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 ), a potent greenhouse gas, can form in the sediments of these ecosystems. Currently, CH emissions are a missing component of VCE C budgets. This review summarizes 97 studies describing CH fluxes from mangrove, salt marsh, and seagrass ecosystems and discusses factors controlling CH flux in these systems. CH 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 m day ). Together CH emissions from mangrove, salt marsh, and seagrass ecosystems are about 0.33-0.39 Tmol CH -C/year-an addition that increases the current global marine CH budget by more than 60%. The majority (~45%) of this increase is driven by mangrove CH fluxes. While organic matter content and quality were commonly reported in individual studies as the most important environmental factors driving CH flux, they were not significant predictors of CH flux when data were combined across studies. Salinity was negatively correlated with CH emissions from salt marshes, but not seagrasses and mangroves. Thus the available data suggest that other environmental drivers are important for predicting CH emissions in vegetated coastal systems. Finally, we examine stressor effects on CH emissions from VCEs and we hypothesize that future changes in temperature and other anthropogenic activites (e.g., nitrogen loading) will likely increase CH emissions from these ecosystems. Overall, this review highlights the current and growing importance of VCEs in the global marine CH budget.