Understanding Anthropogenic Impacts on pH and Aragonite Saturation State in Chesapeake Bay: Insights From a 30-Year Model Study

Ocean acidification (OA) is often defined as the gradual decline in pH and aragonite saturation state (Omega(Ar)) for open ocean waters as a result of increasing atmospheric pCO(2). Potential long-term trends in pH and Omega(Ar) in estuarine environments are often obscured by a variety of other factors, including changes in watershed land use and associated riverine carbonate chemistry and estuarine ecosystem metabolism. In this work, we investigated the anthropogenic impacts on pH and Omega(Ar) over three decades (1986-2015) in Chesapeake Bay using retrospective coupled hydrodynamic-biogeochemical model simulations. Simulation results demonstrated a clear estuarine acidification signal in the midbay region, with a long-term increase in the annual duration of acidified bottom waters (pH < 7.5, similar to 2 days/yr) as well as a shallowing of the saturation horizon (similar to 0.1 m/yr). In contrast, scenario results revealed basification in the upper bay consistent with increased alkalinization of the Susquehanna River. Significant long-term pH and Omega(Ar) declines in the lower bay were driven by nearly equal contributions from OA and lowered surface ecosystem production. The midbay pH variability was primarily influenced by OA and biological processes, while river basification along with OA played a key role in regulating the long-term Omega(Ar) variability. This study quantifies the contributions from multiple anthropogenic drivers to changes in estuarine carbonate chemistry over three decades, highlighting the complex interactions in regulating the dynamics of pH and Omega(Ar) and informing regional natural resource management and ecosystem restoration.