Floods increase carbon dioxide and methane fluxes in agricultural streams

Changes in climate are causing floods to occur more often and more intensely in many parts of the world, including agricultural landscapes of southern Wisconsin (U.S.A.). How flooding and greater flood frequency affect stream carbon dioxide (CO2) and methane (CH4) fluxes and concentrations is not obvious. Thus, we asked how diffusive fluxes of CO(2)and CH(4)varied over time, particularly in response to floods, in agricultural streams, and what were likely causes for observed flood responses. We measured concentrations and diffusive fluxes of CO(2)and CH(4)at 10 stream sites in mixed agricultural and suburban catchments in southern Wisconsin (U.S.A.) during the growing season (March-November) in a year that experienced multiple floods. Habitat, hydrologic, and water chemistry attributes were also quantified to determine likely drivers of changes in gas concentrations and fluxes. Habitat and water chemistry, as well as CO(2)and CH(4)concentrations and fluxes were temporally erratic and lacked any seasonality. Carbon dioxide and CH(4)concentrations and fluxes were higher during floods along with increased water velocity, turbidity, and dissolved organic carbon and decreases in dissolved oxygen, soft sediment depth, and macrophyte cover. Increased gas concentrations and fluxes were probably due to flushing of gases from soils, respiration of organic matter in the channel, and increased gas exchange velocities during floods. Flooding alleviated both supply and transfer limits on CO(2)and CH(4)emissions in these agricultural streams, and frequent and prolonged flooding during the growing season led to sustained high emissions from these streams. We hypothesise that such persistent increases in emissions during floods may be a common response to high precipitation periods for many agricultural streams.

Automated summary

Climate change leads to more frequent and intense floods in agricultural regions of southern Wisconsin. During floods, concentrations and fluxes of CO2 and CH4 in streams are higher, likely due to flushing of gases from soils and respiration of organic matter.