Impacts and uncertainties of climate-induced changes in watershed inputs on estuarine hypoxia

Multiple climate-driven stressors, including warming and increased nutrientdelivery, are exacerbating hypoxia in coastal marine environments. Withincoastal watersheds, environmental managers are particularly interested inclimate impacts on terrestrial processes, which may undermine the efficacyof management actions designed to reduce eutrophication and consequentlow-oxygen conditions in receiving coastal waters. However, substantialuncertainty accompanies the application of Earth system model (ESM)projections to a regional modeling framework when quantifying future changesto estuarine hypoxia due to climate change. In this study, two downscalingmethods are applied to multiple ESMs and used to force two independentwatershed models for Chesapeake Bay, a large coastal-plain estuary of theeastern United States. The projected watershed changes are then used toforce a coupled 3-D hydrodynamic-biogeochemical estuarine model to projectclimate impacts on hypoxia, with particular emphasis on projectionuncertainties. Results indicate that all three factors (ESM, downscalingmethod, and watershed model) are found to contribute substantially to theuncertainty associated with future hypoxia, with the choice of ESM being thelargest contributor. Overall, in the absence of management actions, there isa high likelihood that climate change impacts on the watershed will expandlow-oxygen conditions by 2050 relative to a 1990s baseline period; however,the projected increase in hypoxia is quite small (4 %) because onlyclimate-induced changes in watershed inputs are considered and not those onthe estuary itself. Results also demonstrate that the attainment ofestablished nutrient reduction targets will reduce annual hypoxia by about 50 % compared to the 1990s. Given these estimates, it is virtually certainthat fully implemented management actions reducing excess nutrient loadingswill outweigh hypoxia increases driven by climate-induced changes interrestrial runoff.