From Canals to the Coast: Dissolved Organic Matter and Trace

Worldwide, peatlands are important sources of dissolved organic matter (DOM) and trace metals (TM) 17 to surface waters and these fluxes may increase with peatland degradation. In Southeast Asia, tropical peatlands 18 are being rapidly deforested and drained. The black rivers draining these peatland areas have high concentrations 19 of DOM, and the potential to be hotspots for CO2 release. However, the fate of this fluvial carbon export is 20 uncertain, and its role as a trace metal carrier has never been investigated. This work aims to address these gaps in 21 our understanding of tropical peatland DOM and associated elements in the context of degraded tropical peatlands 22 in Indonesian Borneo. We quantified dissolved organic carbon and trace metal concentrations in the dissolved and 23 fine colloidal (<0.22μm) and coarse colloidal (0.22 – 2.7 μm) fractions and determined the characteristics (δC, 24 Absorbance, Fluorescence: excitation-emission matrix and PARAFAC analysis) of the peatland-derived DOM as 25 it drains from peatland canals, flows along the Ambawang River, a black river, and eventually mixes with the 26 Kapuas Kecil River (white river) before meeting the ocean near the city of Pontianak in West Kalimantan, 27 Indonesia. We observe downstream shifts in indicators of in-stream processing. An increase in the δC of DOC, 28 along with an increase in the C1/C2 ratio of PARAFAC fluorophores, and decrease in SUVA (Specific UV 29 Absorbance) along the continuum suggest the predominance of photo-oxidation. However, very low dissolved 30 oxygen concentrations also suggest that oxygen is quickly consumed by microbial degradation of DOM in the 31 shallow layers of water. Black rivers draining degraded peatlands show significantly higher concentrations of Al, 32 Fe, Pb, As, Ni, and Cd, compared the white river. A strong association is observed between DOM, Fe, As, Cd and 33 Zn in the dissolved and fine colloid fraction, while Al is associated with Pb and Ni and present in a higher 34 proportion in the coarse colloidal fraction. We additionally measured the isotopic composition of lead released 35 from degraded tropical peatlands for the first time and show that Pb originates from anthropogenic atmospheric 36


Introduction 41
Most Southeast Asian tropical peatlands developed as domes beneath ombrotrophic peat swamp forests (Page et 42 al., 2006;Cobb et al. 2017). They store at least 68.5 Pg C, or 15-19% of the global peat carbon stocks (Dargie et  proportion is likely to increase with rapid peatland conversion to agriculture, which destabilizes long-term peat C 58 stocks (Moore et al., 2013). 59 Another implication of DOM transfers from peatlands to surface water is the transport of associated elements, 60 especially trace metals (TM). Tropical peatlands in Southeast Asia are mainly ombrotrophic systems, which 61 receive critical nutrients through atmospheric deposition, and serve as a sink for atmospheric pollutants (Weiss et 62 al., 2002). Northern peatlands have been shown to constitute a source of major and trace elements to surface waters 63 (Broder and Biester, 2017;Jeremiason et al., 2018;Rothwell et al., 2007). This has important implications: as a 64 result of colloidal association between peatland-derived organic molecules and Fe, northern peatlands are 65 responsible for a significant transfer of Fe to the Atlantic Ocean (Krachler et al. 2010(Krachler et al. , 2012. In the UK, peat 66 degradation and erosion has led to the dispersion of lead into watersheds, which previously accumulated through 67 atmospheric deposition over decades (Rothwell et al., 2008). Although drainage of tropical peatlands is occurring 68 at a rapid rate across Southeast Asia, to our knowledge no data are available on trace metal release in black rivers 69 draining tropical peatlands. 70 Black rivers draining peatlands (as defined in Alkhatib et al., (2007)) also have the potential to be hotspots of 71 fluvial carbon degassing Wit et al., 2015). By measuring pCO2 in Indonesian and Malaysian 72 black rivers, Wit et al. (2015) estimated that 53% of DOC entering surface waters was converted to CO2, which is 73 similar to global averages for inland waters. In contrast, black river measurements and incubations by Martin et 74 al., (2018) found a smaller proportion of DOC was processed in rivers. Rixen et al., (2008) also found a large 75 proportion of the DOM was resistant to decomposition in a laboratory incubation study. These studies have focused 76 on CO2 measurements and incubations to assess the potential for DOM processing. opposite directions in response to microbial processing or photo-oxidation. Microbial processing is generally found 88 to increase the aromaticity of DOM by selective processing of less aromatic molecules, while photo-oxidation 89 tends to decrease aromaticity, because of selective photo-oxidation of aromatic moieties (Spencer et al., 2009;90 Hansen et al., 2016). 91 In summary, although there has been an increase in efforts to quantify DOC exports from tropical peatlands, our 92 complementary understanding of the transfer of associated elements and in-stream processing of DOM remains 93 limited. This work aims to address these gaps in our understanding of the composition and evolution of tropical 94 peatland DOM and how it could act as a carrier of trace metals to surface waters, in the context of highly degraded 95 tropical peatlands in Indonesia. We characterize the quality of the peatland-derived DOM and trace metals as they 96 drain from peatland canals, flow along black rivers, and eventually mix with a white water river before meeting 97 the ocean. We assess spatial and seasonal changes in the organic matter quality, and document changes in DOM 98 composition due to transport, mixing, and processing. We also assess black river trace metal release to surface 99 waters, analyzing trace metal concentrations and the isotopic composition of lead released from degraded tropical 100 peatlands for the first time. 101

Study area 103
The study area is located in West Kalimantan, Indonesia, near the city of Pontianak (0.09°N, 109.24°E) on the 104 island of Borneo ( Figure 1). The climate is humid equatorial with 2953±564 mm of rainfall and a mean annual 105 temperature of 27°C (1985-2017 data). The monthly annual rainfall ranges from 170±126 mm (August) to 349±98 106 mm (November). The highest rainfalls are measured from October to January. The mean rainfall is 274 ± 123 mm 107 for January, and 199 ± 106 mm for June. (Figure SI.1). The study focused on the Ambawang River, which flows 108 into the Landak river, which in turn flows into Kapuas Kecil river. It is a black river draining a watershed 109 were analyzed using a High Resolution ICP-MS (Thermo Element II XR; OMP service ICP-MS, Toulouse, 147 France). Measurements were corrected for mass bias using individual sample bracketing with certified and 148 adequately diluted NIST NBS-981 (100 ng L -1 to 500 ng L -1 ) according to Krachler et al. (2004). 149 The UV absorption spectra of pore water were measured with a spectrophotometer (Secoman UVi-lightXT5) from 150 190 to 700 nm in a 1 cm quartz cell. The Specific UV Absorbance at 254 nm (SUVA, L mg -1 m -1 ) was calculated 151 as follows: SUVA= A254/(b*DOC) (Weishaar et al., 2003), where A254 is the sample absorbance at 254 nm (non-152 dimensional), b is the optical path length (m) and DOC is in mg L -1 . The baseline was determined with ultra-pure 153 water. Potential additional absorbance related to Fe content was following the procedure described by Poulin et 154 al., (2014). The additional absorbance was small, and represented only 3.6 ± 1.4% of the total absorbance across 155 all samples and was therefore neglected. 156 Emission Excitation Matrices (EEM) were acquired using a Hitachi F4500 fluorescence spectrometer, and 157 instrument specific correction were applied. Emission spectra were acquired from 250 to 550 nm for excitation 158 ranging from 250 to 550 nm. The slits were set to 5 nm for both the excitation and emission monochromators. The 159 scan speed was 2400 nm min -1 and the integration response was 0.1 s. Fluorescence intensity was corrected from 160 the excitation beam to ensure stability. The inner filter effect water was taken into account using a dilution The isotopic composition (δ 13 C) of DOC was determined at the UC Davis Stable Isotope Facility, following the 167 described procedure (http://stableisotopefacility.ucdavis.edu/doc.html). Briefly, a TOC Analyzer (OI Analytical, 168 College Station, TX) is interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, 169 UK) utilizing a GD-100 Gas Trap Interface (Graden Instruments). 170

Trends in water chemistry from the source of the black river to the ocean 176
The observed water chemistry of the Ambawang river and drainage canals is typical of black rivers draining 177 peatlands (Table 1, Figure 2), and does not show significant differences between the two sampling seasons. It is 178 acidic with a pH of 3.2 ± 0.6 and 3.5 ± 0.3 in the drainage canals (DC) and black river (BR) respectively, has a 179 low conductivity (DC: 89.8 ± 21.4 µS cm -1 , BR: 85.2±21.6 µS cm -1 ), is hypoxic (DC: 2.3 ± 0.3, BR: 1.9 ± 0.7mg 180 L -1 ), and has low nutrient concentrations (DIN < 0.3 mg L -1 and P-PO4 < 0.015 mg L -1 ) but high DOC 181 concentrations (DC: 35.2 ± 5.9, BR: 35.8 ± 3.5 mg L -1 ). The Clconcentrations are low and homogeneous (DC: 182 2.6 ± 0.7, BR: 2.4 ± 0.6 mg L -1 ). After the confluence with the white river, the chemistry of the river radically 183 changes. An abrupt increase in pH is observed (WR: 5.3 ± 0.7). The dissolved oxygen concentration increases to 184 3.7±1.0 mg L -1 , while DOC concentrations drop sharply to 9.2 ± 3.2 mg L -1 . We also observe a slight increase in 185 NO3and decrease in PO4 2-. Across all samples, the DOC concentrations show a significant negative correlation 186 with DO concentrations (r 2 =0.63, n=40, p<4.10 9 ). In contrast, no increase in Clconcentration is observed until 187 close to the ocean (3 samples corresponding to ocean water intrusion were excluded from Figure 2). 188

DOM optical characteristics and stable isotopic signature 189
No systematic differences are observed for the DOM characteristics between the two sampling campaigns. The

Trace element concentrations and physical fractionation 217
Black rivers originating from drained peatlands have a unique composition of inorganic elements. The 218 concentrations of trace metals (Pb, Ni, Zn, Cd) as well as Al and Fe and are significantly higher in the black river 219 and drainage canals than the concentrations in the white river (Table 2, Figure 4). For Al, Fe and As, high 220 concentrations are measured in the black river during the first sampling campaign (drier conditions). In contrast to 221 other TM, higher Cu concentrations are measured in the white river. A PCA analysis ( Figure 5) of TM 222 concentration and DOM properties reveals specific associations between DOC, Fe and As and to a lesser extend 223 Zn and Cd, while another group is formed by Al, Pb and Ni. Cu shows no association with DOM but does show 224 increased concentrations with higher FI. The first axis of PCA (load of DOC, Fe, As) strongly discriminates the 225 black river and drainage canals samples from the white river. 226 The distributions of DOC and TM are presented in Table 3. Dissolved organic carbon is mostly (>98%) dissolved 227 or in the form of fine colloids (<0.22 µm) along the entirety of the studied continuum. Iron and As are mostly 228 present in dissolved form or as fine colloids in the black river and drainage canals (>96%). However, after transfer 229 to the white river, half of Fe and a third of As is present in the coarse colloidal form. Zinc and Cd do not show 230 similar patterns. Aluminium is mostly present in the coarse colloidal phase (>60%) in the black river and drainage 231 canals and this proportion further increases in the white river (>80%). Lead is mostly present in the dissolved and 232 fine colloid phase (>75%) in the drainage canals and black river and shifts to coarse colloidal (>60%) forms after 233 the confluence with the white river. Nickel and Cu are mostly present in the dissolved and fine colloidal phase in 234 the DC and BR but almost entirely in the coarse colloidal fraction in the white river. 235

Pb isotopic composition 236
We observe distinct differences between the lead isotope ratios in the white river and those in the black river and 237 drainage canals. A decrease in the 206 Pb/ 207 Pb isotopic ratio is observed with increasing Pb concentrations in the 238 black river but not the white river ( Figure 6a). Futhermore, the biplot of the 206 Pb/ 207 Pb and the 208 Pb/ 206 Pb 239 signatures illustrate significant differences between the white water and black river/drainage canal groups ( Figure  240 6b). 241 6. Discussion 242

In-stream processing of DOM in black rivers 243
We observe in-stream processing of DOM, but the total DOM exported from tropical peatlands exceeds the 244 processing capacity of the rivers which drain them and a large proportion of DOM is transported to the ocean. We In the future, quantitative assessment of outgassing in tropical peatland drainage canals would improve the 276 evaluation of carbon release following peatland drainage. Overall, more work is needed to understand the extent 277 of upstream processing of peatland DOM. 278

Role of DOM, Al and Fe in trace metal dynamics in peat draining waters 279
This study provides the first record of trace metals in black rivers originating from degraded tropical peatlands.

Peatlands as secondary sources of atmospheric pollutants 301
The isotopic composition of Pb in peat draining water strongly suggests it is of anthropogenic origin. The isotopic 302 signatures measured in river samples are a combination of the signature of undisturbed soils of Borneo (Valentine 303 et al., 2008), and a mix of both present and past anthropogenic inputs. Older anthropogenic inputs are reflected by 304 the signature of atmospheric deposition from Java aerosols (Bollhöfer and Rosman, 2000), while the signature of 305 recent regional anthropogenic inputs was characterized by rain samples collected in Pontianak as part of this study 306 ( Figure 6b). In the black river and drainage canals, the isotopic ratio is close to that of aerosols and recently sampled 307 rainwater and is dominated by anthropogenic inputs, whereas the isotopic ratio in the white river is closer to the 308 natural signal (Figure 6). This isotopic difference is consistent with the difference between the watersheds drained 309 by these two rivers: tropical peatlands are ombrotrophic systems, and the trace metal content in peat soil is derived LG, AMH, GH and CFH designed the study. LG, AMH, MN and GH conducted field campaigns. SM and LG 362 conducted fluorescence analysis. GLR and AC conducted lead isotope analysis. LG and AMH wrote the 363 manuscript, with inputs from all co-authors. 364

Competing interests 365
The authors declare no competing interests. 366