Methane Fluxes from Temperate Eelgrass Meadows

Seagrass meadows play an important role in the marine carbon (C) cycle as they sequester more C than unvegetated areas. However, there is little information about C lost from these ecosystems in the form of methane (CH₄). Yet seagrass meadows are ideal locations for methane formation to occur because their sediments are rich in organic matter and anoxic. To date, the limited number of studies measuring seagrass methane cycling have found that seagrass ecosystems emit methane to the atmosphere, and few have explored environmental drivers of this emission. Here we determine sediment-water column and air-sea methane flux as well as potential environmental drivers of methane emissions from two seagrass (Zostera marina) meadows and adjacent unvegetated areas on the northeast coast of the US (Cape Cod, MA). We used in situ benthic chambers to determine sediment-water column methane fluxes in the dark and light and the discrete water sampling method to determine air-sea methane fluxes during summers 2018 and 2019. We also measured potential environmental drivers (e.g., sediment organic matter content, dissolved inorganic nutrient concentrations). We hypothesized that these sites would be sources of methane to the atmosphere, but that vegetated locations would have higher sediment-water column methane fluxes than unvegetated areas, and that sediment organic matter content would drive methane emissions. During summer 2018, we found that Z. marina meadows emitted 3.5 times more methane from the sediment into the water column than unvegetated areas (Z. marina: 53.7 µmol CH₄ m^-2 d^-1, unvegetated: 15.5 µmol CH₄ m^-2 d^-1; p = 0.026). These initial findings indicate that while temperate seagrasses are a net source of methane to the atmosphere, seagrasses are likely still a net sink of C in the temperate coastal zone. We will present the full two year data set as well as an analysis of environmental drivers. Additionally, we will place our findings into a larger context of methane emissions from vegetated coastal areas.