Stock tris-buffered synthetic seawater (S = 35) was prepared

gravimetrically, following the method of Dickson et al. (2007). The synthetic seawater had an initial pHT of 8.113 (determined using mCP and Selleck Dabrafenib the narrowband spectrophotometer). A series of solutions with pHT values ranging from 7.6 to 8.2 was then prepared by adding 1 N HCl or 1 N NaOH to the buffered synthetic seawater. Indicator (mCP) was added, and RN was measured. Corresponding RB values were measured using the LED photometer at the same sample temperature. An addition of an acid–base pair (e.g., the HL− and L2 − forms of mCP) to natural (unbuffered) seawater creates a small perturbation from the original pH of the sample. For our natural seawater samples, perturbation corrections for RN were determined and applied according to standard procedures ( Clayton and Byrne, 1993 and Dickson et al., 2007). No perturbation corrections were applied to RB. Such perturbations are too small for the photometer to reliably measure, and pHT errors caused by indicator additions (on the order of ~ 0.002

at pHT = 7.9) are much smaller than the target accuracy for the photometer measurements (± 0.01). For our tris-buffered artificial seawater samples, the solutions were sufficiently buffered that indicator-induced pH perturbations were negligible. Laboratory analyses were conducted using surface seawater collected at two locations in the Gulf of Mexico (29°44′N, 86°20′W and 29°50′N, 86°11′W). Original sample salinities were 36.2 and 36.1, respectively. Sample compositions (S and pHT) RG7422 supplier were varied by adding deionized H2O and 1 N HCl. These additions produced a total of 136 seawater samples of 31.0 ≤ S ≤ 36.2 and

7.6 ≤ pHT ≤ 8.2. Paired measurements of sample pHT were then made using the narrowband spectrophotometer and the LED photometer. To evaluate the performance of the LED photometer at different sample temperatures, the laboratory seawater samples were warmed or cooled with a Huber Polystat CC3 Water Bath (Huber Kaltemaschinenbau GmbH, Germany) to achieve a temperature range of 15 ≤ t ≤ 30 °C. At each target temperature, pHT was measured using the narrowband spectrophotometer and the LED mafosfamide photometer. One field test was conducted during an August 2013 R/V Weatherbird II cruise to the Gulf of Mexico. The pHT of seawater samples (collected at 28.75°N, 88.40°W) was measured shipboard (t = 25 °C) using the LED photometer and the Agilent 8453 benchtop spectrophotometer. A second field test was conducted in an aquarium setting. In this case, seawater samples were drawn from a 350-gallon saltwater reef aquarium system. The tank contained artificial seawater with a salinity of approximately 33–34 (made from deionized water and Seachem’s Aqua Vitro Salinity sea salt blend).