Discerning effects of warming, sea level rise and nutrient reduction on long-term hypoxia trend in Chesapeake Bay

Nutrient management strategies have been implemented worldwide in recent decades to improve water quality and mitigate coastal hypoxia (dissolved oxygen<2mg/L). However, the nutrient load reductions that have been achieved in many systems has fallen short of goals, and failed to yield a reversal to pre-eutrophication oxygen conditions. Regional sea level rise and warming resulting from climate change may also have a considerable effect on hypoxia as it responds to altered nutrient loading. Chesapeake Bay has a large seasonal hypoxic volume and a watershed where various nutrient control measures have been put into place since the 1980s. Although a ~15% reduction in nitrate loading from the primary riverine source has been achieved since the early 1990s, observation-based studies have only showed a slight change in early and late summer hypoxia volume with little sign of decline in mid-summer. In the present study, we use a numerical model to discern the individual effects of sea level rise, warming and nutrient reduction on long-term patterns of hypoxia in Chesapeake Bay during 1985-2016. Hindcast and scenario-based simulations removing each individual forcing during this 32 year period are conducted with a coupled physical-biogeochemical model. The trends in dissolved oxygen in base and scenario runs are analyzed with statistical models (GAM) in four regions of main Chesapeake Bay. The results suggest that the increased water temperature (~1.6 °C) dominates the long-term trend of bottom dissolved oxygen in the bay. In comparison, nutrient reduction only plays a minor role and the effect of sea level rise (~0.15 m) is trivial. Both nutrient reduction and warming contribute to the decrease of late summer hypoxia in Chesapeake Bay, but warming appeared to cause significant earlier onset of hypoxia by 5-10 days. In combination, these drivers lead to a decreasing long-term trend in dissolved oxygen, where warming overcame the potential recovery of summer hypoxia due to nutrient load reductions.