Methane and Nitrous Oxide Emissions to the Atmosphere are Low from Two Temperate Seagrass Dominated Ecosystems

Seagrass meadows play an important role in the coastal carbon (C) cycle, as they sequester more C than unvegetated areas. However, to determine the true C storage potential for seagrass ecosystems, their greenhouse gas (i.e., methane: CH4­ and nitrous oxide: N2O) emissions must be quantified. The characteristics (e.g., organic matter rich and anoxic sediments) that make these habitats good at storing C also create ideal conditions for CH4 and N2O production. Yet there is a paucity of data quantifying CH4 and N2O emissions from seagrasses. Thus, the true C budgets of seagrasses are largely unconstrained. Here we used in situ benthic chambers to quantify summer, dark/light CH4 and N2O fluxes across the sediment-water interface from seagrass meadows (Zostera marina) and adjacent bare sediments in two coastal bays on the northeast coast of the US (Cape Cod, MA). In the summer of 2019, we also used the discrete water sampling method to determine system wide air-sea CH4 and N2O fluxes. We hypothesized that vegetated areas would be sources of CH4 and N2O to the water column. We found that CH4 fluxes were 10X higher in Z. marina meadows compared to bare sediments (mean ± se: 1.94 ± 0.43 µmol CH4 m-2 h-1 and 0.19 ± 0.16 µmol CH4 m-2 h-1, respectively) and that light and dark CH4 fluxes were not significantly different. N2O fluxes were very low (mean ± se: -0.005 ± 0.010 µmol N2O m-2 h-1) and did not differ across vegetated and non-vegetated areas or across light and dark treatments. CH4 fluxes to the atmosphere were low in both systems indicating that CH4 consumption through the water column plays a potentially important role in very shallow (< 5 m depth) seagrass ecosystems. N2O emissions from the water column to atmosphere were also low suggesting that N2O emissions are not important in these systems. We will present the two-year data set as well as an analysis of environmental drivers. Additionally, we will place our findings into a larger context of CH4 and N2O emissions from vegetated coastal areas.