Mangroves have been considered for a long time to be a minor methane source, but recent reports have shown that polluted mangroves may emit substantial amounts of methane. In an unpolluted Indian mangrove, we measured annual methane emission rates of 10 g CH4 yr−1 from the stands of Avicennia marina. This rate is of the same order of magnitude as rates from Northern wetlands. Methane emission from a freshwater-influenced area was higher, but was lower from a stunted mangrove growing on a hypersaline soil. Methane emission was mediated by the pneumatophores of Avicennia. This was consistent with the methane concentration in the aerenchyma, which decreased on average from 350 ppmv in the cable roots to 10 ppmv in the emergent part of the pneumatophores. However, the number of pneumatophores varied seasonally. The minimum number occurred during the monsoon season, which reduced methane emissions largely. Ebullition from unvegetated areas may also be important, at least during monsoon season when measured bubble fluxes were occasionally about five times as high as pneumatophore-mediated emissions.
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