The flux of dissolved organic carbon (DOC) from land to sea is an important net transfer within the global carbon cycle. The biogeochemical fate of this terrestrial DOC (tDOC) remains poorly understood and is usually neglected in ocean models. Southeast Asia accounts for around 10% of global tDOC flux, mostly from tropical peatland-draining rivers discharging onto the Sunda Shelf. We developed a new light-driven parameterization of tDOC remineralization that accounts for photochemical, microbial, and interactive photochemical–microbial degradation. Using this, we simulated the transport and remineralization of tDOC through the Sunda Shelf seas using the regional 3D hydrodynamical–biogeochemical models HAMSOM–ECOHAM. Our realistic hindcast simulations for 1958–2022 show that about 50% of riverine tDOC is remineralized before leaving the shelf. This lowers seawater pH across the entire inner Sunda Shelf by an average of 0.005 units (by up to 0.05 units in the Malacca Strait). Correspondingly, seawater pCO2 is raised, increasing CO2 outgassing from the shelf by 3.1 Tg C yr−1 (0.14 mol m−2 yr−1 ) during 2013-2022. Even regional ocean acidification trends increase, because river discharge and tDOC flux increase. Our model reveals large spatial variability with greatest inputs and remineralization of tDOC close to major peatlands, especially off Sumatra and Borneo. The interannual variability in tDOC input and the monsoonal current reversal lead to strong temporal variability in carbonate system parameters in these areas. Our results highlight the importance of representing tDOC in ocean models, and reveal the fate of tropical peatland tDOC.

A document by Patrick Martin. Click on the document to view its contents.

The terrigenous dissolved organic carbon (tDOC) exported from the peatlands in Southeast Asia appears to be extensively remineralized in the shelf sea, but the processes that drive this remineralization remain unclear. Here, we combined incubation experiments and model simulations to quantify the rate and extent of photodegradation of tDOC in the Sunda Shelf Sea. Laboratory photodegradation experiments indicate that up to 74% of the peatland tDOC is potentially labile to photochemical remineralization. Based on our estimated apparent quantum yield for tDOC remineralization, modeled in-situ solar irradiance, and measured inherent optical properties of the water column, we simulated peatland tDOC photoremineralization for two coastal regions of the Sunda Shelf Sea. These simulation results show that natural solar radiation can directly remineralize 20±11% of tDOC over 2 years, which corresponds to the approximate residence time of water in the Sunda Shelf Sea, and that significant photobleaching of tDOC can occur in coastal waters over shorter time-scales. We further derived a simplified photochemical decay constant of 0.008–0.017day-1 for Southeast Asia’s peatland-derived tDOC, which can be used to parameterize the recently proposed UniDOM model framework. We conclude that direct photodegradation may be a greater sink for tDOC in Southeast Asia’s coastal ocean compared to higher latitudes, although it is insufficient to account for the total tDOC remineralization observed in the Sunda Shelf Sea.

A document by Yuan Chen. Click on the document to view its contents.

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.

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 of tDOC biodegradation across peat-draining rivers and coastal waters of Sarawak, Borneo, and report experimental measurements of microbial tDOC remineralization rates from this region. We show first that phenol oxidase assays in aquatic samples are problematic because of the rapid, pH-dependent auto-oxidation of the assay substrate. Our field measurements of phenol oxidase activity detected only substrate auto-oxidation, 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 coloured dissolved organic matter) during several 56-day incubation experiments at 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.

The riverine flux of terrigenous dissolved organic matter (tDOM) to the ocean is a significant contributor to the global carbon cycle. In response to anthropogenic drivers such as land-use change the flux is expected to increase, and this may impact both the availability of sunlight in coastal ecosystems, and the seawater carbonate system and coastal CO2 fluxes. Yet despite its biogeochemical and ecological significance, there are few long-term and high-resolution time series of tDOM parameters. Corals incorporate fluorescent tDOM molecules from the chromophoric dissolved organic matter (CDOM) pool in their skeletons, and the resulting luminescence variability in coral skeleton cores has traditionally been used to reconstruct hydroclimate variation. Here, we use two replicate coral cores and concurrent in-situ biogeochemical data from the Sunda Sea Shelf in Southeast Asia, where coastal peatlands supply high tDOM inputs, to show that variability in coral luminescence green-to-blue ratios (coral G/B) can be used to quantitatively reconstruct the concentration of terrigenous dissolved organic carbon (tDOC). Moreover, coral G/B can be used to reconstruct the full absorption spectrum of CDOM from 230–550 nm, as well as the specific ultraviolet absorbance at 254 nm (SUVA254) of the DOM pool. Comparison to a core from Borneo shows that there may be site-specific offsets in the G/B–CDOM absorption relationship, but that the slope of the relationship is very similar, validating the robustness of the proxy. By demonstrating that coral cores can be used to estimate past changes in coastal tDOC and CDOM, we establish a method to study natural and anthropogenic drivers of land–ocean tDOM fluxes and their ecological consequences in tropical coastal seas over decadal to centennial time scales.

Coastal tropical waters are experiencing rapid increases in anthropogenic pressures, yet coastal biogeochemical dynamics in the tropics are poorly studied. We present a multi-year biogeochemical time series from the Singapore Strait in Southeast Asia’s Sunda Shelf Sea. Despite being highly urbanised and a major shipping port, the strait harbours numerous biologically diverse habitats, and is a valuable system for understanding how tropical marine ecosystems respond to anthropogenic pressures. Our results show strong seasonality driven by the semi-annual reversal of ocean currents: dissolved inorganic nitrogen (DIN) and phosphorus varied from ≤0.05 µmol l-1 during the intermonsoons to ≥4 µmol l-1 and ≥0.25 µmol l-1, respectively, during the southwest monsoon. Si(OH)4 exceeded DIN year-round. Based on nutrient concentrations, their relationships to salinity and coloured dissolved organic matter, and the isotopic composition of NOx-, we infer that terrestrial input from peatlands is the main nutrient source. This input delivered dissolved organic carbon (DOC) and nitrogen, but was notably depleted in dissolved organic phosphorus. In contrast, particulate organic matter showed little seasonality, and the δ13C of particulate organic carbon (-21.0 ± 1.5‰) is consistent with a primarily autochthonous origin. Diel changes in dissolved O2 varied seasonally with a pattern that suggests that light availability controls primary productivity more than nutrient concentrations. However, diel changes in pH were greater during the southwest monsoon, when remineralisation of terrestrial DOC lowers the seawater buffer capacity. We conclude that terrestrial input results in mesotrophic conditions, and that the strait might be vulnerable to further eutrophication if nutrient inputs increase during seasons when light availability is high. Moreover, the seasonality of diel pH variation suggests that coral reefs exposed to terrestrial organic matter in the Sunda Shelf may be at significant risk from future ocean acidification.