Abstract
Abstract. Estuaries are sources of nitrous oxide (N2O) and methane (CH4) to the atmosphere. However, our present knowledge of N2O and CH4 emissions from estuaries in the tropics is very limited because data are scarce. In this study, we present first measurements of dissolved N2O and CH4 from two estuaries in a peat-dominated region of northwestern Borneo. Two campaigns (during the dry season in June 2013 and during the wet season in March 2014) were conducted in the estuaries of the Lupar and Saribas rivers. Median N2O concentrations ranged between 7.2 and 12.3 nmol L−1 and were higher in the marine end-member (13.0 ± 7.0 nmol L−1). CH4 concentrations were low in the coastal ocean (3.6 ± 0.2 nmol L−1) and higher in the estuaries (medians between 10.6 and 64.0 nmol L−1). The respiration of abundant organic matter and presumably anthropogenic input caused slight eutrophication, which did not lead to hypoxia or enhanced N2O concentrations, however. Generally, N2O concentrations were not related to dissolved inorganic nitrogen concentrations. Thus, the use of an emission factor for the calculation of N2O emissions from the inorganic nitrogen load leads to an overestimation of the flux from the Lupar and Saribas estuaries. N2O was negatively correlated with salinity during the dry season, which suggests a riverine source. In contrast, N2O concentrations during the wet season were not correlated with salinity but locally enhanced within the estuaries, implying that there were additional estuarine sources during the wet (i.e., monsoon) season. Estuarine CH4 distributions were not driven by freshwater input but rather by tidal variations. Both N2O and CH4 concentrations were more variable during the wet season. We infer that the wet season dominates the variability of the N2O and CH4 concentrations and subsequent emissions from tropical estuaries. Thus, we speculate that any changes in the Southeast Asian monsoon system will lead to changes in the N2O and CH4 emissions from these systems. We also suggest that the ongoing cultivation of peat soil in Borneo is likely to increase N2O emissions from these estuaries, while the effect on CH4 remains uncertain.
Highlights
Nitrous oxide (N2O) and methane (CH4) are greenhouse gases whose global warming potentials exceed that of carbon dioxide (CO2) by far
Estuarine DIN concentrations were higher than in the unpolluted freshwater end-member of the Lupar River, indicating that the estuary was slightly eutrophic during the time of our measurements. This eutrophication can be attributed both to the release of DIN during respiration of organic matter, which was shown to be pronounced in the Lupar and Saribas estuaries (Müller et al, 2016), especially in the dry season, and to anthropogenic input. These processes were identified as important sources of inorganic nutrients in the Siak River, a eutrophic blackwater river in central Sumatra, Indonesia (Baum and Rixen, 2014)
We argued that floating chamber measurements offered a better representation of the actual flux than gas exchange models that rely on empirical relationships with wind speed, which were initially derived for the open ocean (Wanninkhof, 1992) and do not consider current-induced turbulence as a driver of gas exchange
Summary
Nitrous oxide (N2O) and methane (CH4) are greenhouse gases whose global warming potentials exceed that of carbon dioxide (CO2) by far (a factor of 265 for N2O and 28 for CH4 on a 100-year time horizon; Myhre et al, 2013). An assessment of the natural and anthropogenic sources and sinks as well as the formation pathways of N2O and CH4. D. Müller et al.: Nitrous oxide and methane in two tropical estuaries is essential to understand Earth’s present climate variability and to predict its future development. The world’s oceans, including its coastal zones, are sources of N2O and CH4 to the atmosphere and play a major role in the global budget of atmospheric N2O, but only a minor role in the global budget of atmospheric CH4 (Ciais et al, 2013). Rivers and estuaries are considered hot spots for the production and emission of both N2O and CH4 (Bange, 2006; Bastviken et al, 2011; Borges et al, 2015; Murray et al, 2015; Seitzinger and Kroeze, 1998)
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