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

Abstract

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 (CH 4 ) and nitrous oxide (N 2 O) fluxes. By not accounting for CH 4 and N 2 O fluxes, we may be overestimating or underestimating the true C sequestration capacity of seagrasses. Yet, few observations of seagrass CH 4 and N 2 O fluxes are available. Here, we quantified summer, dark/light CH 4 and N 2 O 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 CH 4 and N 2 O fluxes. We found the CH 4 fluxes across the sediment-water interface varied between the two sites with one site emitting CH 4 from vegetated sediments and a net zero flux at the other site. N 2 O 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 N 2 O. We estimated that both systems were small net sources of CH 4 and N 2 O to the atmosphere; however, the sediments are not likely the source of CH 4 and N 2 O 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 CH 4 and potentially a negligible source or sink of N 2 O.

Methane emissions in seagrass meadows as a small offset to carbon sequestration

“…This has been observed in other tidally-influenced ecosystems such as mangroves and saltmarshes where higher CH 4 concentration in porewater drives the high surface water CH 4 (Call et al, 2018;Santos et al, 2019;Yau et al, 2022). Flanking saltmarshes adjacent to seagrass export CH 4 , elevating CH 4 flux in the seagrass meadows (Al-Haj et al, 2022). Since our system is not directly influenced by flanking marshes, porewater, and freshwater inputs, the relatively low CH 4 air-sea fluxes likely represent emissions from subtidal seagrass habitats.…”

“…Our results show that sediment-water fluxes do not necessarily represent water-air fluxes. Therefore, we updated earlier compilations (Al-Haj et al, 2022) to differentiate between air-sea (8 sites) and sedimentwater CH 4 (20 sites) fluxes in seagrass meadows (Table 4). Both air-sea and sediment-water et al, 2022; 6 This study; 7 Oremland, 1975;8 Moriarty et al, 1984;9 Barber & Carlson, 1993; et al, 1998;11 Alongi et al, 2008;12 Bahlmann et al, 2015;13 Garcias-Bonet, 2017;…”

“…Although measurements of CH 4 fluxes have been widely performed in mangroves (Call et al, 2019), saltmarshes (Yau et al, 2022), and other coastal ecosystems (Borges & Abril, 2011), CH 4 fluxes in seagrass meadows remain poorly constrained across multiple spatial and temporal scales. The air-sea and sediment-water CH 4 fluxes from seagrass ranged from 0 to 400 µmol m -2 d -1 , resulting in global upscaled fluxes of 0.18 Tg CH 4 per year (Al-Haj et al, 2022). Several seagrass meadow CH 4 flux estimates considered sediment-water fluxes, obtained from benthic chambers and sediment incubation approaches, to be equivalent to air-sea fluxes.…”

Carbon sequestration in aquatic ecosystems: Recent advances and challenges

“…The net carbon sequestration ability of aquatic ecosystems depends not only on the net uptake of CO 2 via primary production, but also on the emissions of the powerful greenhouse gases methane (CH 4 ) and nitrous oxide (N 2 O) (Rosentreter et al 2021). Al‐Haj et al (2022) measured negligible emissions of both CH 4 and N 2 O from seagrass meadows, adding further evidence that seagrass beds are effective at sequestering carbon. Hilt et al (2022) reviewed the mechanisms of methane emissions from macrophytes both in freshwater and marine habitats.…”

Methane Emissions in Seagrass Meadows as a Small Offset to Carbon Sequestration

“…Although measurements of CH 4 fluxes have been widely performed in mangroves (Call et al., 2019), saltmarshes (Yau et al., 2022), and other coastal ecosystems (Borges & Abril, 2011), CH 4 fluxes in seagrass meadows remain poorly constrained across multiple spatial and temporal scales. The sea‐air and sediment‐water CH 4 fluxes from seagrass ranged from 0 to 400 μmol m −2 d −1 , resulting in global upscaled fluxes of 0.18 Tg CH 4 per year (Al‐Haj et al., 2022). Several seagrass meadow CH 4 flux estimates considered sediment‐water fluxes, obtained from benthic chambers and sediment incubation approaches, to be equivalent to sea‐air fluxes.…”