Assessing the contribution of porewater discharge in carbon export and CO2 evasion in a mangrove tidal creek (Can Gio, Vietnam)

Although mangrove forests are efficient natural carbon sinks, most of the atmospheric carbon dioxide (CO2) fixed by its vegetation is believed to be exported via tidal exchange, rather than stored in the vegetative biomass and sediment. However, the magnitude of tidal export is largely unknown because direct measurements are scarce. We deployed a novel experimental design that combined automated high-resolution measurements of hydrodynamic, hydrogeochemical and biogeochemical parameters during the dry season in a mangrove tidal creek in the Can Gio Mangrove Forest in Vietnam. The objective was to quantify the tide-controlled water, porewater, DIC and DOC exchange, and estimate the CO2 evasion throughout tidal cycles contrasted by amplitude. Data from three 25-h time series showed a clear peak of DIC, DOC, pCO2, and 222Rn at low tide, particularly during tidal cycles of large amplitude, which directly relate to porewater discharge. Our mass balance models revealed that the tidal creek was a net exporter of dissolved carbon to coastal waters, with an important contribution (38%) coming from DIC in porewater discharge. Porewater exchange varied from 3.1 ± 1.6 to 7.1 ± 2.4 cm day−1. DIC exchange ranged from 352 ± 34 to 678 ± 79 mmolC m−2 day−1; DOC exchange, 20.6 ± 1.9 to 67.7 ± 7.9 mmol C m−2 day−1; and CO2 evasion, 69.9 ± 10.5 to 173.7 ± 26.1 mmolC m−2 day−1. These estimates were in the high range of previous carbon assessments and were explained by (i) the monitoring station being located at equal distance from the head and the mouth of the creek, which minimized carbon degradation and losses associated to transport in water; and (ii) the site being a highly productive mangrove within South East Asia.