Methane (CH4) and nitrous oxide (N2O) are two atmospheric trace gases that have attracted great scientific attention. They are long-lived potent greenhouse gases (GHG) with respective global warming potentials (GWP) of 25 and 300 times that of carbon dioxide (CO2) on a 100-year time horizon. Nitrous oxide is also the single strongest Ozone depleting substance. The atmospheric mixing ratio of CH4 has nearly tripled its pre-industrial level (700 ppb) while N2O has also reached ~120% its pre-industrial level (270 ppb). Of more concern is that the atmospheric concentration of both gases is still on the rise yet the sources are poorly known and quantified. Aquatic systems are likely significant sources but uncertainty surrounds their available emission estimates. Moreover, there is eminent paucity of field measurement data especially for subtropical and tropical systems in the southern latitudes.
This thesis work investigated CH4 and N2O emissions in three distinct subtropical aquatic systems in Southeast Queensland: bays – Bramble Bay and Deception Bay which are part of the greater Moreton Bay, estuary – the Brisbane River estuary and freshwater reservoirs – Lake Wivenhoe (LW), Lake Baroon (LB) and Little Nerang Dam (LND). The study had four main objectives. The first three dealt with assessment of emissions in the selected systems while the fourth was to investigate methane dynamics in the reservoirs. The study was designed to adequately capture both spatial and temporal variability of emissions. Fluxes were estimated using the Thin Boundary Layer (TBL) approach with the gas transfer velocity, k, estimated using six widely used wind-based and four current-based models.
Both investigated bays were strong sources of CH4 and N2O during all seasons with no significant differences of the degree of saturation in the two bays. Both CH4 and N2O concentrations had strong temporal but minimal spatial variability. CH4 varied between 500 and 4,000% saturation while N2O varied between 128 and 255%. Average seasonal CH4 fluxes varied between 0.5±0.2 and 6.0±1.5 mg CH4 m-2 d-1 while N2O varied between 0.4±0.1 and 1.6±0.6 mg N2O m-2 d-1. The estuary is also a strong source of CH4 and N2O all-year-round. CH4 saturation was between 1,210 and 26,430% while N2O varied between 135 and 435% saturation. The degree of N2O saturation significantly increased with nitrate and nitrite (NOx) concentrations (p<0.01, r2=0.55). These results compare well with measurements from tropical estuarine systems. Both CH4 and N2O concentrations increased upstream the estuary indicating likelihood of higher production and/ or inputs in the upper and middle reaches. Although strong temporal variability was observed, seasonal patterns were unclear and were conceivably distorted by the summer 2011 floods. i Seasonal fluxes ranged from 0.5±0.2 to 1.0±0.4 mg N2O m-2 d-1 and 2.7±0.7 to 9.2±5.8 mg CH4 m-2 d-1 while station estimated fluxes varied between 0.3 - 3.4 mg N2O m-2 d-1 and 3.4 - 27.9 mg CH4 m- 2 d-1. Seasonal emissions (t CO2-e) were 63 - 90% and 40 – 80% N2O dominated in the bays and estuary respectively implying that a reduction in N2O inputs and/ or nitrogen availability in both systems would significantly reduce their GHG budgets.
All reservoirs are strong sources of CH4 in all seasons. Surface water CH4 saturation varied between 1,350 and 524,000% and was overall highest in spring and summer, and lowest in winter. No clear temporal patterns common to all reservoirs were, however, observed. N2O saturation varied between 70 and 1,230% with all reservoirs switching from weak sinks in spring to strong sources the rest of the year. There were significant differences for CH4 concentrations and fluxes amongst reservoirs. Within each reservoir, CH4 saturation and fluxes were higher in inflow zones than in the main body but N2O variability was minimal. Area-weighted seasonal fluxes ranged between 2.3 and 20.5 mg CH4 m-2 d- 1 and 0.03 – 0.41 mg N2O m-2 d-1 and annual emissions were 67 to 86% CH4 dominated. The measured degree of CH4 saturation and fluxes are among the highest so far reported in scientific literature indicating that subtropical freshwater reservoirs are likely significant aquatic GHG sources. For all systems, the choice of models/ model combinations for estimation of k constituted huge uncertainty in total emissions highlighting the dire need for harmonisation of existing or development of new and improved k parameterisations.
Water column CH4 concentration varied between 0.03 - 361 µM and was highest in the strongly stratified LND in a more sheltered landscape with a high organic matter input from the Springbrook forest reserve, intermediate in LB with a is partly forested catchment and lowest in the relatively well-mixed LW in a more open grassland landscape. Highest concentrations were measured in summer and spring when the reservoirs were strongly stratified and more favourable for methanogenesis and lowest in winter when the reservoirs were more mixed with deeper dissolved oxygen penetration that conceivably enhanced water column methanotrophy. Methane concentrations were always high in the anoxic water column and often formed minima at the oxycline. Moderate CH4 peaks, that likely increase diffusive CH4 fluxes, consistently formed in the oxic subsurface water in all reservoirs. Microbial community analysis found methanotrophs throughout the water column but methanogenic Archaea mainly proliferated below the oxycline consistent with the measured CH4 profiles. A small population of Archaea, of the order Methanosarcinales, was found in the oxic subsurface water but their possible contribution to the CH4 peaks in this zone remains to be elucidated.