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

2021

JGR Biogeosciences

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Southeast Asia's extensive tropical peatlands account for a significant proportion of the global riverine dissolved organic carbon (DOC) flux to the ocean. Peat‐derived DOC is rich in polyphenolic compounds, the microbial degradation of which is thought to rely on extracellular phenol oxidases. Despite substantial interest in the biogeochemical fate of terrigenous DOC (tDOC), few studies have quantified phenol oxidase activity in aquatic environments, and microbial remineralization rates of tDOC have never been measured in Southeast Asia. Here, we assess the potential for using phenol oxidase assays as a proxy for tDOC biodegradation across peat‐draining rivers and the coastal waters of Sarawak, Borneo, and report experimental measurements of microbial tDOC remineralization rates from this region. We first show that phenol oxidase assays in aquatic samples are problematic because of the rapid, pH‐dependent autoxidation of the assay substrate. Our field measurements of phenol oxidase activity detected only substrate autoxidation, suggesting that real phenol oxidase activity was low or absent. Second, we report that peatland tDOC, collected from one of the few remaining intact peatlands on Borneo, showed at most very limited biodegradation (0%–6% loss of DOC, and 0%–12% loss of colored dissolved organic matter) during several 56‐day incubation experiments at an in situ temperature of ∼30°C, even when diluted with seawater or amended with nutrients. Our results suggest that direct microbial respiration is perhaps not a major pathway for peatland tDOC remineralization in Southeast Asia and that photo‐oxidation is more likely to control the fate of this carbon.

Section: Discussionmentioning

confidence: 69%

Low Biodegradability of Dissolved Organic Matter From Southeast Asian Peat‐Draining Rivers

Nichols

2021

JGR Biogeosciences

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Section: Sources and Seasonality Of Cdommentioning

confidence: 85%

“…In a separate analysis of the carbon biogeochemistry from our time series, Zhou et al (2021) showed that the stable carbon isotope composition of DOC (δ 13 C DOC ) was as low as −25.5 ‰ during the SW Monsoon, which can only be explained by tDOC input. An isotope mass balance of dissolved organic and inorganic C showed that the riverine tDOC source during the SW Monsoon must have a mean (± SE) DOC concentration of 892 ± 104 μmol l −1 (Zhou et al 2021). This is equal to the discharge-weighted mean (± SD) DOC concentration reported by Wit et al (2018) for the 4 main peatland-draining river systems on Sumatra that are closest to our site (890 ± 159 μmol l −1 ), which demonstrates that the tDOC and the terrigenous CDOM in the Singapore Strait during the SW Monsoon must originate from these peatlands.…”

Section: Sources and Seasonality Of Cdommentioning

confidence: 85%

“…The present analysis focusses on December 2018 to August 2020, during which we acquired 77 measurements of CDOM, DOC, and salinity, 60 measurements of particulate absorption, and 36 measurements of backscattering. The CDOM and salinity time-series data are also presented in a separate analysis focussing on carbon biogeochemistry by Zhou et al (2021).…”

Section: Field Sampling and Sensor Measurementsmentioning

confidence: 99%

See 1 more Smart Citation

Dissolved organic matter from tropical peatlands reduces shelf sea light availability in the Singapore Strait, Southeast Asia

Sanwlani

Lee

et al. 2021

Mar. Ecol. Prog. Ser.

Shelf seas provide valuable ecosystem services, but their productivity and ecological functioning depend critically on sunlight transmitted through the water column. Anthropogenic reductions in underwater light availability are thus a serious threat to coastal habitats. The flux of light-absorbing coloured dissolved organic matter (CDOM) from land to sea may have increased world-wide, but how this has altered the availability and spectral quality of light in shelf seas remains poorly known. We present time-series data from the Sunda Shelf in Southeast Asia, where the monsoon-driven reversal in ocean currents supplies water enriched in CDOM from tropical peatlands for part of the year, resulting in 5- to 10-fold seasonal variation in light absorption by CDOM. We show that this terrigenous CDOM can dominate underwater light absorption at wavelengths up to 500 nm, and shift the underwater irradiance spectrum towards longer wavelengths. The seasonal presence of terrigenous CDOM also reduces the 10% light penetration depth by 1-5 m, or 10-45%. We estimate that on average 0.6 m, or 25%, of this terrigenous CDOM-mediated shoaling might be attributable to the enhanced input of dissolved organic matter following peatland disturbance. The seasonal change in the light environment is correlated with changes in phytoplankton absorption spectra that suggest a photo-acclimation response, and we infer that terrigenous CDOM likely contributes to limiting the depth distribution of photosynthetic corals. The results reveal an ecologically important but largely overlooked impact of human modifications to carbon fluxes that is likely increasingly important in coastal seas.

“…Nichols Robert Scott; Patrick Martin, 2021, "Related data for: Low biodegradability of dissolved organic matter from Southeast Asian peat-draining rivers (Nichols & Martin, 2021)," https://doi.org/10.21979/N9/0RIGHW…”

Section: Data Availability Statementmentioning

confidence: 99%

Low biodegradability of dissolved organic matter from Southeast Asian peat-draining rivers

Nichols

2021

Preprint

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Low biodegradability of peatland dissolved organic carbon during 56-day incubations, including when diluted with seawater or amended with nutrients • No measurable phenol oxidase activity in the peat-draining rivers and coastal waters of Sarawak • Autoxidation of the phenol oxidase assay substrate L-3,4dihydroxyphenylalanine occurs at pH ≥ 7; we recommend ultrafiltration to generate enzymefree controls Supporting Information: Supporting Information may be found in the online version of this article.