All greenhouse gas (CH4, CO2, and N2O) fluxes and associated in-chamber environmental parameters (sediment and vegetation C:N, sediment and vegetation moisture, and light.

Vegetated coastal ecosystems (i.e., salt marshes, mangroves, and seagrasses) provide a range of important ecosystems services including significant carbon storage. These ecosystems also export carbon during senescence, however the impact of this carbon export on coastal greenhouse gas budgets remains largely unconstrained. Here we examined the impact of seagrass organic matter deposition on a sandy, temperate beach. We hypothesized that this natural seagrass amendment would increase greenhouse gas fluxes compared to background (i.e., bare sandy beach). To test this hypothesis, we measured methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) fluxes using chamber incubations across three substrate cover types (bare sediment, wrack and sediment, or wrack alone) on a sandy beach near <i>Zostera marina</i> meadows. We also measured environmental parameters to identify potential drivers of GHG emissions from the beach. CO₂, CH_4,and N₂O fluxes did not differ across cover types, with average values of 2,910.73 ± 329.38 µmol CO₂ m^-2 h^-1, 0.33 ± 0.14 µmol CH₄ m^-2 h^-1, and 0.22. ± 0.16 µmol N₂O m^-2 h^-1. CH₄ and N₂O fluxes from seagrass wrack were relatively low when compared to literature values from living seagrass meadows, while CO₂ fluxes were on par with previously reported values. In contrast to our hypothesis, environmental factors, such as sediment moisture and air temperature, played a larger role in driving GHG fluxes than the presence of wrack. This work suggests that seagrass wrack likely plays a minimal role in temperate seagrass meadow GHG budgets.