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

Abstract

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/(m²·day) while N₂O varied between 0.4 ± 0.1 and 1.6 ± 0.6 mg N₂O/(m²·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.

Introduction

Methane (CH₄) and nitrous oxide (N₂O) are two key potent greenhouse gases (GHGs). Both CH₄ and N₂O are long-lived atmospheric trace gases that remain chemically active for 8–12 years and 114–120 years, respectively (Ehhalt et al., 2001). They have respective global warming potentials of around 25 and 300 times that of carbon dioxide (IPCC, 2007, Myhre et al., 2013). N₂O is also currently believed to be the single most important ozone depleting substance, a position it is likely to retain for years to come (Ravishankara et al., 2009). Of great concern is that both CH₄ and N₂O concentrations in the atmosphere have significantly increased since industrialisation and are still on the rise (IPCC, 1990, Stocker et al., 2013, Rigby et al., 2008). Yet the significance of a number of the drivers for this increase remains unclear and quantitatively the strengths of the various sources and sinks are still highly uncertain (Kirschke et al., 2013).

A significant portion of atmospheric and stratospheric CH₄ and N₂O is of biogenic origin and aquatic systems are believed to be significant sources (Bastviken et al., 2011, Bouwman et al., 1995, Khalil and Rasmussen, 1993, Nevison et al., 1995, Walter et al., 2007). Oceans are estimated to contribute 13%–25% of the global N₂O emissions (Bouwman et al., 1995, Nevison et al., 1995) and 1%–4% of the global CH₄ emissions (Cicerone and Oremland, 1988, Karl et al., 2008) with a significant proportion of the emissions in near shore areas likely influenced by discharges from rivers and estuaries (Borges and Abril, 2011). Great uncertainty, however, surrounds these estimates mainly due to scarcity of emission measurement data (Bange et al., 1994, Cicerone and Oremland, 1988, Nevison et al., 1995). Measurement data are particularly rare from tropical and subtropical aquatic systems as most marine and coastal system studies in these areas have mainly concentrated on upwelling regions (Charpentier et al., 2010, Codispoti et al., 1992, Naqvi et al., 2009, Naqvi et al., 2000). Application of emission factors from other climatic regions to these areas during regional and global GHG emissions budgeting is likely to cause uncertainty due to neglect of spatial variability, a well known occurrence even at a small areal extent (Bange et al., 1994, Bange et al., 1998, Zhang et al., 2006). Indeed both oceanic N₂O and CH₄ emissions are not uniformly distributed and it is likely that previous oceanic GHG budgets that were based on open ocean measurements significantly underestimated oceanic emissions (Bange, 2006). Biologically productive coastal and estuarine areas contribute around 60% of the oceanic N₂O budget (Bange et al., 1996, Seitzinger et al., 2000) and up to 75% of the global oceanic CH₄ budget (Bange et al., 1994). Besides biological processes, other causes for the high N₂O and CH₄ concentrations and fluxes in coastal areas may include effluent discharges from wastewater treatment plants (Hashimoto et al., 1999, Zhang et al., 2006), ground water input (LaMontagne et al., 2003, Ronen et al., 1988), input from connected creeks, rivers and estuaries (Ferrón et al., 2007, Musenze et al., 2014, Scranton and McShane, 1991, Zhang et al., 2006) and gas seeps (Amouroux et al., 2002, Reeburgh et al., 1991). Another challenge to regional and global GHG emission budgeting is that existing studies on which such estimates would be based do not adequately capture temporal variability as they are often of measurements done in a single season (Borges et al., 2011, Zhang et al., 2004). Consequently, the lack of information on temporal variability may constitute a large source of uncertainty during global emissions budgeting. There is, therefore, a need for more studies capturing both temporal and spatial variability from systems in diverse climatic divides to support adequate evaluation of regional and global N₂O and CH₄ budgets.

In this study we present a comprehensive assessment of CH₄ and N₂O concentrations and water–air emissions variability from two bays that are part of a subtropical coastal embayment in the southern hemisphere. CH₄ and N₂O concentration measurements were done at 23 stations in different seasons spanning over two years to adequately capture both spatial and temporal variability. Water–air fluxes were estimated using the Thin Boundary Layer Equation (TBLE) with a combination of wind and currents-based models for the estimation of the gas transfer velocity.