Quantifying the direction and magnitude of CO2 flux in estuaries is necessary to constrain the global carbon cycle, yet carbonate systems and CO2 flux in subtropical and urbanized estuaries are not yet fully determined. 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...
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Galveston Bay is a semi-enclosed microtidal estuary located in the northwestern Gulf of Mexico (nwGOM) (Montagna et al., 2013). With an average water depth of 3 m and a surface area of 1554 km², Galveston Bay is the seventh largest estuary in the U.S. and the second largest on the Texas coast (Bass et al., 2018; Morse et al., 1993; Solis & Powell, 1999). The Bay receives freshwater from several rivers, including the Trinity River, San Jacinto River, Clear Creek, and smaller bayous, with the Trinity River contributing 70% of the freshwater (Bass et al., 2018; Dellapenna et al., 2020; Morse et al., 1993). The Bay is separated from the Gulf of Mexico (GOM) by the Bolivar Peninsula and Galveston Island, with water exchange occurring through Bolivar Roads, the mouth of the Bay (Glass et al., 2008).
Monthly cruises were conducted aboard the R/V Trident from October 2017 to September 2018 to examine factors regulating CO2 flux over a year following Hurricane Harvey in August 2017. Although the study began more than 45 days after the hurricane (the residence time of the Bay), salinity recovery was likely still ongoing in the inner and middle sections of the Bay (Du & Park, 2019; Du et al., 2019).
During each survey, a transect was run between five water sampling stations, extending from the Bay mouth (Station 1) to the Five Mile Marker on the Houston Ship Channel (Station 5). An additional offshore cruise in the nwGOM outside Galveston Bay was conducted in October 2018. Underway pCO₂ measurements were taken along the northwesterly transect from Stations 1 through 5 using a SUPER-CO₂ system equipped with a LI-COR® LI-840A infrared gas analyzer to collect both water and air xCO₂ after drying through a Peltier thermoelectric device (Honkanen et al., 2021). The pCO₂ data were converted at sea surface temperature, assuming 100% water vapor pressure (Jiang et al., 2008). Underway seawater was taken from a steel pipe attached to the vessel, as the ship lacked a dedicated water intake system. A diaphragm water pump fed water to the equilibrator. Sea surface temperature and salinity were measured using a SeaBird Scientific SBE45® Thermosalinograph, which was mounted parallel to the equilibrator of the SUPER-CO₂ system. Calibration of the system was done prior to and after each sampling trip using known CO₂ concentration standards (Honkanen et al., 2021; Jiang et al., 2008).
To calculate pCO₂ values for seawater and air, the mole fraction of CO₂ in seawater (xCO₂, water) and the equilibrator barometric pressure and xH₂O were first used to calculate the xCO₂ in dry air (xCO₂, air). xCO₂, air was then converted to pCO₂ using measured temperature of equilibration (Teq) and water vapor pressure of equilibration, following methods outlined by Weiss and Price (1980). Finally, pCO₂, eq was converted to pCO₂, water using sea surface temperature and Teq, according to methods in Jiang et al. (2008).
Meteorological Data
Three National Oceanic and Atmospheric Administration (NOAA) buoys from throughout Galveston Bay provided six-minute interval averages of continuous wind speed data (NOAA, 2022). The average wind speed for all three buoys during the sampling times was calculated and applied to the sampling period. Wind speeds were adjusted to a height of 10 m using the wind profile power law (Hsu et al., 1994):
u1/u2=(z1/z2)Pu1/u2 = (z1/z2)^Pu1/u2=(z1/z2)P
Where u2 is the wind speed at height z2 = 10 m, u1 is the wind speed at height z1, and the exponent P (0.11) for the GOM area is based on the work of Hsu et al. (1994).
United States Geological Survey (USGS) streamgages were used to obtain freshwater discharge data for the Trinity River and San Jacinto River (USGS, 2021). The stations selected were the closest gages to the mouths of the rivers and provided complete discharge data for the study period. Discharges less than or equal to 45 days prior to flux estimates (residence time of the Bay) were used (Bass et al., 2018). River endmember values for dissolved inorganic carbon (DIC) were calculated using total alkalinity (TA) and pH measurements (TCEQ, 2022), with constants from Millero (1982). Seasonally weighted averages of DIC and TA were calculated using discharge-weighted averages for each season.
Historical Data
Results from this study were compared with historical data obtained from the Surface Ocean CO₂ Atlas (SOCAT) database, which includes fCO₂, water, xCO₂, air, and other environmental variables for Galveston Bay from 2006 to 2016 (Bakker et al., 2016). SOCAT transects followed a similar route to the study's transect, starting near Station 4 and extending outward into the GOM. The fCO₂ values from SOCAT were converted to pCO₂ using the R package seacarb (Gattuso et al., 2022). SOCAT data were analyzed independently of the results of this study.
Hu, X., Liu, H., Dias, L. M. (2024) Houston Galveston Bay GPS. Biological and Chemical Oceanography Data Management Office (BCO-DMO). (Version 1) Version Date 2024-11-26 [if applicable, indicate subset used]. http://lod.bco-dmo.org/id/dataset/944542 [access date]
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