Quantifying The Drivers Of Hypoxia Onset Variability In The Chesapeake Bay

Hypoxia, defined as low dissolved oxygen levels, is a widespread environmental issue affecting many coastal ecosystems, including the Chesapeake Bay. Exacerbated by global climate change and local anthropogenic stressors, hypoxia presents concern as it reduces suitable habitats for fish and shellfish, alters food webs and species distributions, and degrades the overall quality of the ecosystem. In the Chesapeake Bay, hypoxia has occurred annually for the past several decades, with the lowest oxygen concentrations occurring during the warm summer months. Despite its consistent yearly occurrence, long-term data analyses revealed substantial year-to-year variations in hypoxia onset timing, with hypoxia onset varying by over a month between years. Multiple environmental factors can contribute to this interannual variability, including wind patterns, air temperatures, and watershed inputs. While the general mechanisms of hypoxia development are understood, the complex and interconnected processes make it a challenge to discern the relative importance of individual factors and their underlying mechanisms as they drive annual hypoxia onset, especially as climate change continues to alter hypoxia timing. In addition, the semi-monthly resolution of the available monitoring data makes it difficult to discern onset variability from observations alone. Therefore, to address this knowledge gap, a 3-D fully coupled hydrodynamic-biogeochemical numerical modeling system was used to systematically modify key environmental factors to quantify their relative importance and to identify the critical time periods of greatest influence on hypoxia initiation. Sensitivity experiments involved modifying wind patterns (wind speed, direction, and waves), air temperatures, and terrestrial inputs (watershed loadings and freshwater discharge) individually to simulate realistic, short-term (i.e., monthly) events based on historical data. The magnitude of change for each factor was based on ±1 standard deviation calculated from a 25-year time series. The results reveal that hypoxia onset is most sensitive to month-long changes in May wind speeds, leading to a ~3-week range in onset timing. May air temperatures follow as the second most important factor, resulting in an onset range of ~1 week. While wind direction and waves also contribute to hypoxia development, their impact is relatively minor. Monthly changes in watershed runoff showed negligible effect on hypoxia onset, but displayed a substantial impact on the severity of total annually integrated hypoxic volume. High freshwater discharge, when isolated from changes in loadings, reduces the hypoxic volume due to enhanced estuarine circulation, despite the opposing effect of increased stratification. Overall, these findings provide new insights into the multiple factors that drive interannual variability in the temporal patterns of hypoxia. Understanding these dynamics is valuable for predicting and managing hypoxia, allowing for more targeted management and forecasting approaches.