Mapping the Present to Recognize the Future: the Magnitude and Frequency of pH and ΩAr Variability in Washington State

Although the chemistry of ocean acidification (OA) is well understood, observing 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 have a significantly larger signal than the global carbon dioxide (CO2) forcing over short timescales. In addition, the dynamic processes common to nearshore environments make it challenging to identify normal carbonate chemistry conditions from a small number of observations. As a result, baseline information for surface ocean pH and other OA relevant parameters, such as aragonite saturation state (ΩAr), is often unavailable in the coastal zone. The lack of modern baseline information challenges our ability to better understand OA in these environments and hampers the ability of decision-making institutions to confidently identify pH-impaired water bodies under the Clean Water Act, which may be necessary to effectively impose conservation measures. To address this issue, we have developed a Washington-specific empirical relationship between salinity and total alkalinity (TA) and have calculated surface ocean pH and ΩAr throughout Washington State’s coastal marine surface waters using estimates of TA paired with in situ CO2 observations from moorings and ships. By compiling and leveraging all of the available surface ocean CO2 observations collected between 1985 and 2015 within Washington waters, we have been able to develop regionally-specific seasonal climatologies of pH, ΩAr, and other OA relevant parameters, providing the first baseline maps for OA in the coastal zone. In addition, three-hour observations of CO2 from four surface moorings in the state are used to evaluate higher-frequency variability and validate the broader regional climatologies. Finally, we use this information to outline strategic monitoring recommendations for detecting OA in Washington, and provide new interpretations of coastal carbonate chemistry variability that will inform future biological OA research in the Pacific Northwest