Carbon dioxide and methane emissions from a temperate salt marsh tidal creek

Abstract

Coastal salt marshes store large amounts of carbon but the magnitude and patterns of greenhouse gas (GHGs; including CO2 and CH4) fluxes are unclear. Information about GHG fluxes from these ecosystems mainly comes from studies of sediments or at the ecosystem-scale (using eddy covariance), but fluxes from tidal creeks are currently unknown. We measured GHG concentrations in water, water quality, meteorology, sediment CO2 efflux, ecosystem-scale GHG fluxes, and plant phenology; all at half-hour time-steps over one year. Manual creek GHG flux measurements were used to parameterize a model of water-to-atmosphere GHG fluxes. The creek was a source of GHGs to the atmosphere where tidal patterns rather than water temperature controlled diel variability. Dissolved oxygen and wind speed were inversely correlated with creek CH4 efflux. Despite lacking a seasonal pattern, creek CO2 efflux was correlated with drivers such as turbidity across phenological phases. Overall, night-time creek CO2 efflux (3.6 ± 0.63 µmol/m2/s) was over two times higher than night-time marsh sediment CO2 efflux (1.5 ± 1.23 µmol/m2/s). Creek CH4 efflux (17.5 ± 6.9 nmol/m2/s) was four times lower than ecosystem-scale CH4 fluxes (68.1 ± 52.3 nmol/m2/s) across the year. These results suggest that salt marsh tidal creeks are potential hotspots for CO2 emissions and (because they are supersaturated with CH4; up to >6000 µmol/mol) could contribute to lateral transport of CH4 to the coastal ocean. This study provides insights for modelling GHG efflux from tidal creeks and suggests that changes in tide stage overshadows water temperature in determining magnitudes of fluxes.