Carbon Budgets in Coastal Estuaries of the Northwestern Gulf of Mexico Under Hydrologic Control

Globally, estuaries are considered as important CO2 sources to the atmosphere. However, previous studies on estuarine carbon fluxes have mostly focused on temperate and high latitude regions, leaving a significant knowledge gap in subtropical and tropical estuaries. In addition, the drivers that cause large spatiotemporal variability in estuarine inorganic and organic carbon fluxes remain insufficiently explored. In this dissertation, carbon budgets in four northwestern Gulf of Mexico (nwGOM) estuaries along a climatic gradient, Lavaca-Colorado Estuary (LCE), Guadalupe Estuary (GE), Mission-Aransas Estuary (MAE), and Nueces Estuary (NE), were -2 -1 evaluated. All these estuaries, with annual CO2 emission ranging 2.7—35.9 mol·C·m ·y , are moderate to strong CO2 sources. However, there was large spatiotemporal variability that corresponded to changes in hydrologic conditions. The two northern estuaries (LCE and GE), due to larger riverine discharges, had an order of magnitude higher CO2 emissions than the southern estuaries (MAE and NE). In addition, episodic flooding made the entire regional CO2 fluxes differ significantly under dry (-0.7—20.9 mmol·C·m-2·d-1) and wet (11.6—170.0 mmol·C·m-2·d-1) conditions. A mass balance model for carbon budget predicted lateral transport of total organic matter (TOC) and dissolved inorganic carbon (DIC) from tidal wetlands, which accounted for ~95% and 70% of total TOC and DIC inputs to the open estuarine water, respectively. However, the loss of coastal saltmarsh-mangrove habitats due to sea level rise could result in ~1% per year decline in estuarine CO2 fluxes at the expense of decreasing lateral carbon transport. Finally, this dissertation suggested that the average estuarine CO2 flux from nwGOM was about 8 times higher than previously estimated North America estuarine CO2 flux. Additionally, flooding condition was estimated to elevate CO2 emission and lateral fluxes by 10 times in this region.