Articles | Volume 13, issue 8
Biogeosciences, 13, 2415–2428, 2016
Biogeosciences, 13, 2415–2428, 2016

Research article 26 Apr 2016

Research article | 26 Apr 2016

Nitrous oxide and methane in two tropical estuaries in a peat-dominated region of northwestern Borneo

Denise Müller1, Hermann W. Bange2, Thorsten Warneke1, Tim Rixen3,4, Moritz Müller5, Aazani Mujahid6, and Justus Notholt1,7 Denise Müller et al.
  • 1Institute of Environmental Physics, University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany
  • 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
  • 3Leibniz Center for Tropical Marine Ecology, Fahrenheitstr. 6, 28359 Bremen, Germany
  • 4Institute of Geology, University of Hamburg, Bundesstr. 55, 20146 Hamburg, Germany
  • 5Swinburne University of Technology, Faculty of Engineering, Computing and Science, Jalan Simpang Tiga, 93350 Kuching, Sarawak, Malaysia
  • 6Department of Aquatic Science, Faculty of Resource Science & Technology, University Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia
  • 7MARUM Center for Marine Environmental Sciences at the University of Bremen, Leobener Str., 28359 Bremen, Germany

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.

Short summary
Estuaries act as sources of the greenhouse gases nitrous oxide (N2O) and methane (CH4) to the atmosphere. We provide first measurements of N2O and CH4 in two estuaries in north-western Borneo, a region which is dominated by peatlands. We show that N2O and CH4 concentrations in these estuaries are moderate despite high organic carbon loads, that nutrient enhancement does not lead to enhanced N2O emissions, and that the wet season dominates the variability of the emissions in these systems.
Final-revised paper