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

Abstract

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 Kiên Vàng) located in the Ca Mau Province (South-West Vietnam). During both seasons, the spatial variations of partial pressure of CO 2 (pCO 2 ) 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, pCO 2 , total alkalinity (TAlk) and oxygen saturation levels (%O 2 ) 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, pCO 2 and TAlk, and an impoverishment in O 2 . The low O 2 and high DIC and pCO 2 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 Kiên Vàng mangrove creeks a distinct increase of pCO 2 and decrease of %O 2 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 CO 2 originating from soil respiration and impoverished in O 2 could also have explained to some extent the patterns observed during the rainy season. Seasonal variations of pCO 2 were more pronounced in the Kiên Vàng mangrove creeks than in the Tam Giang mangrove creeks. The airewater CO 2 fluxes were 5 times higher during the rainy season than during the dry season in the Kiên Vàng mangrove creeks. In the Tam Giang mangrove creeks, the airewater CO 2 fluxes were similar during both seasons. The airewater CO 2 fluxes ranged from 27.1 mmol C m À2 d À1 to 141.5 mmol C m À2 d À1 during the dry season, and from 81.3 mmol m À2 d À1 to 154.7 mmol m À2 d À1 during the rainy season. These values are within the range of values previously reported in other mangrove creeks and confirm that the emission of CO 2 from waters surrounding mangrove forests are meaningful for the carbon budgets of mangrove forests.

Dissolved carbon biogeochemistry and export in mangrove-dominated rivers of the Florida Everglades

“…As shown by Ho et al (2014), pCO 2 observed during SharkTREx 1 and 2 fall in the upper range of those reported in other estuarine (Borges, 2005) and mangrove-dominated systems (Bouillon et al, 2003(Bouillon et al, , 2007aKoné and Borges, 2008;Call et al, 2015). The mean air-water CO 2 fluxes in Shark River for SharkTREx 1 and 2 were 105 ± 9 and 99 ± 6 mmol m −2 d −1 (Ho et al, 2016).…”

“…Bouillon et al (2008a) showed that over 50 % of the carbon fixed by mangroves through photosynthesis could not be accounted for by growth in biomass, accumulation in soils, and export of organic carbon, and suggested that a large fraction of this missing organic carbon may be mineralized to dissolved inorganic carbon (DIC) and either lost to the atmosphere or exported to the surrounding waters. In fact, several studies have shown that the lateral advective transport of interstitial waters through tidal pumping represents a major carbon export pathway from mangroves into adjacent waters, both for DIC (Koné and Borges, 2008;Miyajima et al, 2009; Published by Copernicus Publications on behalf of the European Geosciences Union. 2544 D. T. Ho et al: Dissolved carbon export from mangroves Maher et al, 2013) and dissolved organic carbon (DOC) (Dittmar and Lara, 2001;Bouillon et al, 2007c).…”

Variability and Transport of Inorganic Carbon Dioxide in a Tropical Estuary

“…The annual flux recorded in this work was estimated to be 6 mol/m 2 / year, which is close to the value approximated for the open ocean (<3 mol/m 2 /year) [67]. On the other hand, the values obtained in this work were considerably lower than those estimated for estuarine systems by Koné and Borges [68], Paz et al [69], Cai and Wang [8] and Abril et al [70], which were, respectively: 37 mol/m [70] discussed flux values found by researchers using different sampling techniques. However, the techniques used are not discussed as far as the thermodynamic approach is concerned.…”

Groundwater‐derived dissolved inorganic and organic carbon exports from a mangrove tidal creek: The missing mangrove carbon sink?

“…5). This export of DIC may lead to the elevated PCO 2 and high CO 2 fluxes to the atmosphere reported for waters adjacent to mangroves Bouillon et al 2007a;Koné and Borges 2008). Further work is needed in a range of mangrove systems from various latitudes, with different geomorphic settings, species compositions, and tidal regimes to constrain the magnitude and drivers of DIC exports, an export term that is potentially important globally.…”

“…For example, Bouillon et al (2007b) estimated that pore water contributed between 19% and 87% of a Tanzanian mangrove creek's volume at low tide based on DOC, salinity, and dissolved oxygen concentrations. Koné and Borges (2008) concluded that pore-water DIC, derived from anaerobic organic matter degradation through the sulfate reduction pathway, was a significant contributor to the total DIC pool based on alkalinity and DIC stoichiometry. Dittmar and Lara (2001) suggested that pore-water flow is probably a major driver of nutrients and DOC in a Brazilian mangrove tidal creek.…”