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 of Mexico from October 2017 to September 2018. Underway pCO2 measurements were recorded using a Shipboard Underway pCO2 Environmental Recorder (SUPER-CO2) system. CO2flux was calculated for 0.025° x 0.025° latitude increments along the transect and total CO2 flux for the Bay was estimated. Mean Bay water pCO2 was 384.2 ± 56.7 µatm. A large freshwater inflow event in spring was followed by a period of dilution (low salinity, TA, and DIC) and enhanced primary production (low pCO2, water, CO2 uptake, and high chlorophyll-a levels). CO2 flux exhibited large seasonal and spatial variability, likely primarily due to seasonality in photosynthesis and variability of freshwater inflow events. Overall, Galveston Bay was a sink for CO2, 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.

Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • Problem Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

 

Field Sampling

Galveston Bay is a semi-enclosed microtidal estuary located in the northwestern Gulf of Mexico (nwGOM) (Montagna, Palmer, & Pollack, 2013). With an average water depth of 3 m and a 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 (Bass et al., 2018; Morse, Presley, Taylor, Benoit, Santschi, 1993; Solis & Powell, 1999). 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 (Bass et al., 2018; Solis & Powell, 1999). The Bolivar Peninsula and Galveston Island separate Galveston Bay from the Gulf of Mexico (GOM), with exchange of water between the Bay and the GOM occurring through Bolivar Roads, i.e., the mouth of the Bay (Glass, Rooker, Kraus, & Holt, 2008).

Monthly cruises were conducted between October 2017 and September 2018 on board the R/V Trident. The 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 after Harvey (the residence time of the Bay), salinity recovery of the Bay was likely still ongoing in the inner and middle sections of the Bay (Du & Park, 2019; Du, Park, Dellapenna, & Clay, 2019).

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. Underway pCO₂ measurements were taken along a northwesterly transect extending from stations 1 through 5. A SUPER-CO₂ System equipped with a LI-COR® LI-840A infrared gas analyzer was used to collect both water and air xCO₂ after drying through a Peltier thermoelectric device. The xCO₂ data, after removing residual water vapor (Honkanen et al., 2021), were converted to pCO₂ at sea surface temperature assuming 100% water vapor pressure (Jiang, Cai, Wanninkhof, Wang, & Lu, 2008). Underway seawater was taken from a steel pipe attached to the side of the research vessel, as it did not have a dedicated water intake system, and a diaphragm water pump was used to feed water to the equilibrator. In situ sea surface temperature and salinity were measured with a SeaBird Scientific SBE45® Thermosalinograph mounted parallel to the equilibrator of the SUPER-CO₂ System. Prior to and following each sampling trip, the SUPER-CO₂ System was calibrated using standards of known CO₂ concentrations (273.3, 774.3, and 1468.7 ppm).

To calculate the pCO₂ of seawater and air from measurements, the measured mole fraction of CO₂ in seawater (xCO₂, water) and measured equilibrator barometric pressure and xH₂O were first used to calculate xCO₂ in dry air (xCO₂, air). This xCO₂, air was then converted to pCO₂ of equilibration (pCO₂, eq) using measured temperature of equilibration (Teq) and water vapor pressure of equilibration, which was calculated from salinity and Teq according to methods outlined by Weiss and Price (1980). Next, sea surface temperature (SST) and Teq were used to convert pCO₂, eq to pCO₂, water according to Jiang et al. (2008). For pCO₂, air, xCO₂, air was converted to pCO₂, air using water vapor pressure at SST and salinity, assuming 100% humidity (Weiss & Price, 1980).

Meteorological Data

Three National Oceanic and Atmospheric Administration (NOAA) buoys from throughout Galveston Bay (NOAA, 2022) provided six-minute interval averages of continuous wind speed data. The average wind speed for all three buoys during sampling times was calculated and applied to the timing of sampling in Galveston Bay. Prior to calculations, wind speeds were converted to a height of 10 m (u₁₀) using the wind profile power law (Hsu, Meindl, & Gilhousen, 1994):

u₁/u₂ = (z₁/z₂)^P

In this calculation, u₂ is wind speed at height z₂ = 10 m, u₁ is the collected wind speed data at height z₁, and the exponent P (0.11) around the GOM area is extracted by Hsu et al. (1994).

United States Geological Survey (USGS) stream gages 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 (USGS, 2021). 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 (Solis & Powell, 1999). 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 (TCEQ, 2022). River endmember DIC was calculated from TA and pH measurements using K₁ and K₂ constants from Millero (2010), 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).

Historical Data

Results from this study were compared to historical data for Galveston Bay obtained from the Surface Ocean CO₂ Atlas (SOCAT) database (Bakker et al., 2016), which provided fCO₂, water and xCO₂, air values, along with surface seawater salinity, temperature, and depth, with observations from 2006 and 2010 through 2016, primarily during the month of September. SOCAT transects followed a similar route to our study transect, beginning near station 4 and continuing outward into the GOM, with a side transect through the Galveston Channel, which separates Pelican Island from Galveston Island. fCO₂ values were converted to pCO₂ using the R package seacarb (Gattuso et al., 2022). SOCAT data were analyzed independently from the results of this study. As done before with ship data, SOCAT xCO₂, air was converted to pCO₂, air by accounting for water vapor pressure based on SST and SSS, assuming 100% humidity (Borges et al., 2004).

Air-water CO2 flux calculation

Air-water CO2 flux calculation is performed using the following equation:

F=kK0(pCO2,water−pCO2,air)(3)F = k K_0 (pCO_{2,water} - pCO_{2,air}) \tag{3}F=kK0​(pCO2,water​−pCO2,air​)(3)

where:

• FFF is the air-water CO2 flux,
• kkk (m d⁻¹) is the gas transfer velocity, calculated from wind speed,
• K0K_0K0​ (mol m⁻³ atm⁻¹) is the gas solubility at measured in situ temperature and salinity (Weiss, 1974),
• pCO2,waterpCO_{2,water}pCO2,water​ and pCO2,airpCO_{2,air}pCO2,air​ are the partial pressures of CO₂ in water and air, respectively.

The gas transfer velocity kkk is determined using the equation from Jiang et al. (2008):

k=(0.314×u102−0.436×u10+3.990)×(ScSST600)−0.5(4)k = (0.314 \times u_{10}^2 - 0.436 \times u_{10} + 3.990) \times \left( \frac{ScSST}{600} \right)^{-0.5} \tag{4}k=(0.314×u102​−0.436×u10​+3.990)×(600ScSST​)−0.5(4)

Where:

• u10u_{10}u10​ is the wind speed at 10 m above the water surface (m s⁻¹),
• ScSSTScSSTScSST is the Schmidt number of CO₂ at the in situ temperature of seawater (Wanninkhof, 1992).

Seasonal values are determined by averaging monthly CO2 flux estimates by season (fall, winter, spring, summer). Additionally, missing values for atmospheric pCO₂ are estimated using linear interpolation.

Thermally-adjusted water pCO₂ is calculated using an equation from Takahashi (2002) to account for temperature effects:

pCO2,waterthermally adjusted=(Eq. 1 from Takahashi)(1)pCO_{2,water_{thermally\ adjusted}} = \text{(Eq. 1 from Takahashi)} \tag{1}pCO2,waterthermally adjusted​​=(Eq. 1 from Takahashi)(1)

Statistical analyses

Pearson’s correlation coefficients were calculated to determine the influence of wind speed and direction on pCO₂, air. Variables with a Pearson’s correlation p-value < 0.05 and an absolute correlation coefficient > 0.7 were considered significantly correlated with pCO₂, air.

Since data were non-normal and variances were non-homogeneous, Kruskal-Wallis nonparametric ANOVA tests were used to compare parameters such as DIC, TA, pH, and ΩAr between seasons and stations. Dunn tests were further used to test for pairwise differences (Ruxton & Beauchamp, 2008).

Multiple Linear Regression models were applied to assess biogeochemical influences on pCO₂. The final model included predictor variables such as DIC, TA, ΩAr, and pHT, after excluding salinity difference and SST.

Gaps in sampling were filled with linear interpolation.

Coverage: Northwest Gulf of Mexico estuaries on Texas Coast

NSF Award Abstract:
Hurricane Harvey made landfall Friday 25 August 2017 about 30 miles northeast of Corpus Christi, Texas as a Category 4 hurricane with winds up to 130 mph. This is the strongest hurricane to hit the middle Texas coast since Carla in 1961. After the wind storm and storm surge, coastal flooding occurred due to the storm lingering over Texas for four more days, dumping as much as 50 inches of rain near Houston. This will produce one of the largest floods ever to hit the Texas coast, and it is estimated that the flood will be a one in a thousand year event. The Texas coast is characterized by lagoons behind barrier islands, and their ecology and biogeochemistry are strongly influenced by coastal hydrology. Because this coastline is dominated by open water systems and productivity is driven by the amount of freshwater inflow, Hurricane Harvey represents a massive inflow event that will likely cause tremendous changes to the coastal environments. Therefore, questions arise regarding how biogeochemical cycles of carbon, nutrients, and oxygen will be altered, whether massive phytoplankton blooms will occur, whether estuarine species will die when these systems turn into lakes, and how long recovery will take? The investigators are uniquely situated to mount this study not only because of their location, just south of the path of the storm, but most importantly because the lead investigator has conducted sampling of these bays regularly for the past thirty years, providing a tremendous context in which to interpret the new data gathered. The knowledge gained from this study will provide a broader understanding of the effects of similar high intensity rainfall events, which are expected to increase in frequency and/or intensity in the future.

The primary research hypothesis is that: Increased inflows to estuaries will cause increased loads of inorganic and organic matter, which will in turn drive primary production and biological responses, and at the same time significantly enhance respiration of coastal blue carbon. A secondary hypothesis is that: The large change in salinity and dissolved oxygen deficits will kill or stress many estuarine and marine organisms. To test these hypotheses it is necessary to measure the temporal change in key indicators of biogeochemical processes, and biodiversity shifts. Thus, changes to the carbon, nitrogen and oxygen cycles, and the diversity of benthic organisms will be measured and compared to existing baselines. The PIs propose to sample the Lavaca-Colorado, Guadalupe, Nueces, and Laguna Madre estuaries as follows: 1) continuous sampling (via autonomous instruments) of salinity, temperature, pH, dissolved oxygen, and depth (i.e. tidal elevation); 2) bi-weekly to monthly sampling for dissolved and total organic carbon and organic nitrogen, carbonate system parameters, nutrients, and phytoplankton community composition; 3) quarterly measurements of sediment characteristics and benthic infauna. The project will support two graduate students. The PIs will communicate results to the public and to state agencies through existing collaborations.