Dissolved inorganic carbon dynamics in the waters surrounding forested mangroves of the Ca Mau Province (Vietnam)

Dissolved inorganic carbon (DIC) and ancillary data were obtained during the dry and rainy seasons in the waters surrounding two 10-yearold
forested mangrove sites (Tam Giang and Kieˆn Va`ng) located in the Ca Mau Province (South-West Vietnam). During both seasons, the spatial variations of partial pressure of CO2 (pCO2) were marked, with values ranging from 704 ppm to 11481 ppm during the dry season, and from 1209 ppm to 8136 ppm during the rainy season. During both seasons, DIC, pCO2, total alkalinity (TAlk) and oxygen saturation levels (%O2) were correlated with salinity in the mangrove creeks suggesting that a combination of lower water volume and longer residence time (leading
to an increase in salinity due to evaporation) enhanced the enrichment in DIC, pCO2 and TAlk, and an impoverishment in O2. The low O2 and
high DIC and pCO2 values suggest that heterotrophic processes in the water column and sediments controlled these variables. The latter processes
were meaningful since the high DIC and TAlk values in the creek waters were related to some extent to the influx of pore waters, consistent
with previous observations. This was confirmed by the stochiometric relationship between TAlk and DIC that shows that anaerobic
processes control these variables, although this approach did not allow identifying unambiguously the dominant diagenetic carbon degradation
pathway. During the rainy season, dilution led to significant decreases of salinity, TAlk and DIC in both mangrove creeks and adjacent main
channels. In the Kieˆn Va`ng mangrove creeks a distinct increase of pCO2 and decrease of %O2 were observed. The increase of TSM suggested
enhanced inputs of organic matter probably from land surrounding the mangrove creeks, that could have led to higher benthic and water column
heterotrophy. However, the flushing of water enriched in dissolved CO2 originating from soil respiration and impoverished in O2 could also have
explained to some extent the patterns observed during the rainy season. Seasonal variations of pCO2 were more pronounced in the Kieˆn Va`ng
mangrove creeks than in the Tam Giang mangrove creeks. The airewater CO2 fluxes were 5 times higher during the rainy season than during the
dry season in the Kieˆn Va`ng mangrove creeks. In the Tam Giang mangrove creeks, the airewater CO2 fluxes were similar during both seasons.
The airewater CO2 fluxes ranged from 27.1 mmol C m 2 d 1 to 141.5 mmol C m 2 d 1 during the dry season, and from 81.3 mmol m 2 d 1 to
154.7 mmol m 2 d 1 during the rainy season. These values are within the range of values previously reported in other mangrove creeks and
confirm that the emission of CO2 from waters surrounding mangrove forests are meaningful for the carbon budgets of mangrove forests.