Autonomous in Situ Measurements of Seawater Alkalinity

Article pubs.acs.org/est

Autonomous in Situ Measurements of Seawater Alkalinity Reggie S. Spaulding,*,† Michael D. DeGrandpre,‡ James C. Beck,† Robert D. Hart,‡ Brittany Peterson,‡ Eric H. De Carlo,§ Patrick S. Drupp,§ and Terry R. Hammar⊥ †

Sunburst Sensors, 1226 W Broadway, Missoula, Montana 59802, United States Department of Chemistry and Biochemistry, University of Montana, 32 Campus Drive, Missoula, Montana 59812, United States § Department of Oceanography, University of Hawaii, Manoa, Hawaii 96822, United States ⊥ Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, United States ‡

S Supporting Information *

ABSTRACT: Total alkalinity (AT) is an important parameter for describing the marine inorganic carbon system and understanding the effects of atmospheric CO2 on the oceans. Measurements of AT are limited, however, because of the laborious process of collecting and analyzing samples. In this work we evaluate the performance of an autonomous instrument for high temporal resolution measurements of seawater AT. The Submersible Autonomous Moored Instrument for alkalinity (SAMI-alk) uses a novel tracer monitored titration method where a colorimetric pH indicator quantifies both pH and relative volumes of sample and titrant, circumventing the need for gravimetric or volumetric measurements. The SAMI-alk performance was validated in the laboratory and in situ during two field studies. Overall in situ accuracy was −2.2 ± 13.1 μmol kg−1 (n = 86), on the basis of comparison to discrete samples. Precision on duplicate analyses of a carbonate standard was ±4.7 μmol kg−1 (n = 22). This prototype instrument can measure in situ AT hourly for one month, limited by consumption of reagent and standard solutions.

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INTRODUCTION Atmospheric CO2 has increased from 280 to 400 ppm over the past ∼150 years as a consequence of industrialization.1,2 The oceans have prevented an even greater increase by absorbing a significant amount of anthropogenic CO2.3,4 Seawater pH has decreased by more than 0.1 pH units from conversion of the absorbed CO2 to carbonic acid,5,6 an undesirable side-effect of ocean CO2 uptake. The decreased pH will potentially change biological processes in the oceans. For example, as the oceans become more acidic, aragonite and calcite saturation states become lower,5−7 and it becomes more difficult for calcifying organisms to produce shells.7,8 The predicted detrimental effects on marine calcifiers could propagate through the food chain, altering entire ecosystems.9,10 The inorganic carbon parameters, including saturation states, can be calculated if two of the primary parameters, partial pressure of CO2 (pCO2), total hydrogen ion concentration (pHT), total alkalinity (AT), and total dissolved inorganic carbon (CT), are known. The carbonate system is most accurately defined by measuring either pH or pCO2 in combination with either AT or CT.11,12 In situ instruments are currently available to measure only pH13−15 and pCO2,16−18 although some progress has been made toward in situ CT measurements.19−21 Benchtop automated, flow-through instruments have been developed for AT analysis,22−24 and an in situ potentiometric AT titrator was developed,25 but to our © 2014 American Chemical Society

knowledge no in situ AT data have been reported. Thus, AT data are limited to shipboard determination during cruises or intense efforts with manual or robotic sample collection followed by manual analysis.26 In many cases it is possible to estimate AT using a relationship with salinity27 or salinity and temperature,28 because evaporation, precipitation, and mixing strongly control AT. However, these conservative relationships are not always followed. For example, in the Saragasso Sea, AT is generally conservative with salinity, but a several month-long drawdown of 25−30 μmol kg−1, likely due to coccolithophore calcification, was detected.29 After spring blooms in the northern Bay of Biscay, AT is consistently depleted by coccolithophore calcification, and thus nonconservative with salinity.30 In many coastal areas consistent salinity−AT relationships do not exist because of riverine inputs, CaCO3 dissolution, and anaerobic processes.31 On the Bering Sea shelf the relationship between AT and salinity can be highly variable, due to coccolithophore calcification and CaCO3 mineralization.32 In coral reef ecosystems, AT variability is typically dominated by CaCO3 formation and dissolution.26 Thus, AT−salinity relationReceived: Revised: Accepted: Published: 9573

April 2, 2014 July 13, 2014 July 22, 2014 July 22, 2014 dx.doi.org/10.1021/es501615x | Environ. Sci. Technol. 2014, 48, 9573−9581

Environmental Science & Technology

Article

the total acid added ([H+]added) to its concentration in the a/i solution (Ca/i). The dilution factor is also equivalent to the ratio of the volume of a/i (Va/i) to the total volume of the sampleindicator mixture (VT). The sample dilution factor is 1 − fa/i. The alkalinity balance can be derived using the sample dilution factor as shown below, eliminating the need to measure masses or volumes of sample and titrant. Throughout this paper AT is reported in μmol per kg solution, written as μmol kg−1. At the titration equivalence point, AT is equal to the moles of acid added. However, it is more accurate to measure the pH throughout the titration and calculate AT from the proton balance in eq 6, (ignoring minor protolytic species).

ships should be used with caution in most ocean regions, and are not valid on coral reefs. AT has historically been measured using either a modified Gran titration or a nonlinear least-squares approach, both of which require potentiometric pH measurement and accurate volumetric or gravimetric measurements of sample and titrant.33,34 Similar or better accuracy and precision have been achieved using titrations with spectrophotometric pH measurements.35,36 A novel, simplified titration, termed tracermonitored titration (TMT), has improved on the spectrophotometric method by eliminating the need for accurate volumetric or gravimetric measurements.37 In this method, an indicator tracer is used to quantify the dilution factors for the titrant and sample. We have developed a prototype instrument using the TMT methodology with bromocresol purple as both the tracer and pH indicator. The instrument, named the Submersible Autonomous Moored Instrument for alkalinity (SAMI-alk) has been extensively tested and improved in the laboratory since the original benchtop proof-of-concept design described by Martz et al.37 Field-test results are reported here. The overall accuracy compared to discrete samples was −2.2 μmol kg−1 (n = 86) and the overall precision of duplicate in situ standard analyses was ±4.7 μmol kg −1 (n = 22), respectively. The SAMI-alk uses ∼80 mL of sample and 4.5 mL of titrant (acid + indicator), and takes ∼12 min to measure AT. The instrument can be moored and programmed to analyze standards or certified reference material (CRM) in situ. The length of deployment of the current design is limited to approximately 700 samples when using a 3-L reagent reservoir.

A T + [H+] + [HSO4 −] + [HF] − [OH−] − 2[CO32 −] − [HCO3−] − [B(OH)4 − ] = 0

The equation can be expressed in terms of hydrogen ion concentration, sample dilution factor, and total concentrations of sulfate (ST), fluoride (FT), inorganic carbon (CT), and borate (BT) (eq 7).37 A T(1 − fa/i ) + [H+] + −

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−

METHODS Theory. The indicator used for the SAM-alk, bromocresol purple (BCP), exists in three forms, represented as H2I, HI−, and I2−. H2I is not present in significant concentrations during a titration because its pKa is less than 1. HI− and I2− have absorption maxima at 432 and 589 nm, respectively. At a given wavelength (λ), the indicator absorbance (A) is given by Aλ =

HIε λb[HI

−

] + Iε λb[I2 −]

[I2 −] =

HIε 589Iε 432)

HIε 589Iε 432)

[I]T = [HI−] + [I2 −]

(1)

Va/i [H+]added [I]T = = [I]a/i Ca/i VT

FT (1 − fa/i ) 1 + KF/[H+]

2C T(1 − fa/i )K1K 2 Kw + − + 2 [H ] [H ] + K1[H+] + K1K 2 C T(1 − fa/i )K1[H+] + 2

+

[H ] + K1[H ] + K1K 2

⎛ R−e ⎞ 1 pH = pK a + log⎜ ⎟ ⎝ e 2 − Re3 ⎠

(2)

−

BT (1 − fa/i ) [H+]/KB + 1

− [I2 −] (7)

(8)

Test titrations were performed at different temperatures using seawater with a/i in either a simplified synthetic seawater background39 or a NaCl background. The pKa of BCP and its temperature dependence in these different salt mixtures is not known. We used a pKa that was determined by minimizing the residuals in the NLLS analysis, as described by Martz et al.,37 in conjunction with the salinity (S) dependence defined by Yao and Byrne35 and temperature dependence defined by the Van’t Hoff equation with an enthalpy of reaction (ΔH) of 3000 J. However, the temperature dependence of the measurement is largely accounted for by using temperature-dependent molar extinction coefficients for BCP (see Supporting Information). Using this analysis, the pKa values used were 5.849 and 5.941 at 25 °C and S = 35 in NaCl−seawater and synthetic

(3) (4)

Once [I]T is known, the a/i dilution factor, fa/i, is calculated from eq 5: fa/i =

1 + K s/[H ]

+

ST, FT, and BT are calculated from seawater salinity. K1 and K2 are the temperature- and salinity-dependent first and second ionization constants for H2CO3 and KS, KF, Kw, and KB are the temperature- and salinity-dependent ionization constants for bisulfate, fluoride, water, and borate, respectively. [H+] is the measured [H+]. The proton balance includes the contribution of the indicator [I2−] to the measured AT. All quantities are known except for AT and CT which are solved for using a nonlinear least-squares (NLLS) analysis in Matlab.37 pH is calculated with [H+] in mol kg seawater−1, from eq 8, where pKa is the ionization constant for BCP on the free H+ scale,37 R is the ratio of absorbances at 589 and 432 nm, and e1, e2, and e3 are molar extinction coefficient ratios.38

A589HIε 432 − A432HIε 589 b(HIε 432Iε 589 −

+

34

A432Iε 589 − A589Iε 432 b(HIε 432Iε 589 −

ST(1 − fa/i )

− [H+]added = 0

where ε is the molar extinction coefficient of the species at wavelength λ and b is the optical path length. The concentrations of HI− and I2− can be determined from absorbance measurements at the peak absorbing wavelengths using eqs 2 and 3. Total indicator concentration, [I]T, in the sample−acid/indicator (a/i) mixture is calculated from eq 4. [HI−] =

(6)

(5)

The a/i dilution factor is the total indicator concentration in the sample−a/i mixture ([I]T) relative to its concentration in the a/i solution ([I]a/i). This ratio is equivalent to the ratio of 9574

dx.doi.org/10.1021/es501615x | Environ. Sci. Technol. 2014, 48, 9573−9581


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LEDs is split with a 50:50 beamsplitter (Edmond Optics). Fifty percent of the light is carried to and from the optical cell through 1000-μm core optical fibers. Transmitted light is detected with a photodiode (Hamamatsu). The other 50% of the light is attenuated with a neutral density filter (0.8 optical density, Newport Corp.) and detected with a reference photodiode. SAMI-alk Operation and Calibration. An AT measurement begins by flushing the optical cell with 35−40 mL of seawater or standard, requiring 40−70 s, depending on in situ temperature and pressure. The diaphragm pump is more efficient at lower pressures and higher temperatures, so the flush time is adjusted according to the environment. The cell is then stirred for 30 s and blank (100% transmittance) light signals and reference signals are recorded at each wavelength (I0,λ and I0ref,λ). Next, a/i solution is pumped into the cell with 15−20 pump strokes from the 250-μL pump. After each pump of indicator, the cell contents are stirred for 5 s, light and reference signals (Iλ and Iref,λ) and temperature are recorded, absorbance (A) at wavelength λ is calculated from eq 9 and pH is calculated from eq 8.

seawater−natural seawater mixtures, respectively. These values fall between the values of 5.818 and 5.972, which were reported at 25 °C in 0.528 m NaCl plus 0.060 m MgCl235 and in seawater,40 respectively. SAMI-alk Components. A schematic diagram of the SAMIalk is shown in Figure 1. Tubing is 0.318 cm o.d. × 0.157 cm

⎛I I ⎞ Aλ = −log⎜ λ 0refλ ⎟ ⎝ I0λIrefλ ⎠

(9)

When the pH reaches 5.2, pumping of a/i solution switches to the 10-μL pump. After ∼50 10-μL pump strokes the solution reaches pH ≈ 4.2, the titration is terminated, and the optical cell is flushed with 35−40 mL of sample. The entire measurement takes ∼12 min and uses ∼4.5 mL of a/i. Flushing the cell both at the end and beginning of a titration, and at least a 10 min wait between titrations, are necessary in order to completely remove indicator from the cell. We believe the flushing difficulty is due to indicator adhering to the tubing and optical cell and dead volume areas, for example, at the optical fiber tips and corners of the cell. For calculation of AT, titration points between pH 4.2 and 5.0, with absorbances between 0.1 and 1.1, are used. The final analysis removes points from the NLLS if the squared residual exceeds 1 × 10−11. This decreases the number of titration points that are used in the final AT calculation but does not remove the titration result from the data set. Points in the titration might exceed the squared residual filter if an air bubble or other particle is in the light path or if the cell contents are not completely mixed. The SAMI-alk is also equipped with a solenoid valve that allows periodic analysis of an AT standard or certified reference material (CRM).41 In the field, standards are typically analyzed daily. We observed differences in accuracy as large as 50 μmol kg−1 on samples depending on the sample matrix, that is, NaCl, synthetic seawater, or seawater, apparently caused by small differences in the pKa of BCP in the different matrices. Therefore, it would be ideal to use seawater CRM on the SAMI-alk as an in situ standard so that the sample and standard have consistent matrices. However, the large sample volume used by the SAMI and high cost of CRM makes this an impractical option. Therefore, carbonate AT standards were made in the laboratory and we devised a calibration routine consisting of the following steps: (1) measure the AT of a seawater CRM on the SAMI-alk in the laboratory and set a calibration factor to a number that provides an accurate value for the CRM (the calibration factor is multiplied by Ca/i in eq 5); (2) measure the AT standard using the calibration factor

Figure 1. Schematic diagram of the SAMI-alk. Fluid paths are colorcoded as follows: sample or AT standard (blue), a/i (yellow), mixed sample−a/i (purple). Optical fiber is shown in gray. A magnetic stir bar is housed inside the optical cell for mixing.

i.d. PEEK, with either 0.076 cm i.d. tubing (Oregon study) or a 5-μm filter (Hawaii study) attached to the sample inlet to reduce introduction of particles into the mixing cell. Sample or standard is pumped with a diaphragm pump (NF1.5, KNF Neuberger). A 3-way solenoid valve (The Lee Company) at the pump inlet selects sample or standard. Another 3-way valve at the pump outlet prevents sample flow during titrations and is used to select sample or a/i solution. A/i solution is pumped with 250-μL per stroke (pretitration) and 10-μL per stroke (titration) solenoid pumps (Bio-Chem Fluidics). The pumps and valves are contained within a housing filled with ethylene glycol in contact with the external hydrostatic pressure via a diaphragm. The 3 mL mixing/optical cell is custom-made from Delrin, with a 1 cm optical path length, and mounted on a magnetic stirrer. Three electronic boards are used. A data logger (TFX-11, Onset Corp.) controls the pumps and valves, provides 12-bit analog-to-digital conversion and stores data. A custom-built power board provides power to the pumps, valves, and optical board. A custom-built optical board controls LEDs and measures light signals. The optical system consists of 435 and 588 nm LEDs (Roithner Lasertechnik) filtered at 435.8 and 590 nm, respectively, with 10 nm bandpass filters (full width at half-maximum) (Intor Inc.). Light from the alternately pulsed 9575



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Figure 2. Results from in situ measurements in a test tank at the Hatfield Marine Sciences Center. (Top) Daily AT standard measurements by the SAMI-alk before the adjusted calibration factor was applied, compared to known value of the AT standard; (middle) hourly seawater AT measurements by the SAMI-alk, AT measured by Gran titration on discrete samples, AT calculated from CT and pCO2 measurements on discrete samples, and AT calculated from an AT−salinity relationship; (bottom) temperature and salinity. HCl was added to the tank on 1/31/13. NaOH was added on 2/1/13. SAMI-alk data are missing for ∼2 days due to a problem with the data cable. Salinity was not recorded on the final 2 days. Error bars represent 1 standard deviation.

determined in step 1 (this generally gave an AT value for the standard that was ∼0.2 to 2% different from the known value); and (3) analyze the AT standard twice daily in the field and adjust the calibration factor to maintain an AT value equal to that obtained in step 2. Reagents and Standards. The following reagents were used and purchased from Fisher Scientific unless otherwise specified: indicator grade bromocresol purple sodium salt (BCP lot MKBB6638 V, Sigma-Aldrich), ACS primary standard grade sodium carbonate (Alfa Aesar), certified 0.1 N HCl, certified ACS grade NaCl, reagent grade KCl, anhydrous Na2SO4, CaCl2·2H2O and MgCl2·6H2O. Seawater Certified Reference Material (CRM) was purchased from A.G. Dickson at Scripps Institution of Oceanography.41 All anhydrous salts were dried overnight at 200 °C and cooled to room temperature in a desiccator before use. All solutions were prepared with nanopure water and stored in ∼2.5 L light- and gasimpermeable bags (Pollution Measurement Corp or Hyclone).

For the Hawaii deployment, the reagent was degassed with He before the bags were filled to minimize degassing in the bags. A ∼5.9 × 10−5 mol kg−1 BCP and ∼7.0 × 10−4 mol kg−1 HCl a/i solution was prepared from BCP stock and 0.1 N HCl stock in 0.60 mol NaCl kg−1 (S = 30.1, Oregon) or in synthetic seawater (S = 35.1, Hawaii), where kg is kilogram of solution. Synthetic seawater was made according to DelValls and Dickson.39 AT standards were made with Na2CO3 and 0.1 N HCl in NaCl with final concentrations of 2098 μmol AT kg−1, 2041 μmol CT kg−1 and S = 30.1 (Oregon); or in synthetic seawater with final concentrations of 2200 μmol AT kg−1, 2159 μmol CT kg−1 and S = 34.9 (Hawaii). CRM concentrations were 2221.2 μmol AT kg−1, 2014.8 μmol CT kg−1 and S = 33.3 (Oregon) and 2225.2 μmol AT kg−1, 2022.0 μmol CT kg−1 and S = 33.4 (Hawaii). Oregon Field Study. The initial in situ test of the SAMIalk was conducted at the Hatfield Marine Sciences Center in Newport, Oregon. The SAMI-alk was submerged in an outdoor 9576



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Table 1. AT Measurements of Yaquina Bay Seawater by SAMI-alk, Compared to Gran Titrations on Discrete Samples, AT Calculated from pCO2 and CT Measurements on Discrete Samples, and AT Estimated from Salinity average differencea ± standard deviation (number of samples)

a

sample

Gran titration

calculation from pCO2 and CT

estimation from salinity

seawater seawater + HCl seawater + NaOH

−2.9 ± 6.4 (33) −0.2 ± 4.3 (4) −13.2 ± 4.9 (6)

−6.1 ± 4.5 (3) −16.1 (1) −12.3 ± 7.3 (2)

−5.1 ± 13.5 (101) NA NA

Average difference is (AT from SAMI-alk measurements) − (AT from the method specified in the column heading). NA = not applicable.

100-gallon tank filled with 50-μm filtered Yaquina Bay seawater that was continuously renewed from an inlet flowing at ∼6−7 L min−1, and drained through a standing pipe kept above instrument height. The tank water was mixed with a submersible pump flowing at ∼20 L min−1. This gave us an opportunity to test the SAMI continuously running for 10 days (1/23/13−2/2/13) with variable temperature, but fairly constant salinity, low particulates, and easy access to the SAMI-alk in the event that changes or repairs were necessary. The AT of the native seawater varied by less than 40 μmol kg−1 throughout the experiment. Therefore, in order to validate SAMI-alk accuracy through a range of AT, seawater in the tank was modified. On 1/31/13, the tank inlet water was turned off to stop flow and 0.1 N HCl was added to the tank. After ∼12 h, the tank was flushed with seawater by resuming inlet flow. Although acidified seawater was completely flushed from the tank in less than 2 h, inlet flow remained on for ∼12 h, after which it was again stopped and 0.1 N NaOH was added to the tank. After ∼12 h, inlet flow was again turned on, rapidly flushing the tank with seawater. The SAMI-alk measured tank AT hourly and an AT standard four times per day (two sequential analyses every 12 h). Additionally, a conductivity sensor (Seabird SBE37) was submersed in the tank and measured salinity every 15 min. SAMI-alk measurements were validated by analysis of discrete samples collected approximately every 2−6 h, except during some of the nights. Forty μL of saturated HgCl2 solution was added to each discrete sample to prevent any changes in carbon system parameters due to biological activity. Hawaii Field Study. Kaneohe Bay is a semienclosed bay on the northeast coast of Oahu, Hawaii. The bay has a large barrier reef and many fringing and patch reefs. The Coral Reef Instrumented Monitoring and CO2-Platform (CRIMP-2) sits in ∼3 m of water over sandy sediment on the inside edge of the barrier reef.26,42 The site is ∼20 m inland from the barrier reef, receiving water after it flows ∼2 km across the reef from the ocean. The chemistry at the site is believed to largely represent the chemistry of the reef, free from terrestrial and riverine inputs.42 The site was attractive to us for validation of the SAMI-alk because of the large diurnal AT swings, which are due to calcification and CaCO3 dissolution on the barrier reef,26 constant salinity, and low concentrations of PO43− and Si(OH)4,43 which would otherwise contribute to AT. The SAMI-alk was attached to the bottom of the buoy at ∼1 m depth from June 3−22, 2013. The SAMI-alk was programmed to analyze AT hourly and 2 sequential standards per day. A SAMI-pH44 and a Sea-Bird SBE37-SMP conductivity sensor were also attached to the platform of the buoy. Discrete samples were collected in 300 mL BOD bottles once or twice daily during most of the study period. From June 10−12, discrete samples were collected hourly. Two hundred μL of saturated HgCl2 was added to each discrete sample to prevent biological activity.

Discrete Sample Analysis. AT was measured on discrete samples following the seawater method of Dickson et al.45 These measurements are referred to as Gran titrations throughout this manuscript. Overall accuracy and precision for seawater CRM, which was analyzed daily, were −0.4 and ±4.7 μmol kg−1 (1σ), respectively (n = 44). Most field samples were analyzed in duplicate, with an average standard deviation of ±4.0 μmol kg−1 (n = 70). A subset of the Oregon discrete samples was analyzed for pCO2 and CT by a flow-through infrared method.21 A subset of the Hawaii discrete samples was analyzed for CT by coulometric titration.46 AT was calculated from pCO2 (or pH) and CT data using the equilibrium program CO2SYS47 with carbonate constants (K1 and K2) from Mehrbach,48 refit by Dickson and Millero,49 sulfate constant (KS) from Dickson,50 and pH on the total hydrogen ion scale. For Oregon data a salinity−AT relationship was derived from 2007 cruise data in the salinity range of 30−33.8.51 Throughout this paper accuracy is the average difference between SAMI AT measurements and Gran AT measurements on discrete samples and precision is the standard deviation of the difference.

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RESULTS Oregon Field Study. A total of 196 measurements, including 24 measurements of AT standard, were made by the SAMI-alk over the 10-day period in the seawater tank (Figure 2). The measured value of the AT standard did not drift during this deployment, therefore daily adjustment of the calibration factor was not necessary. However, the measured value of the standard did change after the instrument was removed from the tank to replace a faulty cable in the middle of the deployment. This might have caused the optical fibers to shift, and resulted in the need to adjust the calibration factor for the second half of the deployment (Figure 2, top panel). Outlier data were removed based on laboratory experiments indicating that AT precision depended upon blank drift and reproducibility (see Supporting Information). Using a filter based on change in blank ratio (I0/I0ref) from current to previous titration >0.2% of the blank ratio, 26 titrations were discarded, which decreased the standard deviation of the seawater measurements over the first 4 days, when AT remained nearly constant, from 5.3 to 3.7 μmol kg−1 (n = 45). A filter set to eliminate titrations with changes greater than 0.4% of the blank ratio did not improve precision of the raw data. The middle panel of Figure 2 shows the in situ seawater measurements. Also shown are AT calculated from an AT−salinity relationship,51 AT measured by Gran titration on discrete samples, and AT calculated from CT and pCO2 measured on discrete samples. Temperature varied from ∼8 to 11 °C and salinity decreased from ∼31.6 initially to ∼31.0 (bottom panel). SAMI-alk AT measurements on unaltered seawater were on average slightly lower, but in general agreed well with Gran titrations, AT calculated from 9577



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Figure 3. Data from deployment of the SAMI-alk on the CRIMP-2 buoy in Kaneohe Bay, Hawaii. (Top) Daily measurements of an AT standard by the SAMI-alk before the adjusted calibration factor was applied, compared to the known value of the AT standard; (middle) hourly seawater AT measured by the SAMI-alk, AT measured by Gran titrations on discrete samples, and AT calculated from discrete CT and in situ pH measurements; (bottom) temperature and salinity. Gray shading indicates nighttime. Two days of AT data are missing because of an error starting the SAMI-alk after downloading data. Salinity data is missing because of overload of the data buffer. Error bars represent 1 standard deviation.

pCO2 and CT, and AT calculated from salinity (Table 1). Similar accuracy was obtained when HCl was added to the test tank. When NaOH was added to the test tank, SAMI-alk AT was 12− 13 μmol kg−1 low compared to AT from both Gran titrations and AT from the pCO2−CT calculation. We do not know the source of the offset at high AT in this data set. A similar offset was not observed in the Hawaii field study (see Supporting Information). Hawaii Field Study. A total of 340 SAMI-alk measurements were made, including 30 measurements of the AT standard (Figure 3). On the basis of analysis of the standards, the calibration factor was adjusted by ∼3% over the 18-day deployment (Figure 3, top panel). Using the same 0.2% blank outlier filter that was used for the Oregon data, 91 titrations were discarded, which decreased the standard deviation on duplicate standard analyses from ±10.0 (n = 15) to ±6.6 (n = 13) μmol kg−1. The purpose of analyzing the standard two times sequentially each day was to assess overall precision and the effectiveness of the blank filter. In this case we also had

discrete samples to compare SAMI-alk measurements. The blank filter improved the accuracy and precision on the SAMIalk measurements compared to discrete measurements from −2.3 ± 22.7 (n = 48) to −1.2 ± 15.9 (n = 41) μmol kg−1, indicating that the duplicate standard analyses are an effective indicator of sample precision in cases where frequent collection of discrete samples is not possible. However, we felt that 26% data rejection was too high, and this filter also appeared to remove critical AT data during peaks and lows. A blank filter of >0.4% of the blank ratio removed only 51 of the data points (15% of the data) and maintained ±6.6 μmol kg−1 precision on duplicate standard analyses. The difference between SAMI-alk AT and Gran analysis of discrete samples on the data set with a daily adjusted calibration factor and 0.4% blank filter applied was −0.9 ± 15.7 μmol kg−1(n = 42). The difference between SAMI-alk AT and AT from discrete CT and SAMI-pH was 8.3 ± 23.8 μmol kg−1(n = 10). The SAMI-alk data, using the blank filter and daily-adjusted calibration factor, are shown in the middle panel of Figure 3, along with AT from Gran titrations on 9578



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discrete samples and AT calculated from in situ pH and CT measured on discrete samples. The salinity was fairly constant at ∼34.9−35.1, and the temperature ranged from ∼24.8 to 27 °C. A closer view of a 48-h period during which hourly discrete samples were collected is shown in Figure 4. The SAMI-alk and

noise was higher in Hawaii than in Oregon. The standard deviation of the blank ratio measured over 12 h, reported as a percentage of the initial blank ratio, was ∼0.3% in Hawaii versus ∼0.2% in Oregon. Additionally, the precision on analysis of seawater samples (SAMI − Gran) was not as good in Hawaii (±15.7 μmol kg−1) as it was in Oregon (±6.6 μmol kg−1) and, unlike that in the Oregon field study, the calibration factor had to be adjusted in Hawaii in order to maintain accurate results for the standards. As shown in the Supporting Information, large differences in blank light intensity between samples is often associated with poor reproducibility of AT measurements. The possible causes for blank ratio noise are buildup of small particles, degassing in the flow cell, biofouling in the flow cell, and incomplete flushing of a/i between titrations. A buildup of small particles is unlikely because the seawater in Oregon was filtered to 50 μm and the SAMI inlet had a 5-μm filter in Hawaii. Air bubbles in the cell are the most likely reason for blank instability. Air bubbles result from degassing during the titration and are more problematic at higher temperatures because CO2 is less soluble. Although the a/i solutions were degassed prior to use, the mixing of acid with seawater in the cell during the titration will produce CO2(g). Loss of CO2 to bubbles can cause inaccurate absorbance measurements if an air bubble is in the light path, and will also affect the AT calculation, which assumes that CT is constant. In the lab, we obtained good accuracy and precision at temperatures up to 27 °C, with blank and standard precision degrading considerably at higher temperatures due to degassing. Although biofouling in the flowcell is a possibility, there was no evidence of significant biofilms when the cell was taken apart postdeployment. Poor cell flushing was also probably not the cause of blank instability since pump volume was more than adequate for flushing at deployment conditions. The increased blank noise as well as imperfect timing of the collection of discrete samples in the rapidly changing seawater in Hawaii likely contributed to the lower accuracy and precision in this setting. Probable causes of AT drift as measured by the standard include small changes in the acidity of the a/i during the deployment due to reaction of the reagent with the bag, biological growth in the bag, or gas exchange through the bag. All of these issues are expected to be more problematic at warm temperatures. Overall, the SAMI-alk shows good accuracy and precision at temperatures from 8−27 °C when compared to discrete AT measurements. Instrument drift was corrected for by using an in situ calibration routine. In both studies 15% of the data were rejected based on changing blank light intensities. Although this might seem to be a high data rejection rate, the result is an average of ∼18−20 useable AT measurements per day. With 3L a/i and standard containers, this system can provide ∼1 month of hourly AT measurements, a level of temporal coverage not feasible by conventional manual sampling. Although improved accuracy and precision and lower data rejection are desirable for open ocean studies, ongoing research will improve these shortcomings. Importantly, the SAMI-alk is a valuable tool for studying coral reef ecosystems and their responses to ocean acidification.

Figure 4. In situ SAMI-alk AT and Gran AT from discrete samples that were collected hourly for a 48-h period. Gaps in discrete data indicate the sample was not analyzed or there was an error in the analysis; gaps in SAMI-alk data occur either when an AT standard was analyzed or when a seawater measurement was discarded due to the blank ratio filter.

Gran data during this period closely track each other; however, there are several points with large differences, for example, during early mornings on 6/11 and 6/12, when the discrete measurements show an increase in AT several hours before the same increase is seen in the SAMI-alk measurements. It is not clear whether biological or physical processes, such as CaCO3 dissolution near the surface and poor vertical mixing during these times might contribute to the discrepancies between the measurements, since discrete samples were collected at the surface and the SAMI-alk was deployed below the buoy. This figure highlights the difficulty in field validation of AT data using discrete samples. Overall, the in situ data followed the expected trend, with regular diurnal peaks of 2290−2330 μmol kg−1 between 02:00 and 08:00 local time, and lows of 2190−2230 μmol kg−1 between 12:00 and 18:00, due to coral calcium carbonate dissolution and formation, respectively. This diurnal range is similar to that seen in previous studies on this reef.26,42 These data greatly increase the temporal coverage of AT measurements previously obtained on coral reefs. Net ecosystem calcification can be directly calculated from these data,26,52 providing unprecedented insights into the processes that control coral productivity. These evaluations will be the focus of a forthcoming manuscript.

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DISCUSSION A comparison of the results from the two field studies provides important insights into the performance of the instrument under differing conditions. The mean temperature and salinity were 9.11 ± 0.44 °C and 31.26 ± 0.20, respectively, in Oregon and 25.64 ± 0.65 °C and 35.19 ± 0.04, respectively, in Hawaii. The AT in Oregon was relatively constant while the AT in Hawaii had diurnal changes as large as 100 μmol kg−1. A comparison of the Oregon and Hawaii data shows that blank

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ASSOCIATED CONTENT

S Supporting Information *

Stability of the blanks and the associated AT measurement precision is described. The accuracy of the SAMI-alk across a temperature range of 6−27 °C using the Van’t Hoff equation to 9579



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correct the BCP pKa and a temperature correction for BCP molar extinction coefficients is shown. The linearity of the instrument during the in situ studies is also described. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/fax: 406-532-3246; e-mail: reggie@sunburstsensors. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Cory Beatty and Sarah Hamblock for technical assistance, Burke Hales, Clare Reimers, Kristina McCann, and Rhea Sanders for assistance and use of facilities at the Hatfield Marine Science Center and University of Hawaii divers for assistance at CRIMP-2. Funding was provided by NSF Grants OCE 1051757 (UM) and OCE 1051550 (Sunburst Sensors).

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Article

NOTE ADDED AFTER ASAP PUBLICATION This article published August 7, 2014 with an error in the second term of equation 7. The correct version published August 19, 2014.

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Autonomous in Situ Measurements of Seawater Alkalinity

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