Dataset: Houston Galveston Bay Carbonate

Field sampling

Galveston Bay is a semi-enclosed microtidal estuary located in the nwGOM [42]. With an average water depth of 3 m and surface area covering 1554 km², Galveston Bay is the seventh largest estuary in the U.S. and the second largest estuary on the Texas coast [35, 43, 44]. Galveston Bay receives freshwater from the Trinity River, San Jacinto River, Clear Creek, and smaller bayous and creeks, with the Trinity River providing 70% of the freshwater entering the Bay [35, 45, 43, 44]. The Bolivar Peninsula and Galveston Island separate Galveston Bay from the GOM, with exchange of water between the Bay and the GOM occurring through Bolivar Roads, i.e., the mouth of the Bay [46, 43]. 

Monthly cruises were conducted between October 2017 and September 2018 on board the R/V Trident. Timing of the study allowed for examination of the factors regulating CO₂ flux over the course of a year following Hurricane Harvey in late August of 2017. Although the study began more than 45 days (the residence time of the Bay) after Harvey, salinity recovery of the Bay was likely still ongoing in the inner and middle sections of the Bay [47, 48]. 

During each monthly survey, a transect was run between five water sampling stations, extending northwest from the Bay mouth (Station 1) opening to the Five Mile Marker on the Houston Ship Channel (Station 5). One offshore cruise in the nwGOM outside Galveston Bay was conducted in October of 2018. At each station, surface (~0.5 m below water surface) and bottom water (~0.5 m above the sediment) samples for carbonate analyses were collected. A van Dorn sampler was used to collect unfiltered surface and bottom water into 250 mL borosilicate glass bottles for total alkalinity (TA), dissolved inorganic carbon (DIC), and pH analyses. 100 μL saturated HgCl₂ was added to each water sample to cease biological activity and bottle stoppers were replaced following the application of Apiezon® grease and secured with rubber bands and hose clamps.  The samples were stored at 4 ˚C in the dark until analyses, usually within 2–3 weeks of sample collection. Surface and bottom unpreserved water samples were collected in 125 mL polypropylene bottles for Ca²⁺analysis. 

Discrete sample analyses

Water samples collected at the surface and bottom at each station were analyzed for DIC, TA, pH, and salinity as in [53, 34]. DIC was analyzed by acidifying 0.5 mL water samples with 0.5 mL 10% H₃PO₄ using a 2.5 mL syringe pump on an AS-C3 DIC analyzer (Apollo SciTech Inc.) with a precision of ±0.1%. TA was analyzed at 22.0±0.1⁰C using gran titration of a 25 mL water sample with 0.1 M HCl solution (in 0.5 M NaCl) on an AS-Alk2 alkalinity titrator (Apollo SciTech Inc.), with a precision of ±0.1%. Precisions were estimated based on randomly collected duplicate samples. Reference Material (RM) produced in the lab of Andrew Dickson at Scripps was used in both TA and DIC analysis to ensure data quality [54]. 

A spectrophotometric method with a precision of ±0.0004, and purified m-cresol purple (mCP) obtained from Dr. Robert Byrne’s lab (University of South Florida) [55] was used for pH (on the total scale) analysis [56]. Prior to each sample analysis, a calibrated Orion^TM Ross^TM glass electrode was used to adjust the indicator to pH 7.92 ± 0.01. A 10 cm water-jacketed absorbance cell for pH measurements [56] was kept at 25 ± 0.01 ⁰C. Consecutive runs were done for each sample whereby two volumes (30 μL and 60 μL) of mCP were added to correct the dye effect [57]. Equations from Liu et al. [55] were used when salinity was greater than 20 for the entirety of a sampling trip, and equations from Douglas and Byrne [58], which allows for a wider salinity range (0–40 vs. 20–40) [15] were used when salinity was less than 20 for an entire sampling trip, for calculations of pH. Calculated pH values (on total scale) were converted to in situ temperature using the program CO2SYS with DIC as the other input parameter. 

Salinity was measured with a benchtop salinometer (Orion Star^TM A12, Thermo Scientific), which was calibrated using MilliQ water and known salinity CRM seawater before each sample analysis. Calcium ([Ca²⁺]) concentration was measured using automatic potentiometric titration with ethylene glycol tetraacetic acid (EGTA), with a precision of ±0.2% [59]. A Metrohm® Titrando calcium-selective electrode on a titration system (Metrohm Titrando 888) was used to detect the endpoint. 

Meteorological data

United States Geological Survey [62] streamgages for the Trinity River (gage #08066500) and San Jacinto River, east fork (SJE; gage #08070200) and west fork (SJW; gage #08068000) were used to obtain freshwater discharge. These stations were identified as the closest gages to the mouths of the rivers having complete discharge data for the period of study. Discharges of less than or equal to 45 days (residence time of the Bay) prior to flux estimates were utilized [43, 44]. The Texas Commission on Environmental Quality (TCEQ) performs routine water quality monitoring, and TCEQ water sampling stations were used for river endmember values from the San Jacinto (average of west fork station #11243 and east fork station #11238) and Trinity (station # 10896) rivers [63]. River endmember DIC was calculated from TA and pH measurements using K₁ and K₂ constants from Millero [64], and pH value on the NBS scale. Seasonally weighted averages were calculated by summing the TA or DIC concentration multiplied by daily discharge values for all river measurements of that season and dividing by the sum of all discharge values for all river measurements of that season (using meteorological seasons).