Two temperate seagrass meadows are negligible sources of methane and nitrous oxide

Seagrasses are globally important ecosystems that can help mitigate climate change by sequestering carbon (C). The net impact seagrass meadows have on the climate, however, also depends on methane (CH4) and nitrous oxide (N2O) fluxes. By not accounting for CH4 and N2O fluxes, we may be overestimating or underestimating the true C sequestration capacity of seagrasses. Yet, few observations of seagrass CH4 and N2O fluxes are available. Here, we quantified summer, dark/light CH4 and N2O fluxes across the sediment-water interface from seagrass meadows (Zostera marina) and adjacent nonvegetated sediments in two temperate bays with different environmental characteristics. On two occasions, we also estimated system wide air-sea CH4 and N2O fluxes. We found the CH4 fluxes across the sediment-water interface varied between the two sites with one site emitting CH4 from vegetated sediments and a net zero flux at the other site. N2O fluxes across the sediment-water interface were not different from zero regardless of seagrass presence, although when we did measure a flux, there was more often a net uptake of N2O. We estimated that both systems were small net sources of CH4 and N2O to the atmosphere; however, the sediments are not likely the source of CH4 and N2O emitted to the atmosphere in the systems. Although the diffusive fluxes measured here are lower than those reported in the literature, they are consistent with our current understanding of seagrass sediments being variable sources of CH4 and potentially a negligible source or sink of N2O.