44-5 Physicochemical and Isotopic Evidence for the Controls on Longitudinal Carbon Sourcing and Cycling in a Tidal Creek along the Land-Estuary Interface in a Tropical Mangrove Estuary

Abstract:

We investigated physicochemical and isotopic properties along a 13 km stretch of a tidal creek during tidal ebbing and flooding at the land-estuary interface in a tropical mangrove estuary. We aimed to evaluate the factors controlling longitudinal carbon sourcing and cycling during the dry season. We made physical measurements of water clarity, dissolved oxygen (DO), turbidity, and salinity. We collected samples, which were analyzed for stable isotopes of oxygen and hydrogen (δ¹⁸O and δD), silica, dissolved inorganic carbon (DIC) components (alkalinity, total DIC, and partial pressure of carbon dioxide (pCO₂)), and stable carbon isotopes of DIC (δ¹³C_DIC). We found that water properties exhibited a consistent longitudinal trend from the open estuary into the headwater of the tidal creek. For example, DO, turbidity, salinity, and pH decreased, while clarity, alkalinity, DIC, and the pCO₂ increased. The salinity during tidal flooding was lower than during tidal ebbing, indicating that tidal intrusion brought fresher water into the tidal creek from the open estuary. The small difference in the range of the water isotopes (e.g., 0.4‰ in δ¹⁸O) indicates a dominance of terrestrial (river) water in the tidal creek. A hydrologic mixing model indicates a freshwater-dominated mixed seawater–freshwater source from the open estuary and freshwater in the headwater/interior of the tidal creek mixed with terrestrial groundwater influx at this land-estuary boundary. A Keeling graphical analysis indicates that longitudinal DIC evolution was from mixing between water from the open estuary to 3 to 6 km into the tidal creek and CO₂ exchange between the tidal creek and the atmosphere for the rest of the transverse. Miller-Tans graphical analysis further showed that the isotopic composition of the controlling DIC reservoir was -14.7‰ during tidal flooding and -13.6‰ during tidal ebbing. We attribute the observed longitudinal variability in water properties to the combined influence of mixed seawater-river water influx during tidal flooding, freshwater discharge in the headwaters of the tidal creek, and continuous submarine groundwater discharge from land into the tidal creek. Our findings provide critical insights into the role of tidal creeks in land-estuary carbon flow and in tropical mangrove estuarine carbon studies.

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