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

Abstract

Ocean acidification (OA) is often defined as the gradual decline in pH and aragonite saturation state (ΩAr) for open ocean waters as a result of increasing atmospheric pCO2. Potential long‐term trends in pH and ΩAr 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 ΩAr 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, ~2 days/yr) as well as a shallowing of the saturation horizon (~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 ΩAr 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 ΩAr 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 ΩAr and informing regional natural resource management and ecosystem restoration.

Effects of Wind-Driven Lateral Upwelling on Estuarine Carbonate Chemistry

“…This study showed that winds can drive pH fluctuations of up to 0.3 in Chesapeake Bay over a period of a few days. These pH fluctuations are comparable to or larger than the long-term pH (0.1-0.2) decline that has been observed in Chesapeake Bay over the past three decades (Waldbusser et al, 2011;Shen et al, 2020). This adds to a growing body of evidence for large pH variability in coastal and estuarine systems (Carstensen and Duarte, 2019).…”

“…The RCA biogeochemical model has been validated against biogeochemical data at a number of stations in Chesapeake Bay (including NO 3 , PO 4 , NH 4 , chlorophyll-a, dissolved oxygen, and organic carbon, nitrogen, and phosphorus), integrated metrics of hypoxic volume, rates of water-column primary production and respiration, and nutrient fluxes across the sediment-water surface (Brady et al, 2013;Testa et al, 2013Testa et al, , 2014Testa et al, , 2017Li et al, 2016;Ni et al, 2020). The CC model has been validated against extensive surveys of DIC, TA, and pH collected during ten cruises in 2016 (Shen et al, 2019a) and long term (1985-2015) measurements of pH at a number of monitoring stations (Shen et al, 2019b(Shen et al, , 2020. The model-predicted along-channel distribution of airsea CO 2 flux is also in good agreement with that calculated from the observational data (Shen et al, 2019a).…”

“…Time series of TA measurements in riverine inputs were obtained from the USGS station in the Susquehanna and Potomac Rivers (Raymond et al, 2000). The riverine DIC concentrations were calculated through CO2SYS with the available TA and pH (Shen et al, 2020). The computed DIC is in very good agreement with the observed DIC when direct DIC measurements were made during the field cruises of 2016 ( Supplementary Figure S1).…”

“…Shen et al (2019a) focused their modeling analysis on the large-scale carbonate chemistry dynamics and validation against field observations. Furthermore, Shen et al (2019b) and Shen et al (2020) conducted 30-year hindcast simulations to examine ecosystem metabolism and carbon balance and investigated the anthropogenic impacts on pH and aragonite saturation state. This modeling study takes the next step to investigate how physical processes affect carbonate chemistry in this estuary, seeking to test the lateral upwelling mechanism over a range of wind conditions, in particular during the up-estuary wind condition not observed in Huang et al (2019).…”

Effects of Infectious Diseases on Population Dynamics of Marine Organisms in Chesapeake Bay

“…As excess CO 2 is absorbed into the Chesapeake Bay, it will change the pH and alkalinity of the waters resulting in acidification. Chesapeake Bay pH changes are likely to be spatially dependent; model simulations of the Chesapeake Bay found that pH increased in the upper Bay, decreased in the lower Bay, and was variable in the mid-Bay from 1986 to 2015 (Shen et al 2020).These changes to water chemistry are expected to affect the physiology of many important Chesapeake Bay marine organisms. However, there is a general lack of information on the effects of acidification on disease susceptibility of estuarine organisms.…”