BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-11-5245-2014 Methane and nitrous oxide sources and emissions in a subtropical freshwater reservoir, South East Queensland, Australia Sturm K. 1 Yuan Z. 1 Gibbes B. 2 Werner U. 1 Grinham A. https://orcid.org/0000-0001-8313-2276 1 2 Advanced Water Management Centre (AWMC), The University of Queensland, Level 4, Gehrmann Building, Brisbane, Queensland 4072, Australia School of Civil Engineering, The University of Queensland, Level 5, Advanced Engineering Building, Brisbane, Queensland 4072, Australia 30 09 2014 11 18 5245 5258 Copyright: © 2014 K. Sturm et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/11/5245/2014/bg-11-5245-2014.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/11/5245/2014/bg-11-5245-2014.pdf

Reservoirs have been identified as an important source of non-carbon dioxide (CO<sub>2</sub>) greenhouse gases with wide ranging fluxes for reported methane (CH<sub>4</sub>); however, fluxes for nitrous oxide (N<sub>2</sub>O) are rarely quantified. This study investigates CH<sub>4</sub> and N<sub>2</sub>O 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 CH<sub>4</sub> and N<sub>2</sub>O. Atmospheric CH<sub>4</sub> fluxes were dominated by ebullition (60 to 99%) relative to diffusive fluxes and ranged from 4.14 × 10<sup>2</sup> to 3.06 × 10<sup>5</sup> μmol CH<sub>4</sub> m<sup>−2</sup> day<sup>−1</sup> across the sampling sites. Dissolved CH<sub>4</sub> concentrations were highest in the anoxic water column and sediment pore waters (approximately 5 000 000% supersaturated). CH<sub>4</sub> production rates of up to 3616 ± 395 μmol CH<sub>4</sub> m<sup>−2</sup> day<sup>−1</sup> were found during sediment incubations in anoxic conditions. These findings are in contrast to N<sub>2</sub>O where no production was detected during sediment incubations and the highest dissolved N<sub>2</sub>O concentrations were found in the oxic water column which was 110 to 220% supersaturated with N<sub>2</sub>O. N<sub>2</sub>O 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 CH<sub>4</sub> fluxes with an appropriate spatial resolution to ensure capture of ebullition zones, whereas assessment of N<sub>2</sub>O fluxes should focus on the diffusive pathway.

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