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 CH4and 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 CH4and N2O. Atmospheric CH4fluxes were dominated by ebullition (60 to 99%) relative to diffusive fluxes and ranged from 4.14 × 102to 3.06 × 105μmol CH4mg-2dayg-1across the sampling sites. Dissolved CH4concentrations were highest in the anoxic water column and sediment pore waters (approximately 5 000 000% supersaturated). CH4production rates of up to 3616 ± 395 μmol CH4mg-2dayg-1were 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 CH4fluxes with an appropriate spatial resolution to ensure capture of ebullition zones, whereas assessment of N2O fluxes should focus on the diffusive pathway.