Abstract. Worldwide, peatlands are important sources of dissolved organic
matter (DOM) and trace metals (TMs) to surface waters, and these fluxes may
increase with peatland degradation. In Southeast Asia, tropical peatlands
are being rapidly deforested and drained. The blackwater rivers draining these
peatland areas have high concentrations of DOM and the potential to be
hotspots for CO2 release. However, the fate of this fluvial carbon
export is uncertain, and its role as a trace metal carrier has never been
investigated. This work aims to address these gaps in our understanding of
tropical peatland DOM and associated elements in the context of degraded
tropical peatlands in Indonesian Borneo. We quantified dissolved organic
carbon and trace metal concentrations in the dissolved and fine colloidal
(<0.22 µm) and coarse colloidal (0.22–2.7 µm)
fractions and determined the characteristics (δ13C, absorbance,
fluorescence: excitation-emission matrix and parallel factor – PARAFAC – analysis) of the
peatland-derived DOM as it drains from peatland canals, flows along the
Ambawang River (blackwater river) and eventually mixes with the Kapuas Kecil
River (whitewater river) before meeting the ocean near the city of Pontianak in
West Kalimantan, Indonesia. We observe downstream shifts in indicators of
in-stream processing. An increase in the δ13C of dissolved organic carbon (DOC), along
with an increase in the C1∕C2 ratio of PARAFAC fluorophores, and a decrease in
SUVA (specific UV absorbance) along the continuum suggest the predominance
of photo-oxidation. However, very low dissolved oxygen concentrations also
suggest that oxygen is quickly consumed by microbial degradation of DOM in
the shallow layers of water. Blackwater rivers draining degraded peatlands show
significantly higher concentrations of Al, Fe, Pb, As, Ni and Cd compared
to the whitewater river. A strong association is observed between DOM, Fe, As, Cd
and Zn in the dissolved and fine colloid fraction, while Al is associated
with Pb and Ni and present in a higher proportion in the coarse colloidal
fraction. We additionally measured the isotopic composition of lead released
from degraded tropical peatlands for the first time and show that Pb
originates from anthropogenic atmospheric deposition. Degraded tropical
peatlands are important sources of DOM and trace metals to rivers and a
secondary source of atmospherically deposited contaminants.
