Dataset: Bowdoin buoy discrete sampling

Sample Collection
Discrete samples were collected from Niskin bottles or surface buckets in the estuary, and transferred to sample bottles through silicone tubing to prevent bubbling. Samples for pH, total alkalinity (TAlk), and dissolved inorganic carbon (DIC) were transferred without bubbling into 60-ml glass BOD bottles with greased stoppers. These were filled to leave less than 1% headspace in the bottle, preserved with 0.1 ml of saturated mercuric chloride solution, and immediately cooled. Samples for TAlk and DIC were generally stored for several weeks before analysis, allowing for settling of particulate material, with the supernatant sample drawn for analysis. DIC was measured first from each sample bottle, followed sequentially by pH and TAlk.

Temperature and Salinity Measurement
Temperature and salinity were measured in situ with either a SBE-37 thermosalinograph (Sea-Bird Electronics, Bellevue, WA) deployed as part of a profiling package (conductivity accuracy +/- 0.0003 S/m, temperature accuracy +/- 0.002 degrees C), or measured in situ with a handheld meter (YSI, Yellow Springs, Ohio) with a manufacturer-stated accuracy +/- 0.2 degrees C and +/- 1.0 uS.

DIC Analysis
DIC of unfiltered water was determined using an automated analyzer built by Apollo SciTech (Bogart, GA). Immediately after opening the sample bottle, a digital syringe withdrew a small amount of sample (0.5 mL), acidified it with 10% phosphoric acid and subsequently measured the evolved CO2 with a Li-Cor 6262 non-dispersive infrared gas analyzer (similar to the method described by Cai and Wang 1998). Certified seawater reference materials from Dr. A. Dickson were used to determine DIC concentration by preparing a calibration curve covering the range of DIC from 200-2000 umol/kg (Dickson et al., 2003), with a resulting precision ranging from 0.05-0.5% (or 0.1-10 umol/kg), with an average of ~0.1% (2 umol/kg).

TAlk and pH Analysis
TAlk and pH of unfiltered water were simultaneously measured by the same instrument, and thus pH and TAlk measurements are both based on the same pH electrode. The pH electrode used in the TAlk titration (Orion 3-Star, Thermo Fisher Inc.) was calibrated using three low ionic strength pH buffers certified on the U.S. National Bureau of Standards (NBS) scale to +/- 0.01, and the initial reading before the addition of acid titrant was taken as the sample pHNBS (pH on the NBS scale, hereafter simply referred to as pH).

TAlk was measured by Gran titration (Gran 1952) with 0.1N HCl using an automated titrator. This method adds an initial aliquot of acid to the sample in an open cell, generally lowering the sample pHNBS below 3.8, and then adds subsequent aliquots of titrant until the pHNBS changes linearly with the volume of acid added. The TAlk endpoint is then obtained from linear regression of the change of pHNBS against the volume of acid added, according to a Gran transformation:

GF = (v + Vo)/Vo * 10^(-pH)

where GF is the resulting Gran Function, v is the volume of acid added to the sample, Vo is the original sample volume, and pH is the pH value (in this case, on the NBS scale) measured after each successive addition of volume v. Multiple analyses of the Dickson reference material resulted in a calculated precision of this method of about 0.1% (or ~ +/- 2 umol/kg). The accuracy of the TAlk automated system, also calibrated with multiple batches of the Dickson CRM as discussed above, was +/- 3-4 umol/kg.

References:
Cai, W.-J. and Wang, Y. 1998. The chemistry, fluxes and sources of carbon dioxide in the estuarine waters of the Satilla and Altamaha Rivers, Georgia. Limnology and Oceanography 43: 657-668.

Dickson, A.G., Afghan, J.D. and G.C. Anderson. 2003. Reference materials for oceanic CO2 analysis: a method for the certification of total alkalinity. Marine Chemistry 80: 185-197. doi:10.1016/S0304-4203(02)00133-0

Gran, G.  1952.  Determination of the equivalence point in potentiometric Titrations, Part II, Analyst 77: 661-671. doi:10.1039/AN9527700661