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

Abstract

<p><strong>Abstract.</strong> South-east Asian peatland-draining rivers have attracted much attention due to their high dissolved organic carbon (DOC) yield and high CO<sub>2</sub> 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 δ<sup>13</sup>C 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 δ<sup>13</sup>C ranged between −29.4 ‰ to −31.1 ‰ in the river and a clear increasing trend towards more δ<sup>13</sup>C -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 δ<sup>13</sup>C, 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, <sup>13</sup>C-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.</p>

Biogeographical distribution of microbial communities along the Rajang River–South China Sea continuum

“…Next-generation sequencing technologies have enabled a better understanding of the rare or unculturable biosphere which traditional culture methods would not have been able to elucidate (Boughner and Singh, 2016;Cao et al, 2017). Only a few studies assessing bacterial community composition have been undertaken in lotic or riverine environments (Fortunato et al, 2012;Ladau et al, 2013;Zwart et al, 2002), with even fewer focusing on the diversity of surfaceattached biofilms in lotic environments, particularly in comparison to biofilm studies in benthic habitats (Zeglin, 2015). Furthermore, bacterial assemblages on suspended particles were shown to differ from free-living bacterioplankton in a number of studies (Bidle and Fletcher, 1995;Crump et al, 1999) in which the ratios between both fractions are often influenced by the quality of suspended particulate matter (Doxaran et al, 2012).…”

“…Furthermore, bacterial assemblages on suspended particles were shown to differ from free-living bacterioplankton in a number of studies (Bidle and Fletcher, 1995;Crump et al, 1999) in which the ratios between both fractions are often influenced by the quality of suspended particulate matter (Doxaran et al, 2012). Even fewer studies attempt to map bacterial community composition in a river-to-sea contin-uum across multiple seasons and habitats (Fortunato et al, 2012), and it was only recently reported that the most abundant riverine bacterioplankton resemble lake bacteria and can be regarded as "typical" freshwater bacteria (Lozupone and Knight, 2007;Zwart et al, 2002). Metagenomics studies substantiated the dominance of Proteobacteria and Actinobacteria while Bacteroidetes, Cyanobacteria and Verrucomicrobia were also found to be abundant in rivers (Cottrell et al, 2005;Kolmakova et al, 2014;Lemke et al, 2009;Newton et al, 2011;Read et al, 2015;Staley et al, 2013).…”