Extensive remineralization of peatland-derived dissolved organic carbon
and acidification in the Sunda Shelf Sea, Southeast Asia
Abstract
Southeast 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 dissolved organic carbon, 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 by up to 0.10
pH units. The persistent CO2 oversaturation drives a CO2 efflux of 4.1
– 8.2 mol C 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.