Methane offsets to carbon sequestration in coastal ecosystems

Mangroves, saltmarshes, and seagrasses are known as blue carbon ecosystems because of their high rates of productivity and efficiency in sequestering carbon. Carbon can be stored in sediments, biomass, exported laterally to the coastal ocean, or returned to the atmosphere as carbon dioxide (CO2) and methane (CH4). CH4 is a greenhouse gas with global warming potential 34-86 times higher than CO2 on a mass basis. CH4 is often produced in anoxic, carbon-rich sediments during carbon degradation. CH4 emissions to the atmosphere potentially offset some of the carbon sequestration in blue carbon ecosystems. A broad literature compilation showed that aquatic CH4 fluxes may reduce their net sediment carbon sequestration by up to 7000% in saltmarshes, 3500% in mangroves, and 900% in seagrass meadows. These earlier observations have great spatial variability and large uncertainties, and often do not separate local methane from upstream freshwater sources. In this thesis, I performed high resolution continuous dissolved CH4 measurements in surface water from various coastal marine ecosystems in Brazil, Spain, China, and Iceland. I estimated sea-air CH4 fluxes and evaluated the biogeochemical processes driving the production and oxidation of CH4. The investigated coastal vegetated ecosystems inputs acted as a net CH4 source (mangroves: 75 ± 190 µmol m-2 d-1; saltmarsh: 3.0 ± 2.0 µmol m-2 d-1; seagrass: 0.1 ± 0.1 µmol m-2 d-1). Tidally-driven porewater exchange was an important source of CH4 in mangroves and saltmarshes, but not in seagrasses. The lack of major external freshwater inputs led to relatively small methane fluxes in the investigated blue carbon ecosystems. Overall, my observations revealed that water-air CH4 emissions offset only 1 to 8% of the sediment carbon sequestration rates in these marine ecosystems, underscoring the important role of coastal vegetated ecosystem as a carbon sink.