Floods release greenhouse gases from wetlands

Scientists from Southern Cross University (SCU) have discovered that flood events dramatically increase the release of greenhouse gases from coastal floodplains into the atmosphere where the landscape’s hydrology has been modified by human activity.

Writing in the Journal of Geophysical Research, the study authors said, “Floods frequently produce deoxygenation and acidification in waters of artificially drained coastal acid sulfate soil (CASS) wetlands. These conditions are ideal for carbon dioxide and methane production.”

PhD student Jackie Gatland, from the university’s School of Environment, Science and Engineering, explained, “Natural wetlands store large amounts of carbon; however, artificial drainage of wetlands modifies the carbon balance.

“A flood event in February 2013 in Rocky Mouth Creek at Woodburn stimulated huge losses of carbon from the wetland. Most of this carbon was lost in the form of carbon dioxide and, more importantly, methane - an extremely potent greenhouse gas.

“Drains, which are widespread throughout the catchment, accelerated water loss from the wetland into the creek, which resulted in an overflow of decomposing vegetation and some of the highest carbon dioxide and methane levels ever observed in waterways.”

The study was the first research looking at the impact of flooding on carbon dioxide (CO₂) and methane (CO₄) in CASS wetlands, which are widespread along the Australian coastline. The scientists said they “investigated CO₂ and CO₄ dynamics and quantified carbon loss within an artificially drained CASS wetland during and after a flood”.

“CO₂ and CH₄ emissions from the floodplain to the atmosphere were 17-fold and 170-fold higher during the flooded period compared to the post-flood period, respectively,” the authors revealed. “CO₂ emissions accounted for about 90% of total floodplain mass carbon losses during both the flooded and post-flood periods. Assuming a 20- and 100-year global warming potential (GWP) for CH₄ of 105 and 27 CO₂-equivalents, CH₄ emission contributed to 85% and 60% of total floodplain CO₂-equivalent emissions, respectively.”

Dr Damien Maher, from the university’s Centre for Coastal Biogeochemistry Research (CCBR), said the study has widespread implications on how much carbon wetlands could sequester. He noted, “The intensity and frequency of floods is predicted to increase due to climate change, and our study shows that more floods will likely provide a positive feedback to global warming.”

Associate Professor Isaac Santos, also from the CCBR, said modifying the hydrology of wetlands seemed to decrease their ability to retain carbon in the soil; therefore, restoring hydrological function of drained wetlands may be important from water quality, greenhouse gas and carbon accounting perspectives.