2021 Experimental Discrete Field and Laboratory CO2 System Measurements from the Hillsborough River, Florida

Discrete water samples from the Hillsborough River study site were collected on January 26, 2021, from the boardwalk along Lowry Park Riverside Trail (28.0138° N, -82.4646° W) from two 30 L Niskin bottles that were used to collect surficial river water. For additional details on experiment setup, see (Moore and others, 2022b). Prior to the Niskin bottle water sample collection, surficial river water temperature and salinity were measured and recorded in the field using a Seabird Scientific Micro Thermosalinograph. Once surficial water was collected in each Niskin, the water from each Niskin was filtered, promptly analyzed to establish the samples’ initial spectrophotometric pHT, and then collected for analysis of major inorganic nutrients. The filtration was performed under positive pressure using a 12V peristaltic pump tubing. The tubing was connected to a 142 mm diameter acrylic filter housing that contained a 0.45 µm cellulose nitrate filter. Water collected for pH analysis followed the methods of Clay and Byrne (1993) where samples were analyzed spectrophotometrically using an Ocean Optics USB2000 spectrometer and Ocean Optics LS-1 light source with purified meta-Cresol purple. pH was calculated on the total scale using the algorithms of Müller and Rehder (2018) and temperature-corrected using the algorithms of Waters and others (2014) using CO2sys.m (van Heuven and others, 2011). Water collected for analysis of major inorganic nutrients followed the methods of Gordon and others (1993) and were additionally filtered using a sterile 0.22 µm pressure filter attached by Luer lock to a 60 mL syringe and then stored in a 30 mL acid-washed high-density polyethylene bottle. Once each nutrient sample bottle was sealed, the bottle was stored on ice, transported to the laboratory, and then frozen until analysis (Dore and others, 1996). Next, water samples from each Niskin were consecutively filtered into a total of 70 borosilicate glass bottles (300 mL total volume) using the methods described by Dickson et al. (2007). Half of the samples (i.e., 35 samples from one Niskin bottle) were poisoned with 100 µL of 6.5% HgCl2. The 35 filtered water samples from the second Niskin were not poisoned. Both sets of bottles were promptly sealed with grease for transport. After these samples were collected and processed, additional water from each Niskin was filtered, analyzed spectrophotometrically to establish the samples’ final pHT, and then collected for analysis of major inorganic nutrients. All water samples were then transported to the Carbon Laboratory at the U.S. Geological Survey (600 4th St. S St. Petersburg, FL 33701) where the nutrient samples were frozen for later analysis. All borosilicate glass bottles were stored in the dark at room temperature (25°C) and sequentially analyzed, beginning hours after collection and subsequently over a period of three months, for pHT (total scale), total dissolved inorganic carbon (CT) and total alkalinity (AT).

Water samples were sequentially analyzed, beginning hours after collection, over a period of three months for pHT (total scale), total dissolved inorganic carbon (CT) and total alkalinity (AT) respectively in the USGS SPCMSC Carbon Laboratory. During each sample analysis, duplicate water samples of HgCl2 poisoned and unpoisoned bottles were first analyzed for pH, where sample water was poured into two 10-centimeter (cm) pathlength cells that were placed in a water bath set to 20°C. Once temperature equilibrated, pH analysis was performed using the methods of Clayton and Byrne (1993) using an Ocean Optics USB2000 spectrometer and Ocean Optics LS-1 light source with purified meta-Cresol purple indicator dye. pH was calculated on the total scale using the algorithms of Müller and Rehder (2018) and temperature corrected using the algorithms of Waters and others (2014) using CO2sys.m (van Heuven and others, 2011). Next, total dissolved inorganic carbon analysis was performed using a CM5017 CO2 carbon coulometer coupled to a CM5330 acidification module following the methods described by Dickson and Goyet (1994). Each water sample (~20 mL) was drawn from a bottle after pH analysis using a 60 mL syringe attached to a three-way Luer lock valve and stopper to minimize gas exchange. Sample weight was determined from the difference in syringe weight before and after the sample was injected through a septum on the stripping chamber of the acidification module, using a Denver Instruments PI-214 analytical balance. Samples were acidified with ~10 mL of 8.5% of phosphoric acid (H3PO4-). Analytical grade nitrogen (N2) gas was used as the carrier gas of CO2 from the acidified sample to the coulometer. The titration endpoint was determined by the coulometer, and CT was calculated using software from UIC, Inc. Accuracy and precision was determined from the analysis of certified reference materials (CRM) Dickson (2010). Lastly, total alkalinity analysis was performed, using the spectrophotometric methods of Yao and Byrne (1998) and Liu and others (2015). Each water sample (~100 mL) was poured from a bottle after CT analysis into an open glass cell (Hellma Cells, Inc.). Mass of the sample was determined gravimetrically using a Denver Instruments PI-214 analytical balance and placed within a custom plastic frame connected to an Ocean Optics LS-1 tungsten light source and Ocean Optics USB2000 spectrometer. Bromocresol purple indicator dye (4 millimolar (mM)) was administered by pipette and the sample was titrated using 0.100 Normal (N) standardized hydrochloric acid (HCl) using a plastic 10 mL syringe fitted with a Teflon needle. Absorbance measurements were made using the Ocean Optics software package OOIBase 32. Solution pHT was measured continuously throughout the titration to and end endpoint of near pH 4.3. The total weight of the added acid was determined from the difference in the syringe weight before and after acid addition. At the end of each titration, the solution was purged with a stream of N2 gas pre-saturated with water (H2O). After purging, final absorbance measurements were made, and solution temperature was determined using a Fluke 51 II Handheld Digital Probe Thermometer. AT was calculated using the equations of Hudson-Heck and others (2021). Accuracy and precision were determined from analysis of CRMs from Dickson (2010).