Primary stressors and mechanisms impacting the long-term variability of the Chesapeake Bay carbonate system

Multiple anthropogenic stressors have been impacting the Chesapeake Bay carbonate (CO₂) system over the past several decades. These include global-scale increases in atmospheric temperature and CO₂ concentration, as well as sea level, and local-scale changes in riverine nutrients and alkalinity entering the Bay. Because these stressors will impact both physical (i.e. water temperature, salinity, circulation and air-sea CO₂ exchange) and biogeochemical (i.e. production and respiration) mechanisms, the Chesapeake Bay CO₂ system is likely to undergo complex long-term variability that is challenging to study via analysis of in-situ data alone. For example, preliminary analyses of historical pH data collected since 1996 indicate spatially and temporally varying trends throughout the mainstem of the Bay. Significant decreasing trends in surface pH are observed from May to September throughout the majority of the mesohaline and polyhaline Bay, with rates of up to -0.2 pH unit decade^-1. From October to March, however, increasing trends in surface pH of up to +0.3 pH unit decade^-1 are observed. Bottom water pH data over the same period indicate increasing trends on the order of +0.2 pH unit decade^-1 regardless of season. Significant increase in water temperature in the summer is expected to decrease pH by up to -0.03 pH unit decade^-1; however, it is difficult to quantify the impacts of other stressors (e.g. changing nutrients) on pH trends. We thus examine and identify the primary drivers of this interdecadal variability in the Chesapeake Bay CO₂ system using a three-dimensional physical-biogeochemical estuarine model. Specifically, a reference simulation over the years 1985 to 2018 is used to better understand how the CO₂ system is changing in time. Sensitivity simulations are used to identify the dominant physical and biogeochemical mechanisms and investigate the global vs. local anthropogenic stressors that are responsible for these long-term CO₂ system changes.