Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 13, 691-705, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
04 Feb 2016
Fate of terrestrial organic carbon and associated CO2 and CO emissions from two Southeast Asian estuaries
D. Müller1,2, T. Warneke1, T. Rixen2,3, M. Müller4, A. Mujahid5, H. W. Bange6, and J. Notholt1,7 1Institute of Environmental Physics, University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany
2Leibniz Center for Tropical Marine Ecology, Fahrenheitstr. 6, 28359 Bremen, Germany
3Institute of Geology, University of Hamburg, Bundesstr. 55, 20146 Hamburg, Germany
4Swinburne University of Technology, Faculty of Engineering, Computing and Science, Jalan Simpang Tiga, 93350 Kuching, Sarawak, Malaysia
5Department of Aquatic Science, Faculty of Resource Science and Technology, University Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia
6GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
7MARUM Center for Marine Environmental Sciences, University of Bremen, Leobener Str., 28359 Bremen, Germany
Abstract. Southeast Asian rivers convey large amounts of organic carbon, but little is known about the fate of this terrestrial material in estuaries. Although Southeast Asia is, by area, considered a hotspot of estuarine carbon dioxide (CO2) emissions, studies in this region are very scarce. We measured dissolved and particulate organic carbon, as well as CO2 partial pressures and carbon monoxide (CO) concentrations in two tropical estuaries in Sarawak, Malaysia, whose coastal area is covered by carbon-rich peatlands. We surveyed the estuaries of the rivers Lupar and Saribas during the wet and dry season, respectively. Carbon-to-nitrogen ratios suggest that dissolved organic matter (DOM) is largely of terrestrial origin. We found evidence that a large fraction of this carbon is respired. The median pCO2 in the estuaries ranged between 640 and 5065 µatm with little seasonal variation. CO2 fluxes were determined with a floating chamber and estimated to amount to 14–268 mol m−2 yr−1, which is high compared to other studies from tropical and subtropical sites. Estimates derived from a merely wind-driven turbulent diffusivity model were considerably lower, indicating that these models might be inappropriate in estuaries, where tidal currents and river discharge make an important contribution to the turbulence driving water–air gas exchange. Although an observed diurnal variability of CO concentrations suggested that CO was photochemically produced, the overall concentrations and fluxes were relatively moderate (0.4–1.3 nmol L−1 and 0.7–1.8 mmol m−2 yr−1) if compared to published data for oceanic or upwelling systems. We attributed this to the large amounts of suspended matter (4–5004 mg L−1), limiting the light penetration depth and thereby inhibiting CO photoproduction. We concluded that estuaries in this region function as an efficient filter for terrestrial organic carbon and release large amounts of CO2 to the atmosphere. The Lupar and Saribas rivers deliver 0.3 ± 0.2 Tg C yr−1 to the South China Sea as organic carbon and their mid-estuaries release approximately 0.4 ± 0.2 Tg C yr−1 into the atmosphere as CO2.

Citation: Müller, D., Warneke, T., Rixen, T., Müller, M., Mujahid, A., Bange, H. W., and Notholt, J.: Fate of terrestrial organic carbon and associated CO2 and CO emissions from two Southeast Asian estuaries, Biogeosciences, 13, 691-705,, 2016.
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Short summary
We studied organic carbon and the dissolved greenhouse gases carbon dioxide (CO2) and carbon monoxide (CO) in two estuaries in Sarawak, Malaysia, whose coast is covered by carbon-rich peatlands. The estuaries received terrestrial organic carbon from peat-draining tributaries. A large fraction was converted to CO2 and a minor fraction to CO. Both gases were released to the atmosphere. This shows how these estuaries function as efficient filters between land and ocean in this important region.
We studied organic carbon and the dissolved greenhouse gases carbon dioxide (CO2) and carbon...