Carbon dynamics and CO2 and CH4 outgassing in the Mekong delta
Authors: Borges Alberto V. , Abril Gwenaël , Bouillon Steven 
Published: February 2018

Abstract. 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 nmol L−1) was higher in the Ca Mau province mangrove
creeks than in the Mekong delta (Be^´n Tre, maximum
222 nmol L−1) during the October 2004 cruise (rainy season and high
freshwater discharge period). In April 2004 (dry season and low freshwater
discharge period), the CH4 values were much lower than in October 2004
(average 19 ± 13 and 210 ± 158 nmol L−1, respectively) in
the Ca Mau province mangrove creeks, owing to the higher salinity (average
33.2 ± 0.6 and 14.1 ± 1.2, respectively) that probably led to
higher sediment sulfate reduction, leading to inhibition of sediment
methanogenesis and higher anaerobic CH4 oxidation. In the inner
estuarine region (three branches of the Mekong delta), CO2 emissions to
the atmosphere averaged 121 mmol m−2 d−1, and the CH4
emissions averaged 118 µmol m−2 d−1. The CO2
emission to the atmosphere from the Mekong inner estuary was higher than
reported in the Yangtze and Pearl river inner estuaries. This was probably
due to the lower salinity in the Mekong delta branches, possibly due to
different morphology: relatively linear channels in the Mekong delta versus
funnel-shaped estuaries for the Yangtze and Pearl river inner estuaries.