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

Abstract

Abstract. Southeast Asian peatland-draining rivers have attracted much attention due to their high dissolved organic carbon (DOC) yield and high CO2 emissions under anthropogenic influences. In August 2016, we carried out a field investigation of the Rajang River and its 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 ‰, 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, and there was a clear increasing trend towards more enriched δ13C values with higher salinity. In the estuary, there was a linear conservative dilution pattern for dissolved organic matter composition (as quantified by D- and L-amino acid enantiomers) plotted against DOC-δ13C, whereas when plotted against salinity, dissolved D- and L-amino acid enantiomer values were higher than the theoretical dilution value. Together, these data indicate that the addition of DOC to the estuary (by peatland) not only increased the DOC concentration but also altered its composition, by adding more biodegraded, 13C-depleted organic matter into the bulk dissolved organic matter. Alteration of organic matter composition (addition of a more degraded subpart) was also apparent for the particulate phase, but patterns were less clear. The Rajang was characterized by DOC to DON (dissolved organic nitrogen) ratios of 50 in the river section, with loss of DON in the estuary increasing the ratio to 140, suggesting an unbalanced export of organic carbon and nitrogen. Where affected by anthropogenic activities, further assessment of organic carbon to nitrogen ratios is needed.

Rising dissolved organic carbon concentrations in coastal waters of northwestern Borneo related to tropical peatland conversion

“…These spatial patterns of CDOM, γ 0 , and DOC point to peatlands as one of the main sources of DOC in these coastal waters, which is consistent with in situ spectral measurements of CDOM and fluorescent dissolved organic matter (FDOM) in this area ( 38 , 39 ). Moreover, chemical and isotopic data from the Rajang River showed that peatland tDOC input causes the riverine DOC concentration to double along the delta ( 38 , 40 ), even though the deltaic peatlands contribute less than 7% of the total catchment area ( 41 ). In contrast to CDOM and DOC, TSM concentrations decreased rapidly beyond the immediate vicinity of river mouths (Fig.…”

Extensive Remineralization of Peatland‐Derived Dissolved Organic Carbon and Ocean Acidification in the Sunda Shelf Sea, Southeast Asia

“…The δ 13 C DOC data were not available for rivers on Sumatra, but the δ 13 C DOC values of the peatland‐derived tDOC across peatlands on Sumatra and rivers on Borneo and Peninsular Malaysia mostly range between −30 and −28‰ (Baum, 2008; Evans et al., 2014; Gandois et al., 2014; Zhu et al., 2020). We therefore adopted −29‰ as the riverine endmember value of δ 13 C DOC for the peatland‐derived tDOC.…”

Extensive remineralization of peatland-derived dissolved organic carbon and acidification in the Sunda Shelf Sea, Southeast Asia

“…The δ 13 C DOC data were not available for rivers on Sumatra, but the δ 13 C-DOC values of the peatland-derived tDOC across peatlands on Sumatra and rivers on Borneo and Peninsular Malaysia mostly range between −30 and −28‰ (Baum, 2008;Evans et al, 2014;Gandois et al, 2014;Zhu et al, 2020). We therefore adopted −29‰ as the riverine endmember value of δ 13 C DOC for the peatland-derived tDOC.…”