Hydrological controls on the distribution of dissolved inorganic carbon and total alkalinity in a northwestern Gulf of Mexico estuary

Coastal regions are recognized as areas of significant biogeochemical activities, primarily due to the influx of nutrients from rivers. However, under the influence of climatic changes, these riverine fluxes are diminishing, significantly altering the biogeochemistry of coastal regions. Such changes could also impact carbon chemistry in these regions, affecting the health of ecosystems. Therefore, it is imperative to conduct long-term studies to monitor the impact of these hydrological changes on the carbon chemistry of coastal systems. In this study, we conducted an analysis of a decadal time series data on dissolved inorganic carbon (DIC) and total alkalinity (TA), along with physicochemical parameters (including pH, Temperature, Salinity), in the Mission-Aransas estuary, located in the northwestern Gulf of Mexico. These samples were collected at five stations from April 2014 to August 2025. Our focus was on characterizing the sources and sinks of DIC and TA and evaluating the hydrological impacts on carbonate parameters. The results show both non-conservative removal and gain of DIC and TA from the mixing line between river water and seawater. Gain of DIC and TA is mainly observed <~25 salinity, whereas removal is observed at higher salinities. Additional supply of DIC and TA can be attributed to the fluxes at the sediment-water interface and the dissolution of carbonates. The most significant increase is observed during periods of high-water flow, which suggests that carbonate dissolution is facilitated by a decrease in pH resulting from the mixing of low pH and high pCO₂ water. Additionally, the remineralization of organic matter transported by rivers contributes to this process by lowering the pH. Removal of DIC and TA is due to the biological activities, for example calcification by oysters, which are abundant in this estuary. Although these water samples predominantly exhibit supersaturation with respect to both calcite and aragonite, this suggests the potential for abiotic precipitation of carbonate minerals. Furthermore, the oxidation of reducing agents such as sedimentary sulfide, introduced through the sediment-water interface because of significant resuspension activities, may potentially decrease the TA concentration. This study suggests that DIC and TA cycling in coastal regions is greatly influenced by hydrological changes, and more global studies are needed to predict carbon behaviors in these biogeochemically active regions to accurately quantify the impact on net export fluxes of DIC and TA to the coastal ocean.