Methane and nitrous oxide sources and emissions in a subtropical freshwater reservoir, South East Queensland, Australia

Abstract. Reservoirs have been identified as an important source of non-carbon dioxide (CO₂) greenhouse gases with wide ranging fluxes for reported methane (CH₄); however, fluxes for nitrous oxide (N₂O) are rarely quantified. This study investigates CH₄ and N₂O sources and emissions in a subtropical freshwater Gold Creek Reservoir, Australia, using a combination of water–air and sediment–water flux measurements and water column and pore water analyses. The reservoir was clearly a source of these gases as surface waters were supersaturated with CH₄ and N₂O. Atmospheric CH₄ fluxes were dominated by ebullition (60 to 99%) relative to diffusive fluxes and ranged from 4.14 × 10² to 3.06 × 10⁵ μmol CH₄ m⁻² day⁻¹ across the sampling sites. Dissolved CH₄ concentrations were highest in the anoxic water column and sediment pore waters (approximately 5 000 000% supersaturated). CH₄ production rates of up to 3616 ± 395 μmol CH₄ m⁻² day⁻¹ were found during sediment incubations in anoxic conditions. These findings are in contrast to N₂O where no production was detected during sediment incubations and the highest dissolved N₂O concentrations were found in the oxic water column which was 110 to 220% supersaturated with N₂O. N₂O fluxes to the atmosphere were primarily through the diffusive pathway, mainly driven by diffusive fluxes from the water column and by a minor contribution from sediment diffusion and ebullition. Results suggest that future studies of subtropical reservoirs should monitor CH₄ fluxes with an appropriate spatial resolution to ensure capture of ebullition zones, whereas assessment of N₂O fluxes should focus on the diffusive pathway.