Mechanisms controlling hypoxia trend in Chesapeake Bay from 1985 to 2005: Results from A Terrestrial-Estuarine-Ocean Biogeochemical Modeling System

Hypoxia in Chesapeake Bay has been intensively observed and simulated over the past three decades. However, the long-term trends and controlling mechanisms are still in argument. In this study, we run a previous built Terrestrial-Estuarine-Ocean Biogeochemical Modeling system (DLEM-ChesROMS-ECB) for Chesapeake Bay from 1985 to 2005 to examine key factors controlling cumulative hypoxic volume (CHV). The modeling system can well reproduce the DO seasonal cycle, vertical gradient, as well as CHV derived from observational data. Then, we applied Ensemble Empirical Mode Decomposition (EEMD) to the simulated monthly CHV. The residual trend demonstrates that CHV increased rapidly from 1985 to 1995, gradually from 1995 to 2000, and remained consistently high from 2000 to 2005. A concurrent EEMD analysis of external forcing factors shows that both river discharge and nutrient loading decreased after 2000, but temperature and SLR remained high. In addition, the sub-regional analysis reveals that the Potomac riverine variables explained more mid-bay CHV interannual variability than the Susquehanna River. Our results suggest that the worsened physical condition due to climate change contributes significantly to the hypoxia in Chesapeake Bay.