Drivers of Seasonal and Diel Methane Emissions From a Seagrass Ecosystem

Seagrass meadows are effective sinks of atmospheric carbon dioxide (CO₂). However, there is little insight on how methane (CH₄) emissions may potentially offset carbon sequestration in seagrass meadows. Here, we resolve diel and seasonal dynamics of CH₄ and CO₂ water-air fluxes over a cold-temperate Zostera marina seagrass meadow using high-resolution timeseries observations in seawater. CH₄ was emitted from the seagrass-dominated coastal bay year-round to atmosphere with CH₄ fluxes ranging from 0.2 to 2.6 μmol m⁻² d⁻¹. These fluxes are at the lower end of earlier estimates based mostly on short-term (i.e., 1 day) observations. The 13-fold seasonal fluctuations in CH₄ emissions were greater than the 6-fold diel fluctuation. Radon observations imply that dissolved CH₄ was primarily originated from sediment porewater. The main fate of CH₄ in the water was outgassing to the atmosphere via wind forcing. Oxygen and temperature partially controlled dissolved CH₄ seasonal dynamics. There was an annual average uptake of CO₂ from the atmosphere (−0.9 ± 1.5 mmol m⁻² d⁻¹) driven by enhanced photosynthesis in the spring and summer. The CO₂-equivalent CH₄ outgassing (0.5 ± 0.6 g CO₂ eq m⁻² yr⁻¹) offsets only 0.8% of the sediment carbon accumulation in this cold-temperate Z. marina meadows over a 20-year time horizon. The CO₂-equivalent CH₄ flux was 6% of the average annual CO₂ uptake. Hence, CH₄ emissions from this cold-temperate seagrass meadow acted as a minor offset to carbon sequestration.