Spatial and seasonal variability of CO2 flux and carbonate chemistry in a subtropical estuary

Quantifying the direction and magnitude of CO2 flux in estuaries is necessary to better constrain the global carbon budget, yet carbonate systems and CO2 flux in subtropical and urbanized estuaries are not well characterized. To estimate the CO2 flux for Galveston Bay, a subtropical estuary located in the northwestern Gulf of Mexico proximal to the Houston-Galveston metroplex, monthly cruises were conducted along a transect extending from the Houston ship channel to the mouth of Galveston Bay and Gulf of Mexico from October 2017 to September 2018. Underway surface water partial pressure of CO2 (pCO2) measurements were conducted using a Shipboard Underway pCO2 Environmental Recorder (SUPER-CO2) system. CO2 flux was calculated for 0.025 degrees x 0.025 degrees latitude increments along the transect and total CO2 flux for the Bay was estimated. On these cruises, discrete water samples were collected for laboratory analyses of total alkalinity (TA), total dissolved inorganic carbon (DIC), calcium (Ca2+), and pH (on the total scale, pHT); and aragonite saturation state (Omega Ar) was calculated using pH and DIC as well as measured Ca2+. Annual mean Bay waterpCO2 was 384.2 +/- 56.7 mu atm, TA was 2062.0 +/- 213.2 mu mol kg-1, DIC was 1863.7 +/- 160.9 mu mol kg-1, pHT was 8.09 +/- 0.17, and Omega Arwas 2.43 +/- 0.96. When compared to theoretical river-ocean mixing scenarios, TA values fell above the mixing line in winter and summer, indicating production, while DIC values displayed more spatial variability that included both production and consumption in different seasons. A large freshwater inflow event in spring was followed by a period of dilution (low salinity, TA, and DIC) and enhanced primary production (low waterpCO2 along with CO2 uptake and high chlorophyll-a levels). CO2 flux exhibited large seasonal and spatial variability, primarily due to seasonality in photosynthesis and variability of freshwater inflow events. Overall, Galveston Bay was a sink for CO2 during the study period, with a mean air-sea CO2 flux of -8.3 +/- 17.3 mmol m-2 d-1, and carbonate chemistry in Galveston Bay was regulated by an interaction between hydrology and biogeochemistry. The carbonate chemistry and CO2 uptake patterns of Galveston Bay differ from those that are common in temperate estuaries, which reiterates the need for further research in subtropical estuaries.