Impacts And Uncertainties Of Climate Change On The Chesapeake Bay

Climate change impacts in the Chesapeake Bay will limit the efficacy of nutrient reduction efforts and decrease dissolved oxygen, but uncertainties associated with the magnitude of these effects remain. An understanding of underlying mechanisms that have driven recent warming trends will narrow uncertainties for future pathways of temperature change. Additionally, future simulations of climate impacts in the estuary are dependent on multiple different sources of uncertainty, many of which have not yet been fully evaluated. This dissertation used a three- dimensional coupled hydrodynamic-biogeochemical model to investigate recent warming trends as well as underlying uncertainties likely to influence regional projections of changes to estuarine dissolved oxygen.Recent warming trends over the past 35 years were analyzed using a combination of long- term observations, hindcast simulations, and model sensitivity tests. Robust agreement between model results and in situ sampled temperatures supported the use of the numerical model to calculate warming trends. Additional sensitivity tests that isolated the impact of different factors on long-term temperature trends demonstrated the dominance of atmospheric warming on the Bay, while also identifying the importance of ocean warming on summer temperature increases within the estuary’s southern reaches. The relationship between future climate impacts on watershed processes and estuarine hypoxia were also investigated by varying a multitude of climate input factors. These factors that modified watershed forcings for the estuarine model included the choice of Earth System Model, downscaling methodology, and watershed model. Results showed that each of these factors contributed substantially to the total uncertainty with respect to changes to hypoxia. Simulations also showed that the largest remaining uncertainty for dissolved oxygen is tied to the successful implementation of watershed nutrient reductions, which will decrease hypoxia by an order of magnitude more than increases due to watershed climate impacts alone. Further uncertainties associated with estimates of future Chesapeake Bay hypoxia are due to climate projection scenario design. These were studied by analyzing multiple common approaches for generating climate scenarios. Simulations of climate impacts on mid-21st century hypoxia derived using continuous, delta, and time slice approaches were compared. Key findings demonstrated that the commonly used delta method doubled the projected change in hypoxia relative to the continuous and time slice approaches. Differences in experimental results suggest that when assessing changes in estuarine hypoxia continuous simulations should be favored over time slice experiments whenever possible, and delta approaches should be avoided. This dissertation set out to generate scientific knowledge applicable to environmental managers working to integrate the unknowns of a future climate into decision-making for a more resilient ecosystem. The evidence produced by this research affirms the capabilities of numerical modeling techniques to identify unobservable causal mechanisms and constrain a future range of uncertain changes to dissolved oxygen levels. Hopefully, this work will also help to optimize future observational sampling efforts, and identify nutrient reduction priorities in the face of mounting climate stressors