Dataset: Houston Galveston Bay Pigments
Deployment: Galveston_Bay_Cruises

Field Sampling

Galveston Bay is a semi-enclosed microtidal estuary located in the northwest Gulf of Mexico (Montagna et al., 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 et al., 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; Dellapenna et al., 2020; Morse et al., 1993; Solis & Powell, 1999). The Bolivar Peninsula and Galveston Island separate Galveston Bay from the Gulf of Mexico, with the exchange of water between the Bay and the Gulf occurring through Bolivar Roads, i.e., the mouth of the Bay (Glass et al., 2008; Morse et al., 1993).

Monthly cruises were conducted between October 2017 and September 2018 onboard the R/V Trident. Timing of the study allowed for the examination of the factors regulating CO₂ flux over the course of a year following Hurricane Harvey in late August 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 (Du & Park, 2019a; Du et al., 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 northwest Gulf of Mexico outside Galveston Bay was conducted in October 2018.

Pigments

Chlorophyll-a concentrations were analyzed from surface water samples collected at each station in the Bay as described by Liu and Xue (2020). Surface waters were filtered through GF/F filters, which were frozen immediately in liquid nitrogen and later stored in the freezer at −80°C until analysis. Extraction of pigments from filters followed procedures outlined by Liu et al. (2006) and Sun et al. (1991), whereby filters were extracted in acetone in polypropylene centrifuge tubes, which were sonicated for 15 minutes in a sonicator (Model FS 60, Fisher Scientific). Acetone extract was filtered through a syringe filter (0–2 μm Nylon filter). Procedures were repeated for sample filters, and the two extracts (total 6 mL) were combined and blown with nitrogen gas under ice to dryness (Chen et al., 2005). Acetone (0.5 mL) was then added to dissolve the residue before high-performance liquid chromatography (HPLC) analysis (Liu & Xue, 2020).

Pigments were analyzed using a Shimadzu HPLC system with a reverse-phase column (Agilent Eclipse XDB-C8, 3.5 μm particle size, 150 mm length × 4.6 mm diameter) and a photodiode array (PDA) detector set at 450 nm. The mobile phases included A (70:30 v/v methanol: 28 mM tetrabutylammonium acetate; pH 6.5) and B methanol (100%). After sample injection (400 μL, mixing 0.5 mL acetone extract and 1.25 mL 28 mM tetrabutylammonium acetate), a gradient program (1.0 mL/min) began with 5% B and increased to 95% B in 22 minutes, then to 95% B isocratically over 30 minutes. All chromatographic separations were performed in a column oven set at 60°C. Pigments were identified by comparing retention times with authentic standards purchased from DHI (Denmark) or Sigma-Aldrich (USA). Peak areas were converted to concentrations based on response factors calculated from authentic standards. Duplicate analyses of the same extract generally agreed within 20%.