Methane dynamics in a temperate seagrass meadow

Seagrass meadows are essential coastal ecosystems that play a crucial role in carbon sequestration and greenhouse gas regulation. However, our understanding of methane (CH₄) production and emission from these important carbon sinks remains limited. This study investigates CH₄ dynamics in a temperate seagrass meadow in Swan Lake (Shandong, China), with a focus on the production and emission of CH₄. The addition of ¹³C-labelled substrates revealed that CH₄ production rate constant in sediments ranged from 0.072 to 2.2 day⁻¹, with methylotrophic methanogenesis predominating, accounting for over 96% of the total CH₄ production, while hydrogenotrophic methanogenesis contributed less than 4%. These rate constants were significantly lower (up to 20 times) compared to those observed in tropical seagrass meadows, likely due to the lower temperatures in temperate ecosystems. Additionally, anaerobic oxidation of CH₄ was not detected based on the ¹³CH₄ incubation experiments. Time-series observations of ²²²Rn, CH₄ and various hydrological parameters indicated that the CH₄ emission fluxes from sediment-water interface were 1065±176 μmol m^-2 day^-1 in the summer and 1415±233 μmol m^-2 day^-1 in the winter, exceeding the range of CH₄ fluxes previously reported from other seagrass meadows. The CH₄ outgassing fluxes were 184±55 μmol m^-2 day^-1 in the summer and 216±65 μmol m^-2 day^-1 in the winter. Notably, over 80% of the CH₄ was oxidized in the water column before reaching the atmosphere. The higher CH₄ emissions observed in winter were attributed to the seasonal presence of swans in Swan Lake. Swan excreta and the food provided to them significantly increased the availability of dissolved organic carbon (DOC), which, in turn, supplied ample substrates for CH₄ production, consistent with the higher DOC concentrations observed in the winter. Our study provides valuable insights into CH₄ production and emission dynamics, highlighting the seagrass meadow as a source of atmospheric CH₄.