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. Worldwide, peatlands are important sources of dissolved organicmatter (DOM) and trace metals (TMs) to surface waters, and these fluxes mayincrease with peatland degradation. In Southeast Asia, tropical peatlandsare being rapidly deforested and drained. The blackwater rivers draining thesepeatland areas have high concentrations of DOM and the potential to behotspots for CO2 release. However, the fate of this fluvial carbonexport is uncertain, and its role as a trace metal carrier has never beeninvestigated. This work aims to address these gaps in our understanding oftropical peatland DOM and associated elements in the context of degradedtropical peatlands in Indonesian Borneo. We quantified dissolved organiccarbon and trace metal concentrations in the dissolved and fine colloidal( µ m) and coarse colloidal (0.22–2.7  µ m)fractions and determined the characteristics ( δ13 C, absorbance,fluorescence: excitation-emission matrix and parallel factor – PARAFAC – analysis) of thepeatland-derived DOM as it drains from peatland canals, flows along theAmbawang River (blackwater river) and eventually mixes with the Kapuas KecilRiver (whitewater river) before meeting the ocean near the city of Pontianak inWest Kalimantan, Indonesia. We observe downstream shifts in indicators ofin-stream processing. An increase in the δ13 C of dissolved organic carbon (DOC), alongwith an increase in the C1∕C2 ratio of PARAFAC fluorophores, and a decrease inSUVA (specific UV absorbance) along the continuum suggest the predominanceof photo-oxidation. However, very low dissolved oxygen concentrations alsosuggest that oxygen is quickly consumed by microbial degradation of DOM inthe shallow layers of water. Blackwater rivers draining degraded peatlands showsignificantly higher concentrations of Al, Fe, Pb, As, Ni and Cd comparedto the whitewater river. A strong association is observed between DOM, Fe, As, Cdand Zn in the dissolved and fine colloid fraction, while Al is associatedwith Pb and Ni and present in a higher proportion in the coarse colloidalfraction. We additionally measured the isotopic composition of lead releasedfrom degraded tropical peatlands for the first time and show that Pboriginates from anthropogenic atmospheric deposition. Degraded tropicalpeatlands are important sources of DOM and trace metals to rivers and asecondary source of atmospherically deposited contaminants.

From canals to the coast: dissolved organic matter and trace metal composition in rivers draining degraded tropical peatlands in Indonesia