Abstract
AbstractSoutheast Asia is a hotspot of riverine export of terrigenous organic carbon to the ocean, accounting for ∼10% of the global land‐to‐ocean riverine flux of terrigenous dissolved organic carbon (tDOC). While anthropogenic disturbance is thought to have increased the tDOC loss from peatlands in Southeast Asia, the fate of this tDOC in the marine environment and the potential impacts of its remineralization on coastal ecosystems remain poorly understood. We collected a multi‐year biogeochemical time series in the central Sunda Shelf (Singapore Strait), where the seasonal reversal of ocean currents delivers water masses from the South China Sea first before (during Northeast Monsoon) and then after (during Southwest Monsoon) they have mixed with run‐off from peatlands on Sumatra. The concentration and stable isotope composition of DOC, and colored dissolved organic matter spectra, reveal a large input of tDOC to our site during Southwest Monsoon. Using isotope mass balance calculations, we show that 60%–70% of the original tDOC input is remineralized in the coastal waters of the Sunda Shelf, causing seasonal acidification. The persistent CO2 oversaturation drives a CO2 efflux of 2.4–4.9 mol m−2 yr−1 from the Singapore Strait, suggesting that a large proportion of the remineralized peatland tDOC is ultimately emitted to the atmosphere. However, incubation experiments show that the remaining 30%–40% tDOC exhibits surprisingly low lability to microbial and photochemical degradation, suggesting that up to 20%–30% of peatland tDOC might be relatively refractory and exported to the open ocean.
Highlights
Southeast Asia harbors the largest area of the world's tropical peatlands, which are widely distributed in coastal areas of Sumatra and Borneo and store around 69 Pg C of terrestrial organic carbon (Dommain et al, 2014; Page et al, 2011)
The SW Monsoon exhibited the lowest spectral slope of colored dissolved organic matter (CDOM) (S275–295) (0.016–0.020 nm−1), the highest SUVA254 (2–3 L mg−1 m−1), and the lightest δ13CDOC (−25.5 to −24‰) of the year, all of which indicate a large input of terrigenous dissolved organic carbon (tDOC) (Figures 3e–3g)
Given that the extent of tDOC remineralization that we infer within the shelf is close to our experimental measurements of tDOC decomposability from an intact peatland catchment, our data indicate that the majority of any anthropogenically mobilized peatland tDOC must be largely remineralized within the shelf sea, and outgassed to the atmosphere. This is consistent with existing recommendations for estimating downstream anthropogenic CO2 emissions from peatland dissolved organic carbon (DOC) remineralization (Evans et al, 2016; Intergovernmental Panel on Climate Change (IPCC), 2014), our results suggest that the extent of peatland tDOC remineralization might be slightly lower than the value of 90% that is currently used for peatland emissions reporting (IPCC, 2014)
Summary
Southeast Asia harbors the largest area of the world's tropical peatlands, which are widely distributed in coastal areas of Sumatra and Borneo and store around 69 Pg C of terrestrial organic carbon (Dommain et al, 2014; Page et al, 2011). Southeast Asia is a hotspot of terrestrial organic carbon export to the ocean: the fluvial flux of terrigenous dissolved organic carbon (tDOC) to the Sunda Shelf Sea is ∼21 Tg C yr−1, which accounts for ∼10% of the annual global land-to-ocean tDOC flux by the world's rivers (Baum et al, 2007; Moore et al, 2011). Patrick Martin Visualization: Yongli Zhou, Yuan Chen Writing – original draft: Yongli Zhou, Christopher D. This ongoing anthropogenic pressure on peatlands could have impacts extending into the marine environment of Southeast Asia, especially on sensitive ecosystems such as coral reefs
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