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Ocean acidification may shift coral reefs from a state of net ecosystem calcification (+NEC) to net ecosystem dissolution (–NEC). Changes in NEC are typically inferred from either measured or calculated total alkalinity (TA) or the dissolved calcium (Ca) to salinity ratio relative to a reference value. The alkalinity anomaly technique has historically been the primary method to estimate NEC due to the greater analytical challenges and uncertainty associated with dissolved Ca measurements in seawater. However, this method assumes that changes in salinity-normalized TA are exclusively the result of calcification and dissolution processes. In many cases, this assumption is valid, but in some environments additional processes can significantly influence seawater TA (e.g., nutrient fluxes, redox processes). Seawater Ca is unaffected or less sensitive to these processes, and therefore, Ca and TA anomalies can be used to estimate absolute or relative changes in NEC with greater confidence. Here, we present a two-year time series of monthly seawater Ca and TA measurements across the Bermuda carbonate platform and the nearby Bermuda Atlantic Time-series Study (BATS) location offshore. High precision Ca measurements (±6 μmol kg−1) were conducted using an improved spectrophotometric titration system and showed mostly good agreement with changes in TA over the same spatial and temporal scales. Ca and TA measurements across the Bermuda platform showed seasonal fluctuations relative to offshore waters, with +NEC during summer months and near-zero or possible –NEC (net dissolution) during winter months. These seasonal patterns were most pronounced at the inshore locations with the longest residence times (10+ days), which allow stronger biogeochemical signals to develop relative to the offshore source water. Although obtaining high accuracy and precision Ca measurements remains challenging, parallel measurements of Ca and TA from both inshore and offshore waters over a multi-annual timescale strengthen the validity of predictions for when, where, and why a reef system, such as the Bermuda platform, may shift from +NEC to –NEC.

Equimolar Tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer prepared in artificial seawater media is a widely accepted pH standard for oceanographic pH measurements, though its change in pH over pressure is largely unknown. The change in volume (Δ V ) of dissociation reactions can be used to estimate the effects of pressure on the dissociation constant of weak acid and bases. The Δ V of Tris in seawater media of salinity 35 (Δ V Tris ⁎ ) was determined between 10 and 30 °C using potentiometry. The potentiometric cell consisted of a modified high pressure tolerant Ion Sensitive Field Effect Transistor pH sensor and a Chloride-Ion Selective Electrode directly exposed to solution. The effects of pressure on the potentiometric cell were quantified in aqueous HCl solution prior to measurements in Tris buffer. The experimentally determined Δ V Tris ⁎ were fitted to the equation Δ V Tris ⁎ = 4.528 + 0.04912 t where t is temperature in Celsius; the resultant fit agreed to experimental data within uncertainty of the measurements, which was estimated to be 0.9 cm − 3 mol − 1 . Using the results presented here, change in pH of Tris buffer due to pressure can be constrained to better than 0.003 at 200 bar, and can be expressed as: ∆ pH Tris = − 4.528 + 0.04912 t P ln 10 RT . where T is temperature in Kelvin, R is the universal gas constant (83.145 cm 3 bar K − 1 mol − 1 ), and P is gauge pressure in bar. On average, pH of Tris buffer changes by approximately − 0.02 at 200 bar.

This study examines the utility of combining pH measurements with other inorganic carbon parameters for autonomous mooring-based carbon cycle research. Determination of the full suite of inorganic carbon species in the oceans has previously been restricted to ship-based studies. Now with the availability of autonomous sensors for pH and the partial pressure of CO 2 ( p CO 2 ), it is possible to characterize the inorganic carbon system on moorings and other unmanned platforms. The indicator-based pH instrument, SAMI-pH, was deployed with an autonomous equilibrator-infrared p CO 2 system in Monterey Bay, California USA from June to August 2007. The two-month time-series show a high degree of short-term variability, with pH and p CO 2 changing by as much as 0.32 pH units and 240 μatm, respectively, during upwelling periods. The pH and salinity-derived alkalinity (A Tsalin ) were used to calculate the other inorganic carbon parameters, including p CO 2 , total dissolved inorganic carbon (DIC) and CaCO 3 saturation states. The calculated p CO 2 was within 2 μatm of the measured p CO 2 during the first day of the deployment and within 8 μatm over the first month. The DIC calculated from pH–A Tsalin and p CO 2 –A Tsalin were within 5 μmol kg −1 of each other during the first month. However, DIC calculated from pH– p CO 2 differed by ~ 50 μmol kg −1 from the other estimates over the same period, reflecting the sensitivity of the pH– p CO 2 calculation to measurement error. The data continued to diverge during the final month and this difference was likely driven by extensive biofouling. Because of the relative insensitivity of CO 3 2− concentration to these errors, aragonite saturation calculated from the pH– p CO 2 pair was within 0.15 of the pH–A Tsalin values over the entire deployment. These results show that in situ pH, when combined with other CO 2 parameters, can provide valuable insights into both data quality and inorganic carbon cycling.

We describe a novel method for determination of carbonate ion concentration in seawater by acidimetric titration with UV detection. Because CO 3 2− absorbs light at wavelengths of less than ~ 250 nm, it is feasible to titrate most carbonate-containing natural waters with acid and observe an increase in %Transmittance. The observed signal is proportional to the concentration of carbonate ion in the original sample. Present technology permits a theoretical precision in the determination of [CO 3 2− ] in natural seawater background of ~ 0.5% (at 10 cm pathlength, 200 μM CO 3 2− , ± 0.0001 AU). The procedure has been tested at 1 and 10 cm pathlengths using single and multipoint titration methods, respectively. Results using natural seawater test solutions indicate a resolution in [CO 3 2− ] of 3.6% in a standard 1 cm cuvette using a very simple manual method, and 0.7% using a custom-built 10 cm closed titration cell. Estimates of the relative distribution of CO 3 2− between inner and outer sphere complexes with Mg 2+ and Na + were also determined and the equilibrium constants agree with published values. This method provides a new tool for studies of several fundamental aspects CO 2 chemistry, including the second dissociation constant of carbonic acid, CO 3 2− ion pairing, and can be used to directly measure the distribution of carbonate ion in seawater and many other types of natural waters.