Ocean acidification, which is manifested by the long-term trend of decreasing pH and carbonate ion concentrations in seawater, has become a research hotspot in the field of ocean science. The changing inorganic carbon system equilibrium caused by this process can be characterized by four parameters, including partial pressure of carbon dioxide (pCO2), pH, dissolved inorganic carbon (DIC) and total alkalinity (TA). Carbonate ion concentrations ([CO₃^2-]) can be calculated from any two of the four parameters. Except for pCO2, samples for other parameters are still collected discretely in most studies, and then analyzed either in shipboard or land-based laboratories. These techniques are labor and cost intensive. Furthermore, sample transferring and storage needs particular considered. Consequently, development of automated instruments for real-time monitoring of seawater carbonate systems will benefit greatly for ocean acidification and ocean carbon cycle research.

Until now, all of the carbonate system parameters can be measured spectrophotometrically with minor modifications. Spectrophotometry has the unique advantage of easiness to be automated. In order to improve instruments’ diversity, reliability and cost effectiveness, this work focused on the development and application of automated instruments for determing pH and CO₃^2- in seawater. The main results are summarized below:

(1) An automated instrument for pH measurement based on visible spectrophotometry was described. The instrument consisted of a syringe pump equipped with a multiport valve for liquid delivery, LEDs and charge-coupled device for absorbance measurement. Absorbance values of seawater and indicator (meta-Cresol Purple, mCP) mixture at specific wavelengths allow direct measuremnt of pH. Laboratory test showed that the instrument has a precision of ~0.001. The instrument was used to on-line monitor pH variations continuously in a coral reef tank. During the 5-day field test, the pH automated measurement instrument operated properly, and the measurement error was -0.015±0.014 (n=38) compared to calculated values with TA-DIC pair.

(2) The automated CO₃^2- measurement instrument was built with similar manifold of pH measurement instrument except ultraviolet detection components. Carbonate ion concentrations can be determined from ultraviolet spectra of Pb(II) and seawater mixture. Laboratory test showed that the instrument has a precision of 1.1% (n=13). During the coral reef tank field test, the measurement error was -2.4±15.7 μmol/kg (n=14). The instrument was tested at sea to perform underway measurements and discrete samples measurement from two vertical profiles. The measurement error in the first stage was -0.5±5.0 μmol/kg (n=31) and 2.1±5.7 μmol/kg (n=22), respectively.