Fate of terrestrial organic carbon and associated CO2 and CO emissions from two Southeast Asian estuaries

[摘要] Southeast Asian rivers convey large amounts of organic carbon, but little isknown about the fate of this terrestrial material in estuaries. AlthoughSoutheast Asia is, by area, considered a hotspot of estuarine carbon dioxide(CO₂) emissions, studies inthis region are very scarce. We measured dissolved and particulateorganic carbon, as well as CO₂ partial pressures andcarbon monoxide (CO) concentrations in two tropical estuaries inSarawak, Malaysia, whose coastal area is covered by carbon-rich peatlands. Wesurveyed the estuaries of the rivers Lupar and Saribas during the wetand dry season, respectively. Carbon-to-nitrogen ratios suggest that dissolved organic matter(DOM)is largely of terrestrial origin. We found evidence that a large fraction of this carbon isrespired. The median pCO₂ in the estuaries ranged between640 and 5065 µatm with little seasonalvariation. CO₂ fluxes were determined with a floating chamberand estimated to amount to 14–268 mol m⁻² yr⁻¹, whichis high compared to other studies from tropical and subtropicalsites. Estimates derived from a merely wind-driven turbulent diffusivity model wereconsiderably lower, indicating that these models might be inappropriate in estuaries,where tidal currents and river discharge make an important contribution tothe turbulence driving water–air gas exchange.Although an observed diurnal variability of CO concentrations suggested that CO was photochemicallyproduced, the overall concentrations and fluxes were relatively moderate (0.4–1.3 nmol L⁻¹ and0.7–1.8 mmol m⁻² yr⁻¹) if compared to published data for oceanic or upwelling systems.We attributed this to the large amounts of suspended matter (4–5004 mg L⁻¹), limiting thelight penetration depth and thereby inhibiting CO photoproduction.Weconcluded that estuaries in this region function asan efficient filter for terrestrial organic carbon and release large amounts ofCO₂ to the atmosphere. The Lupar and Saribas riversdeliver 0.3 ± 0.2 Tg C yr⁻¹ to the South China Sea as organiccarbon and their mid-estuaries release approximately 0.4 ± 0.2 Tg C yr⁻¹ into the atmosphere as CO₂.

[学科分类] 地球化学与岩石