Carbon dynamics and CO2 and CH4 outgassing in the Mekong delta

• Published in: Biogeosciences 2018-01, Vol.15 (4), p.1093-1114
• Main Authors: Borges, Alberto V, Abril, Gwenaël, Bouillon, Steven
• Format: Article
• Language: English
• Subjects: Alkalinity; Aquaculture; Aquatic sciences & oceanology; Atmosphere; Biogeochemistry; Brackishwater environment; Carbon; Carbon cycle; Carbon dioxide; Carbon dioxide atmospheric concentrations; Carbon dioxide emissions; Channels; Chemical composition; Creeks; Creeks & streams; Data; Datasets; Deltas; Discharge; Dissolved inorganic carbon; Dry season; Dynamics; Emissions; Estimates; Estuaries; Estuarine dynamics; Extreme values; Farm buildings; Fresh water; Freshwater; Growth; Inland water environment; Isotope composition; Life sciences; Maxima; Methane; Methanogenesis; Mineral nutrients; Morphology; Nutrient enrichment; Offshore; Organic carbon; Organic matter; Outgassing; Oxidation; Partial pressure; Particulate organic carbon; Phytoplankton; Plankton; Ponds; Rainy season; Reduction; Residence time; River channels; River plumes; Rivers; Salinity; Salinity effects; Sciences aquatiques & océanologie; Sciences du vivant; Seasonal variation; Seasonal variations; Seasons; Sediment; Sediments; Shellfish farming; Stable isotopes; Sulfate reduction; Sulfates; Sulphate reduction; Suspended matter; Wind
• ISSN: 1726-4170; 1726-4189; 1726-4189
• DOI: 10.5194/bg-15-1093-2018

We report a data set of biogeochemical variables related to carbon cycling obtained in the three branches (Mỹ Tho, Hàm Luông, Co^´ Chiên) of the Mekong delta (Be^´n Tre province, Vietnam) in December 2003, April 2004, and October 2004. Both the inner estuary (upstream of the mouth) and the outer estuary (river plume) were sampled, as well as side channels. The values of the partial pressure of CO2 (pCO2) ranged between 232 and 4085 ppm, O2 saturation level (%O2) between 63 and 114 %, and CH4 between 2 and 2217 nmol L−1, within the ranges of values previously reported in temperate and tropical meso- and macro-tidal estuaries. Strong seasonal variations were observed. In the upper oligohaline estuary, low pCO2 (479–753 ppm) and high %O2 (98–106 %) values were observed in April 2004 most probably related to freshwater phytoplankton growth owing to low freshwater discharge (1400 m3 s−1) and increase in water residence time; during the two other sampling periods with a higher freshwater discharge (9300–17 900 m3 s−1), higher pCO2 (1895–2664 ppm) and lower %O2 (69–84 %) values were observed in the oligohaline part of the estuary. In October 2004, important phytoplankton growth occurred in the offshore part of the river plume as attested by changes in the contribution of particulate organic carbon (POC) to total suspended matter (TSM) (%POC) and the stable isotope composition of POC (δ13C-POC), possibly related to low TSM values (improvement of light conditions for phytoplankton development), leading to low pCO2 (232 ppm) and high %O2 (114 %) values. Water in the side channels in the Mekong delta was strongly impacted by inputs from the extensive shrimp farming ponds. The values of pCO2, CH4, %O2, and the stable isotope composition of dissolved inorganic carbon (δ13C-DIC) indicated intense organic matter degradation that was partly mediated by sulfate reduction in sediments, as revealed by the slope of total alkalinity (TA) and DIC covariations. The δ13C-POC variations also indicated intense phytoplankton growth in the side channels, presumably due to nutrient enrichment related to the shrimp farming ponds. A data set in the mangrove creeks of the Ca Mau province (part of the Mekong delta) was also acquired in April and October 2004. These data extended the range of variability in pCO2 and %O2 with more extreme values than in the Mekong delta (Be^´n Tre), with maxima and minima of 6912 ppm and 37 %, respectively. Similarly, the maximum CH4 concentration (686 nmo