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

Ocean acidification (OA) in the open ocean has been observed widely; however, detecting pH declines in the coastal zone remains a challenge due to the high-magnitude, high-frequency, and irregular pH variations caused by local processes (natural and anthropogenic), which are significantly larger than the global CO₂ forcing. In these regions, where management strategies could be employed, more information is needed to identify where, why, and how fast water chemistry is changing. In order to develop the modern bassline information needed to answer these questions, continuous observations are often required; however, in most coastal regions where carbonate system observations do exist, the data have been collected monthly or seasonally and provide only low temporal resolution information in these dynamic environments. In addition, it is often the case that only one carbonate system parameter is measured while two are required to constrain seawater carbonate chemistry. Here we propose use of the quasi-conservative carbonate system parameter total alkalinity (TA) as a means to expand existing OA information in time and space, and display the significant and surprising utility of TA for OA research. Although there are well documented challenges to using TA-salinity relationships in the coastal zone, we find that one relationship holds for the highly variable Washington State coastline (given the available data) and that computations of pH from salinity-based TA estimates coupled with high-quality CO₂ observations from moorings have similar or lower uncertainties than what can be achieved presently with autonomous pH sensors. We share these results to highlight the surprising utility of the TA-salinity relationship approach in coastal waters with salinities ranging from 20 to 35, and suggest that this method may be an effective way to leverage single parameter carbon data for ocean acidification research in other coastal regions.