A synthesis of methane emissions from shallow vegetated coastal ecosystems

Abstract

Vegetated coastal ecosystems (VCEs; i.e., mangroves, salt marshes, and seagrasses) play a critical role in global carbon (C) cycling, storing 10× more C than temperate forests. Methane (CH 4 ), a potent greenhouse gas, can form in the sediments of these ecosystems. Currently, CH 4 emissions are a missing component of VCE C budgets. This review summarizes 97 studies describing CH 4 fluxes from mangrove, salt marsh, and seagrass ecosystems and discusses factors controlling CH 4 flux in these systems. CH 4 fluxes from these ecosystems were highly variable yet they all act as net methane sources (median, range; mangrove: 279.17, −67.33 to 72,867.83; salt marsh: 224.44, −92.60 to 94,129.68; seagrass: 64.80, 1.25-401.50 µmol CH 4 m −2 day −1 ). Together CH 4 emissions from mangrove, salt marsh, and seagrass ecosystems are about 0.33-0.39 Tmol CH 4 -C/year-an addition that increases the current global marine CH 4 budget by more than 60%. The majority (~45%) of this increase is driven by mangrove CH 4 fluxes. While organic matter content and quality were commonly reported in individual studies as the most important environmental factors driving CH 4 flux, they were not significant predictors of CH 4 flux when data were combined across studies. Salinity was negatively correlated with CH 4 emissions from salt marshes, but not seagrasses and mangroves. Thus the available data suggest that other environmental drivers are important for predicting CH 4 emissions in vegetated coastal systems. Finally, we examine stressor effects on CH 4 emissions from VCEs and we hypothesize that future changes in temperature and other anthropogenic activites (e.g., nitrogen loading) will likely increase CH 4 emissions from these ecosystems. Overall, this review highlights the current and growing importance of VCEs in the global marine CH 4 budget. K E Y W O R D S global carbon budget, mangrove, methanogenesis, methanotrophy, salt marsh, seagrass, synthesis | 2989 AL-HAJ And FULWEILER About two-thirds of the total global CH 4 emissions can be attributed to human activity (e.g., agriculture, sewage and landfill waste, fossil fuel consumption; Saunois et al., 2016). Vegetated coastal ecosystems (VCEs; i.e., mangroves, salt marshes, and seagrasses) play a critical role in global carbon (C) cy-S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section. How to cite this article: Al-Haj AN, Fulweiler RW. A synthesis of methane emissions from shallow vegetated coastal ecosystems. Glob Change Biol. 2020;26:2988-3005. https://

Plant species determine tidal wetland methane response to sea level rise

“…The difference between lowest and highest mean CH 4 emissions was 31 g CH 4 m −2 year −1 (Fig. 2a), corresponding to c. 6% of the total range of CH 4 emissions reported for tidal marshes globally 7 and to c. 95% of the total range reported for differences between meso-and polyhaline tidal marshes based on the salinity-CH 4 model of Poffenbarger et al 6 . We furthermore show strong positive effects of elevated CO 2 which increased CH 4 emissions an amount similar to sea level effects.…”

“…Lower CH 4 emissions in tidal vs. nontidal wetlands result from higher soil concentrations of sulfate, which acts as a terminal electron acceptor and allows sulfate-reducing bacteria to outcompete methanogenic communities for electron donors 5 , 6 . Site salinity, a proxy for sulfate availability, is the best-established predictor of CH 4 emissions from tidal wetlands, but it weakly constrains emission rates 6 , 7 . Overall, CH 4 emissions from tidal wetlands are extremely variable, and many sites emit CH 4 at rates that exceed C sequestration in terms of CO 2 equivalents 2 , 8 , 9 .…”

“…Overall, CH 4 emissions from tidal wetlands are extremely variable, and many sites emit CH 4 at rates that exceed C sequestration in terms of CO 2 equivalents 2,8,9 . Drivers of variability in CH 4 emissions other than sulfate are poorly understood 7,10 . Only few case studies have elucidated other important drivers of CH 4 emissions, such as sedimentation dynamics 11 , organic matter quality and quantity 7 , tidal pumping 12 , and functional trait composition of plant communities [13][14][15] .…”

Ideas and perspectives: A strategic assessment of methane and nitrous oxide measurements in the marine environment

“…Such data are important because for example, the magnitude of CH 4 and N O fluxes vary over a diel period depending on the redox environment as a result of tidal effects and changes in inorganic N and O 2 availability (Seitzinger and Kroeze, 1998;Call et al, 2015;Vieillard and Fulweiler, 2014;Maher et al, 2015;Murray et al, 2015;Foster and Fulweiler, 2019). The magnitude of CH 4 and N 2 O fluxes also varies over longer temporal scales (seasonally to yearly) due to additional factors such as groundwater inputs, adjacent land-use, dissolved O 2 , organic matter content and quality, and macrofaunal distributions (Barnes and Upstill-Goddard, 2011; Upstill-Goddard and Barnes, 2016;Gelesh et al, 2016;Bonaglia et al, 2017;Borges et al, 2018;Wells et al, 2018;Ray et al, 2019;Al-Haj and Fulweiler, 2020;Reading et al, 2020). To determine the contributing factors and resolve the spatial distributions, mobile sampling platforms such as small vessels (Müller et al, 2016;Brase et al, 2017;Tait et al, 2017), and autonomous vehicles (Manning et al, 2019) are essential.…”

Corrigendum

“…In the paper by Al‐Haj and Fulweiler (2020), global warming potential (GWP) and sustained flux global warming potential (SGWP) were calculated incorrectly. GWP should be calculated using gas emissions in gas mass units (Neubauer & Megonigal, 2015).…”