pH

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	To assist in the interpretation of the data, it can be displayed using the Hawaii Ocean Time-series Data Organization & Graphical System (HOT-DOGS©).

Analytical Method

Beginning in 1992, pH was determined spectrophotometrically using the indicator m-cresol purple following the methods described in Tupas et al. (1993). The absorbance of the mixture was measured at wavelengths of 578 and 434 nm on a Perkin Elmer Model 3 dual-beam spectrophotometer and converted to pH on the seawater scale according to Clayton and Byrne (1993). The mean and standard deviation of the difference of sets of duplicate pH measurements was 0.0009 and 0.00077 respectively.

Results

The structure of pH (Figure 19) in the upper water column closely resembles that of dissolved inorganic carbon (Figure 17). There appears to be a slight increase in pH during the winter months and gradually decreases after that. This is directly related to the drawdown of inorganic carbon in the water column during the spring and summer periods.