LAUSR

Vegetated coastal ecosystems (i.e., mangroves, salt marshes, and seagrasses) play a critical role in global carbon (C) cycling, storing 10x more C than temperate forests. Methane (CH4 ), a potent greenhouse gas, can form in the sediments of these ecosystems. Currently, CH4 emissions are a missing component of vegetated coastal ecosystem C budgets. This review summarizes 97 studies describing CH4 fluxes from mangrove, salt marsh, and seagrass ecosystems and discusses factors controlling CH4 flux in these systems. CH4 fluxes from these ecosystems were highly variable yet they all act as net methane sources (median, range; mangrove: 279.17, -67.33 - 72867.83; salt marsh: 224.44, -92.60 - 94129.68; seagrass: 64.80, 1.25 - 401.50 µmol CH4 m-2 d-1 ). Together CH4 emissions from mangrove, salt marsh, and seagrass ecosystems is about 0.33 to 0.39 Tmol CH4 -C yr-1 - an addition that increases the current global marine CH4 budget by more than 60%. The majority (~45%) of this increase is driven by mangrove CH4 fluxes. While organic matter (OM) content and quality was commonly reported in individual studies as the most important environmental factor driving CH4 flux, when data were combined across studies, it was not a significant predictor of CH4 flux. Salinity was negatively correlated with CH4 emissions from salt marshes, but not seagrasses and mangroves. Thus the available data suggest that other environmental drivers are important for predicting CH4 emissions in vegetated coastal systems. Finally, we examine stressor effects on CH4 emissions from vegetated coastal ecosystems and we hypothesize that future changes in temperature and other anthropogenic activites (e.g., nitrogen loading) will likely increase CH4 emissions from these ecosystems. Overall, this review highlights the current and growing importance of vegetated coastal ecosystems in the global marine CH4 budget.