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Abstract. Southeast Asian peatland-draining rivers have attracted much attention dueto their high dissolved organic carbon (DOC) yield and high CO2 emissions under anthropogenic influences. In August 2016, we carried out afield investigation of the Rajang River and its estuary, a tropical systemlocated in Sarawak, Malaysia. The Rajang has peatland in its estuary, whilethe river basin is covered by tropical rainforest. DOC- δ13C inthe Rajang ranged from −28.7  ‰ to −20.1  ‰, with a U-shaped trend from river to estuary.For particulate organic carbon (POC), δ13C ranged between −29.4  ‰ and −31.1  ‰ in the river, andthere was a clear increasing trend towards more enriched δ13C values with higher salinity. In the estuary, there was a linear conservativedilution pattern for dissolved organic matter composition (as quantified byD- and L-amino acid enantiomers) plotted against DOC- δ13C , whereaswhen plotted against salinity, dissolved D- and L-amino acid enantiomer valueswere higher than the theoretical dilution value. Together, these dataindicate that the addition of DOC to the estuary (by peatland) not onlyincreased the DOC concentration but also altered its composition, by addingmore biodegraded, 13C -depleted organic matter into the bulk dissolvedorganic matter. Alteration of organic matter composition (addition of a moredegraded subpart) was also apparent for the particulate phase, but patternswere less clear. The Rajang was characterized by DOC to DON (dissolvedorganic nitrogen) ratios of 50 in the river section, with loss of DON in theestuary increasing the ratio to 140, suggesting an unbalanced export of organiccarbon and nitrogen. Where affected by anthropogenic activities, further assessment oforganic carbon to nitrogen ratios is needed.

The nonconservative distribution pattern of organic matter in the Rajang, a tropical river with peatland in its estuary