Spatio‐temporal variation of gross CO₂ and CH₄ diffusive emissions from Australian reservoirs and natural aquatic ecosystems, and estimation of net reservoir emissions

Abstract

Carbon dioxide (CO 2 ) and methane (CH 4 ) diffusive emissions were measured during two field surveys in Queensland and Tasmania, Australia, using the floating chamber method. Bubbling and degassing emissions in 2010 were estimated in Koombooloomba Dam reservoir using only inverted funnels and gas concentrations, respectively. A total of 14 reservoirs and 16 rivers and lakes were sampled from 2006 to 2010. Spatial variation was substantial within each water body, as well as between them. The main drivers of diffusive emission variation were physiographic region and climate, with a clear demarcation being observed between diffusive emissions from tropical Queensland and temperate Tasmania, and between the humid West Coast Range (Tasmania) and dry Central Plateau (Tasmania). Higher CO 2 and CH 4 diffusive emissions were observed during the dry season, when long water residence times would promote organic matter degradation. Estimated total gross emissions, including diffusive, bubbling and degassing emissions, for Koombooloomba Dam reservoir were about 1.5 9 10 6 t CO 2 eq km 2 per year, or 24 9 10 6 t CO 2 eq per year. This corresponds to a plant emission factor of 3.18 kg CO 2 eq MWh À1 . Using an estimate of terrestrial emissions derived from literature data for the Tully River catchment area, rough estimated net emissions from the catchment area are about 44 kt CO 2 eq per year, or 5.83 kg CO 2 eq MWh À1 , which is in the lower range of the studied reservoirs.

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

“…Fewer studies conducted worldwide have analysed the contribution of N 2 O to GHG emissions from reservoirs (Guerin et al, 2008;Mengis et al, 1997;Tremblay et al, 2005) despite N 2 O having a higher GWP than CH 4 . There are currently only two studies (Bastien and Demarty, 2013;Grinham et al, 2011) reporting CH 4 emissions and none for N 2 O from reservoirs in Australia -a country with over 2300 reservoirs covering a surface area in excess of 5700 km 2 at full supply (Geoscience Australia, 2004).…”

“…These reservoirs enable storage and greater certainty of supply compared to river and groundwater sources in Australia. In reservoirs without continuous water release, the primary CH 4 emission pathways to the atmosphere are ebullition from sediments, diffusion over the water-air interface and plant-mediated transport from littoral zones (Bastviken et al, 2004). Ebullition has been shown to be the dominant CH 4 emission pathway in many tropical systems (DelSontro et al, 2011;Devol et al, 1988;Grinham et al, 2011;Joyce and Jewell, 2003;Keller and Stallard, 1994;Soumis et al, 2005).…”

“…Ebullition has been shown to be the dominant CH 4 emission pathway in many tropical systems (DelSontro et al, 2011;Devol et al, 1988;Grinham et al, 2011;Joyce and Jewell, 2003;Keller and Stallard, 1994;Soumis et al, 2005). Factors controlling CH 4 ebullition in lake systems are relatively well known (Bastviken et al, 2004;Joyce and Jewell, 2003;Ortiz-Llorente and Alvarez-Cobelas, 2012); however, the dynamics and the spatial distribution of ebullition are not well understood (DelSontro et al, 2011;Ostrovsky et al, 2008;Ramos et al, 2006). CH 4 is typically produced by the process of methanogenesis under anoxic conditions (Canfield et al, 2005) as found in the sediment and hypolimnetic zones of a reservoir.…”

The carbon footprint of a Malaysian tropical reservoir: measured versus modelled estimates highlight the underestimated key role of downstream processes

“…3). Given the very small isotopic fractionation (0.9992) of CH 4 during gas evasion (Knox et al, 1992), the only process that can explain the observed δ 13 CH 4 increase is CH 4 oxidation (Bastviken et al, 2002;Thottathil et al, 2018). We estimated that riverine CH 4 oxidation ranged from 0.38 to 1.80 mmol m −2 d −1 (expressed per squared metre of reservoir area for comparison), transforming 18 % to 32 % (depending on the sampling campaign) of the CH 4 to CO 2 within the first 19 km of the outflow.…”

The carbon footprint of a Malaysian tropical reservoir: measured versus modeled estimates highlight the underestimated key role of downstream processes

“…While diffusion is the most frequently sampled, it is often not the main emission pathway (Demarty and Bastien, 2011). Indeed, measured ebullition, degassing, and downstream emissions range from negligible to several order of magnitude higher than diffusion in different reservoirs (Bastien and Demarty, 2013;DelSontro et al, 2010;Galy-Lacaux et al, 1997;Keller and Stallard, 1994;Kemenes et al, 2007;Teodoru et al, 2012;Venkiteswaran et al, 2013), making it a challenge to model total reservoirs GHG emissions.…”