Surface sediments were sampled from 19 sites in the Mission Aransas Estuary using home-made core tubes, then sectioned and frozen immediately at -80 ºC until analysis. Sediment core samples were analyzed for mineral grain size, organic carbon and nitrogen, stable isotopic composition, pigments, plus sediment alkane and polycyclic aromatic hydrocarbons.

Mineral grain size
The grain size of surface sediment was measured using a Beckman-Coulter laser particle size analyzer (Wang et al., 2014). Briefly, 20 mL deionized water was added to 1 gram of freeze dried sediment in a beaker. After soaking for 24 hours, the sediment was subjected to vortex mixing for 5 minutes to disaggregate loosely-attached aggregates. Neither organic matter nor carbonate was removed for the laser grain size analysis. The detected size range is from 0.02 to 2000 µm.

Organic carbon and nitrogen, and stable isotopes
Surface sediment (~2 g) was first freeze dried using Labconco FreeZone, and then also acidified to remove inorganic carbon. The organic carbon and nitrogen content and δ13C in these samples was measured using a Thermo FLASH 2000 CHN Elemental Analyzer coupled with a Thermo Delta V Plus isotope ratio mass spectrometer. The δ13C values were expressed relative to Vienna Pee Dee Belemnite standard. Precision for the C/N content is within 5% and for δ13C within 0.2‰.

Pigment analysis
Approximately 2 grams of frozen sediment was transferred into a 15 mL polypropylene centrifuge tube, to which 3 mL acetone was added for pigment extraction (Sun et al., 1991). The mixture was sonicated for 15 minutes, and then centrifuged for another 10 minutes. The acetone extract was filtered with a syringe filter (0.2 µm Nylon filter). The remaining sediment in the centrifuge was extracted again by the same procedure using fresh acetone, and the two extracts were combined before the high performance liquid chromatography (HPLC) analysis. Quantitative analysis of all pigments was conducted 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). Sediment water contents were used to convert the concentrations of pigment into micrograms per gram (µg/g) of dry sediment.

Sediment alkane and polycyclic aromatic hydrocarbons (PAHs)
PAHs extraction and analysis followed Rhind et al. (2009) and Wang et al. (2014). Briefly, about 1 gram sediment (dry weight) was added with surrogate standards (Ace-d10, Phe-d10, BaP-d12) and 8 mL ethanoic potassium hydroxide (1 M). The samples were heated to 90°C for 8 hours. The analytes were extracted by hexane and then purified with a column packed with activated silica gel and topped with 1 cm anhydrous sodium sulfate. The PAHs were then eluted with dichloromethane/hexane (1:4, v/v). The eluted solution was concentrated and exchanged by hexane to 1 mL with a rotary evaporator, and stored at 4°C until further analysis. PAHs were analyzed by gas chromatography– mass spectrometry (GC-MS, Shimadzu QP2010 plus). The GC-MS is equipped with a RXi-1MS capillary column (20 mÅ~0.18 mm i.d., film thickness 0.18 μ m), with helium as the carrier gas at a flow rate of 0.8 mL min−1, using a selective ion monitoring mode to detect PAH. The scan ions ranged from 126 to 279 atomic mass units, and the dwell time per ion was 200 milliseconds. The oven temperature was held at 60°C for 1 min, increased to 240°C at a rate of 10°C min−1, and then increased to 280°C at a rate of 4°C min−1 and held for 3 min. The temperatures of the injector and detector were 260°C and 275°C, respectively. The injection volume was 1 μl with a split ratio of 1/20. All of the 16 PAHs were eluted from 5 to 30 min in the GC column.

Acquisition notes:
Stations S15 and S19 were not sampled in April 2018 due to extremely low water level which impeded boat passage to the two sampling sites