Effects of Sea Level Rise on the Seasonal Hypoxia of Chesapeake Bay

The largest estuary in the continental US, the Chesapeake Bay, provides crucial habitat and natural resources for native and migratory species. It also experiences recurrent hypoxia (dissolved oxygen <2mg/L) during the summer with hypoxic volumes reaching ~8km³ in July. The zones that are most affected by hypoxia are within a deep channel (depth ~40m) with a relatively long residence time (~240 days). Although multiple modeling studies have examined future changes in the Bay's hypoxia, these studies showed mixed responses to sea level rise (both improvements and degradation in hypoxia), and the differences in methodology (climate scenarios, years investigated) made comparisons difficult. The present study focuses on the effect of sea level on the Bay's hypoxia in absence of other climate drivers. The experimental plan includes two comparable hydrodynamical-biogeochemical models of the Bay (ChesROMS-ECB and UMCES-ROMS-RCA), reference simulations for the years 1991-1995, and sensitivity experiments where sea level at the oceanic boundary of the models is representative of levels expected for the mid-to-late 21st century. The two models suggest significant physical changes in response to increased sea level. Salinities increase throughout the Bay and the water column, with similar changes throughout the year. Water temperatures slightly increase in winter months and cool slightly in summer months (+/-0.5C by late 21st century). Summertime dissolved oxygen concentrations generally decrease in the upper part of the water column as sea level rise causes the oxycline to migrate upward. In the deep channel where hypoxia is most prevalent, both models suggest some improvements in summertime bottom dissolved oxygen (at least partially due to the cooler temperatures). However, the magnitude and duration of this improvement vary substantially between the two models. We explore the causes of these differences and discuss their potential importance for other estuarine systems.