Drivers of Seasonal and Diel Methane Emissions From a Seagrass Ecosystem

Abstract Seagrass meadows are effective sinks of atmospheric carbon dioxide (CO 2 ). However, there is little insight on how methane (CH 4 ) emissions may potentially offset carbon sequestration in seagrass meadows. Here, we resolve diel and seasonal dynamics of CH 4 and CO 2 water‐air fluxes over a cold‐temperate Zostera marina seagrass meadow using high‐resolution timeseries observations in seawater. CH 4 was emitted from the seagrass‐dominated coastal bay year‐round to atmosphere with CH 4 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 CH 4 emissions were greater than the 6‐fold diel fluctuation. Radon observations imply that dissolved CH 4 was primarily originated from sediment porewater. The main fate of CH 4 in the water was outgassing to the atmosphere via wind forcing. Oxygen and temperature partially controlled dissolved CH 4 seasonal dynamics. There was an annual average uptake of CO 2 from the atmosphere (−0.9 ± 1.5 mmol m −2 d −1 ) driven by enhanced photosynthesis in the spring and summer. The CO 2 ‐equivalent CH 4 outgassing (0.5 ± 0.6 g CO 2 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 CO 2 ‐equivalent CH 4 flux was 6% of the average annual CO 2 uptake. Hence, CH 4 emissions from this cold‐temperate seagrass meadow acted as a minor offset to carbon sequestration.