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 45.3±1.3nM or 135 to 435% of atmospheric saturation level, while CH₄ concentrations varied between 31.1±3.7 to 578.4±58.8nM 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 NO_x concentrations (r²=0.55). The estimated water-air fluxes varied between 0.1 and 3.4mgN₂Om^-2d^-1 and 0.3 to 27.9mgCH₄m^-2d^-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. © 2013 Elsevier B.V.