Methanogens in Tanzanian mangrove sediments along Dar es Salaam coast

Methane is known to be produced in anaerobic aquatic habitats including the mangrove sediments. Some of the gas is oxidized by methanotrophs and the rest escapes to the atmosphere. A number of aquatic plants have been shown to facilitate methane oxidation by creating an aerobic rhizosphere within the anaerobic sediments. The extent of methane emission and oxidation in tropical mangrove sediments, and the role played by mangrove pneumatophores in reducing the rate of methane emission are yet to be established. This study therefore estimated in situ methane emission rates by mangrove sediments from impacted (cleared cut) mangrove trees and non-impacted sites (intact) mangrove trees. The in situ rates were compared to ex situ production rates. 'nie organic matter content in the sediments ranged from 4 to 28 % and methanogens counts ranged from I x 104 to 7.4 x 106 cells/gram fresh weights of sediments. Methane emission rates varied considerably between and among sampling sites and were higher during the rainy season than the dry season. The highest in situ emission rate (83 1 . 8 µmoles/m2/h) was measured flom an impacted site with the highest organic matter content and the lowest rate (0.22 µmoles/m2/h) measured in sediments from a non-impacted site with the lowest organic matter content. There was no correlation between methane emission rates and mangrove species. Ex situ methane oxidation rates ranged between 0.9 and 71.6 nmoles/gfw/h. These rates vary heterogeneously between and among sampled yeas. Generally clear cut areas had higher oxidation rates than intact areas due to the fact that mangrove clear cutting reduces rhizosphere oxygenation through pneumatophores and thus favour the methanogenic process (substrate availability for methanotrophs). The lower methane emission in intact areas (with pneumatophores) indicated that the gas was oxidized in the rooted (rhizosphere) zone.