Methane and nitrous oxide emissions from a subtropical coastal embayment (Moreton Bay, Australia)

Surface water methane (CH4) and nitrous oxide (N2O) concentrations and fluxes were investigated in two subtropical coastal embayments (Bramble Bay and Deception Bay, which are part of the greater Moreton Bay, Australia). Measurements were done at 23 stations in seven campaigns covering different seasons during 2010-2012. Water-air fluxes were estimated using the Thin Boundary Layer approach with a combination of wind and currents-based models for the estimation of the gas transfer velocities. The two bays were strong sources of both CH4 and N2O with no significant differences in the degree of saturation of both gases between them during all measurement campaigns. Both CH4 and N2O concentrations had strong temporal but minimal spatial variability in both bays. During the seven seasons, CH4 varied between 500% and 4000% saturation while N2O varied between 128 and 255% in the two bays. Average seasonal CH4 fluxes for the two bays varied between 0.5 +/- 0.2 and 6.0 +/- 1.5 mg CH4/(m(2).day) while N2O varied between 0.4 +/- 0.1 and 1.6 +/- 0.6 mg N2O/(m(2).day). Weighted emissions (t CO2-e) were 63%-90% N2O dominated implying that a reduction in N2O inputs and/or nitrogen availability in the bays may significantly reduce the bays' greenhouse gas (GHG) budget. Emissions data for tropical and subtropical systems is still scarce. This work found subtropical bays to be significant aquatic sources of both CH4 and N2O and puts the estimated fluxes into the global context with measurements done from other climatic regions. (C) 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences

Methane dynamics in subtropical freshwater reservoirs and the mediating microbial communities

Methane (CH4) is a potent greenhouse gas of primarily biogenic origin. Aquatic systems are significant CH4 sources but uncertainty surrounds their estimated source strengths. There is limited understanding of CH4 concentrations variability in both space and time (dynamics) and the responsible drivers. Water column CH4 dynamics were investigated in three subtropical freshwater reservoirs in Southeast Queensland, Australia, during four seasons in 2011-2012. The reservoirs (Little Nerang Dam-LND, Lake Baroon-LB and Lake Wivenhoe-LW) are of different sizes and characteristics. Overall, CH4 concentrations were significantly higher in the strongly stratified LND (0.03-361 A mu M) with a more forested catchment and sheltered landscape, intermediate in LB (0.05-220 A mu M) and lowest in the relatively well-mixed LW (0.04-150 A mu M) in a more open grassland landscape. CH4 concentrations were highest in summer and spring when the reservoirs were strongly stratified and lowest in winter when the reservoirs were more mixed with deeper dissolved oxygen penetration. Concentrations were also highest in the anoxic water column and often had minima at the oxycline when stratification occurred. Highest CH4 concentrations were predominantly at the sediment-water interface but there were indications of high CH4 production within the anoxic water column in both LB and LND. Moderate CH4 peaks were consistently observed in the oxic subsurface water in all reservoirs likely increasing diffusive CH4 fluxes. DO was a strong proxy for methanotrophy while methanogenesis was best indicated by the oxidation-reduction potential of the water column. Microbial community analysis revealed methanotrophs throughout the water column but methanogenic Archaea mainly proliferated below the oxycline. This is the first comprehensive field-based study towards understanding of CH4 dynamics in subtropical freshwater systems

Methane and nitrous oxide emissions from a subtropical estuary(the Brisbane River estuary, Australia)

Methane (CH4) and nitrous oxide (N2O) are two key greenhouse gases. Their global atmospheric budgeting is, however, flout with challenges partly due to lack of adequate field studies determining the source strengths. Knowledge and data limitations exist for subtropical and tropical regions especially in the southern latitudes. Surface water methane and nitrous oxide concentrations were measured in a subtropical estuarine system in the southern latitudes in an extensive field study from 2010 to 2012 and water-air fluxes estimated using models considering the effects of both wind and flow induced turbulence. The estuary was found to be a strong net source of both CH4 and N2O all-year-round. Dissolved N2O concentrations ranged between 9.1 +/- 0.4 to 4.53 +/- 1.3 nM or 135 to 435% of atmospheric saturation level, while CH4 concentrations varied between 31.1 +/- 3.7 to 578.4 +/- 58.8 nM or 1210 to 26,430% of atmospheric saturation level. These results compare well with measurements from tropical estuarine systems. There was strong spatial variability with both CH4 and N2O concentrations increasing upstream the estuary. Strong temporal variability was also observed but there were no clear seasonal patterns. The degree of N2O saturation significantly increased with NOx concentrations (r(2) = 0.55). The estimated water-air fluxes varied between 0.1 and 3.4 mg N2O m(-2) d(-1) and 0.3 to 27.9 mg CH4 m(-2) d(-1). Total emissions (CO2-e) were N2O (64%) dominated, highlighting the need for reduced nitrogen inputs into the estuary. Choice of the model(s) for estimation of the gas transfer velocity had a big bearing on the estimated total emissions. (C) 2013 Elsevier B.V. All rights reserved