides on CH4 emissions along a salinity gradient of the subtropical Jiulong River estuary (JRE) during spring, neap tides and over 24 h tidal cycles in 2017 and 2018. Spatially, the CH4 concentrations increased from the lower reaches to the upper reaches, with values varying widely from 58 to 1726 (393 ± 336) nmol/L, which correspond to saturations of 2513–59,371 (15,549 ± 11,341)%. The CH4 concentration was inversely and significantly correlated with salinity, indicating the dilution and sulfate inhibition effects of the adjacent saline water. Temporally, the highest CH4 concentrations and fluxes were detected in the spring tide period, which can enhance the resuspension of particles and CH4 from sediments, and were accompanied by high turbidity values. Elevated CH4 concentrations were found during low tide, and horizontal advection of low salinity water could be the driver of the increased CH4 concentrations during low tide. Due to the lack of agreement among current diffusive models, three different parameterizations were used to assess the water-air fluxes of CH4 in this study. The flux range was estimated to be 193–18,585 (2793 ± 3812) μmol/m2/d, which is well above that reported for most estuaries worldwide.