Quantifying the Impact of Nutrient Reductions on Dissolved Oxygen in the Chesapeake Bay: Has the Bay Become More Resilient?

Hypoxia resulting from anthropogenic eutrophication has been the focus of management actions in the Chesapeake Bay over the past three decades due to the importance of dissolved oxygen to estuarine ecosystem health. Despite successful reductions in flow-weighted nutrient concentrations in the three largest rivers entering the Bay, high precipitation and riverine discharge in recent years have fueled some of the largest hypoxic volumes in the Bay on record. In an effort to quantify the impact of nutrient reductions on oxygen levels in the Chesapeake Bay during a wet year with a large amount of hypoxia, a 3-D Estuarine Carbon and Biogeochemistry numerical model embedded in the Regional Ocean Modeling System (ChesROMS-ECB) is used to conduct simulations of 2018, one of the wettest years experienced in this region over the past century. Simulations with altered riverine nitrogen concentrations representative of 1985 concentrations are compared to a reference simulation for 2018 where nitrogen concentrations are unchanged. Total anoxic volume (O₂ < 0.2 mg/L) in the Chesapeake Bay for the year 2018 is roughly 50% greater in the altered simulations compared to the reference simulation, suggesting that the extent of anoxia in the Bay would have been 1.5 times greater in 2018 if nitrogen reductions had not taken place. Total hypoxic volume (O₂ < 2 mg/L) and low-oxygen volume (O₂ < 5 mg/L) also display greater values in the altered simulation compared to the reference, suggesting that the reductions have also curtailed the extent of hypoxic waters in the Bay. These results are encouraging for Chesapeake Bay managers, demonstrating that had it not been for the progress made in reducing nutrient pollution in the watershed, the high precipitation rates experienced in 2018 would have had a larger negative impact on the estuarine ecosystem. This suggests that the observed nutrient reductions are helping to make the Bay more resilient to hypoxia during wet years.