Export of inorganic carbon from two Southeast Asian mangrove forests to adjacent estuaries as estimated by the stable isotope composition of dissolved inorganic carbon

Abstract

[1] The influence of mangrove forests on the dynamics of dissolved inorganic carbon (DIC) in tropical estuaries was estimated quantitatively using newly developed isotope (d 13 C) mass balance models that take into account both the input of DIC and the air-water exchange of CO 2 . To this aim, the concentration and d 13C of DIC were determined across the salinity gradient of two river estuaries facing the Andaman Sea. The longitudinal distribution of DIC could be explained by conservative mixing of the river water and seawater DIC in the low-discharge period (March 2006), while a net accumulation of DIC up to 190 mmol L À1 was observed in the high-discharge period (December 2006). d 13 C DIC values were generally lower than expected for the mixing of the river water and seawater DIC, due to the 13 C-depleted DIC inputs from the riverside mangroves. The concentration of mangrove-derived DIC in the estuarine waters was estimated by the proposed models to be as much as 856 mmol L À1 , and was higher during the low-discharge period. This suggested that the mangroves exported much higher levels of DIC to the estuaries than indicated by the net accumulation of DIC. Our results confirm that mangroves function as an effective CO 2 pump that takes CO 2 from the atmosphere and releases it into estuarine waters. This study illustrates that d 13 C DIC is a sensitive and quantitative indicator for DIC emission to the sea from coastal wetlands including mangroves.

Uncoupled surface and below-ground soil respiration in mangroves: implications for estimates of dissolved inorganic carbon export

“…Our finding that significant amounts of mineralized carbon are lost via subsurface flow supports the recent studies of DIC flow from mangrove creek waters (Bouillon et al 2007;Biswas et al 2004;Koné and Borges 2008;Miyajima et al 2009) as well as carbon flow models for mangrove forests (Bouillon et al 2008a;Alongi 2009). The waterway studies consistently found that mangrove waters are oversaturated in CO 2 as a direct result of pelagic respiration and porewater DIC being transported laterally by tidal pumping to the adjacent tidal creeks.…”

Longitudinal distribution of nitrate δ15N and δ18O in two contrasting tropical rivers: implications for instream nitrogen cycling

“…Although the d 13 C DIC in the salinewater section rapidly increased due to the mixing with isotopically heavier marine DIC (Fig. 3e, f), massbalance calculations have demonstrated that the measured d 13 C in this zone was significantly deviated negatively by up to 5.7 and 2.4% in the Khura and Trang Rivers, respectively, from the values that were expected by the conservative mixing of river and marine DIC (Miyajima et al 2009). These characteristic profiles apparently reflect the influence of the mangroves that were abundant in the intertidal area along the estuary of the Khura and Trang Rivers.…”

Marine water from mid-Holocene sea level highstand trapped in a coastal aquifer: Evidence from groundwater isotopes, and environmental significance

“…On the basis of the observed δ 13 C values we propose that the additional DIC is derived from the carbon fixed by mangroves and other salt-tolerant coastal vegetation that has long been native to the region. Miyajima et al (2009);Maher et al (2013) and Alongi (2014) showed that coastal mangrove communities contribute substantial DIC (as well as DOC) to water and soils in coastal swamp environments, acting as an effective CO 2 pump from the atmosphere to estuaries through surface and subsurface pathways. Saintilan et al (2013) measured δ 13 C compositions of soils and plant matter in southeast Australian mangrove swamps including Western Port, and found values of~−27‰ for …”