Estimating Total Alkalinity in the Washington State Coastal Zone: Complexities and Surprising Utility for Ocean Acidification Research

Evidence of ocean acidification (OA) throughout the global ocean has galvanized some coastal communities to evaluate carbonate chemistry variations closer to home. An impediment to doing this effectively is that, often, only one carbonate system parameter is measured at a time, while two are required to fully constrain the inorganic carbon chemistry of seawater. In order to leverage the abundant singlecaibonate-parameter datasets in Washington State for more rigorous OA research, we have characterized an empirical relationship between total alkalinity (TA) and salinity (TA = 47.7 × S + 647; lσ = ±17 μmol kg⁻¹) for regional surface waters (≤25 m) that is robust in the salinity range from 20 to 35 for all seasons. The relationship was evaluated using 5 years of 3-h contemporaneous observations of salinity, carbon dioxide partial pressure (pCO₂), and pH from a surface mooring on the outer coast of Washington. In situ pCO₂ observations and salinity-based estimates of TA were used to calculate pH for comparison with in situ pH measurements. On average, the calculated pH values were 0.02 units lower than the measured pH values across multiple pH sensor deployments and showed extremely high fidelity in tracking the measured high-frequency pH variations. Our results indicate that the TA-salinity relationship will be a useful tool for expanding single-carbonate-parameter datasets in Washington State and quality controlling dual pCO₂-pH time senes.