A synthesis of methane emissions from shallow vegetated coastal ecosystems

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.