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

Abstract

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° × 0.025° 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, pH_T); and aragonite saturation state (Ω_Ar) was calculated using pH and DIC as well as measured Ca²⁺. Annual mean Bay water pCO₂ was 384.2 ± 56.7 μatm, TA was 2062.0 ± 213.2 μmol kg⁻¹, DIC was 1863.7 ± 160.9 μmol kg⁻¹, pH_T was 8.09 ± 0.17, and Ω_Ar was 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 water pCO₂ 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.