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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 05 Aug 2019

Submitted as: research article | 05 Aug 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

The non-conservative distribution pattern of organic matter in Rajang, a tropical river with peatland in its estuary

Zhuoyi Zhu1, Joanne Oakes2, Bradley Eyre2, Youyou Hao1, Edwin Sien Aun Sia3, Shan Jiang1, Moritz Müller3, and Jing Zhang1 Zhuoyi Zhu et al.
  • 1State Key Laboratory of Estuarine and Coastal Research, School of Oceanography, East China Normal University, Shanghai, 200241, China
  • 2Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore NSW, 2480, Australia
  • 3Swinburne University of Technology, Faculty of Engineering, Computing and Science, Jalan Simpang Tiga, Kuching, 93350, Sarawak, Malaysia

Abstract. South-east Asian peatland-draining rivers have attracted much attention due to their high dissolved organic carbon (DOC) yield and high CO2 emissions under anthropogenic activities. In August 2016, we carried out a field investigation of the Rajang river and estuary, a tropical system located in Sarawak, Malaysia. The Rajang has peatland in its estuary while the river basin is covered by tropical rainforest. DOC δ13C in the Rajang ranged from −28.7 ‰ to −20.1 ‰ and a U-shaped trend from river to estuary was identified. For particulate organic carbon (POC), the δ13C ranged between −29.4 ‰ to −31.1 ‰ in the river and a clear increasing trend towards more δ13C -enriched with higher salinity existed in the estuary. In the estuary, there was a linear conservative dilution pattern for dissolved organic matter composition (as quantified by D/L amino acids enantiomers) plotted against DOC δ13C, whereas when plotted against salinity dissolved D/L amino acids enantiomers values were higher than the theoretical dilution value. Together, these data indicate that the addition of DOC in estuary (by peatland) not only increased the DOC concentration, but also altered its composition, by adding more bio-degraded, 13C-depleted organic matter into the bulk dissolved organic matter. Alteration of organic matter composition (adding of more degraded subpart) was also apparent for the particulate phase, but patterns were less clear. The Rajang was characterized by DOC / DON ratios of 50 in the river section, with loss of DON in the estuary increased the ratio to 140, suggesting the unbalanced export pattern for organic carbon and nitrogen, respectively. Under anthropogenic activities, further assessment of organic carbon to nitrogen ratio is needed.

Zhuoyi Zhu et al.
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Status: final response (author comments only)
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Zhuoyi Zhu et al.
Zhuoyi Zhu et al.
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Short summary
Samples were collected in August 2016 in the Rajang river and estuary, with tropical forest in its river basin and peatland in the estuary. Organic matter composition was influenced by transportation in the river basin, whereas peatland can added clear bio-degraded part to the fluvial organic matter, which implies modification of the initial lability and/or starting points in subsequent degradation/alternation process after the organic matter entering into the sea.
Samples were collected in August 2016 in the Rajang river and estuary, with tropical forest in...