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

The influence of mangrove forests on the dynamics of dissolved inorganic carbon (DIC) in tropical estuaries was estimated quantitatively using newly developed isotope (δ13C) mass balance models that take into account both the input of DIC and the air‐water exchange of CO2. To this aim, the concentration and δ13C 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 μmol L−1 was observed in the high‐discharge period (December 2006). δ13CDIC values were generally lower than expected for the mixing of the river water and seawater DIC, due to the 13C‐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 μmol L−1, 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 CO2 pump that takes CO2 from the atmosphere and releases it into estuarine waters. This study illustrates that δ13CDIC is a sensitive and quantitative indicator for DIC emission to the sea from coastal wetlands including mangroves.