Mechanisms Driving Decadal Changes in the Carbonate System of a Coastal Plain Estuary

Understanding decadal changes in the coastal carbonate system is essential for predicting how the health of these waters responds to anthropogenic drivers, such as changing atmospheric conditions and riverine inputs. However, studies that quantify the relative impacts of these drivers are lacking. In this study, the primary drivers of decadal trends in the surface carbonate system, and the spatiotemporal variability in these trends, are identified for a large coastal plain estuary: the Chesapeake Bay. Experiments using a coupled three‐dimensional hydrodynamic‐biogeochemical model highlight that, over the past three decades, the changes in the surface carbonate system of Chesapeake Bay have strong seasonal and spatial variability. The greatest surface pH and aragonite saturation state (ΩAR) reductions have occurred in the summer in the middle (mesohaline) Bay: −0.24 and −0.9 per 30 years, respectively, with increases in atmospheric CO2 and reductions in nitrate loading both being primary drivers. Reductions in nitrate loading have a strong seasonal influence on the carbonate system, with the most pronounced decadal decreases in pH and ΩAR occurring during the summer when primary production is strongly dependent on nutrient availability. Increases in riverine total alkalinity and dissolved inorganic carbon have raised surface pH in the upper oligohaline Bay, while other drivers such as atmospheric warming and input of acidified ocean water through the Bay mouth have had comparatively minor impacts on the estuarine carbonate system. This work has significant implications for estuarine ecosystem services, which are typically most sensitive to surface acidification in the spring and summer seasons. Plain Language Summary Seawater pH, a measure of how acidic or basic water is, is a crucial water quality parameter influencing the growth and health of marine organisms, such as oysters, fishes and crabs. Decreasing pH, commonly referred to as acidification, is a severe environmental issue that has been exacerbated by human activities since the industrial revolution. In the open ocean, elevated atmospheric carbon dioxide is the key driver of acidification. However, in coastal environments the drivers are particularly complex due to changing human influences on land. In this study the primary drivers of acidification in the Chesapeake Bay over the past three decades are identified via the application of a three‐dimensional ecosystem model. Increased atmospheric CO2 c