Dead-Zone Report Card

The forecast and report card use multiple metrics to quantify and compare hypoxic severity:

• Maximum Daily Hypoxic Volume (km³): The maximum volume of Chesapeake Bay water experiencing hypoxic conditions on any given day
• Average Summer Hypoxic Volume (km³): The average volume of hypoxic water from June through September
• Hypoxic Duration (days): The number of days in a given year between the first and last day of hypoxic conditions exceeding 2 km³ in volume
• Total Annual Hypoxic Volume (km³ days): The total amount of hypoxia in the Bay for a given year, calculated by summing the hypoxic volume on each day

Springtime inflows from the Susquehanna River were high in 2019, resulting in the prediction that 2019 would have the 4^th largest July hypoxic volume in the last 20 years. However, summer winds and temperatures also play large roles in the severity of hypoxia. Through mid-July 2019, total annual hypoxic volume was on the high end of 2014 to 2018. Weak winds and high temperatures from the end of June to mid-August allowed hypoxia to increase to higher levels. This is different from 2018, when strong winds reduced the amount of mid-summer hypoxia. In 2019, hypoxia decreased quickly in late August and early September (Hurricane Dorian) as winds increased; however, hypoxia returned with the high temperatures in late September and early October until strong winds mixed the Bay water and ended hypoxia in the mainstem of the Bay for the year. Overall, the total amount of hypoxia in 2019 was estimated to be on the high end of the normal range for 1985 to 2018, and higher than in the recent past. As in 2018, hypoxia also lasted longer than in other recent years.

Even with environmental conditions that favor severe hypoxia (high riverine input to the Bay and light winds), the total amount of hypoxia in 2019 was within the normal range, suggesting nutrient reductions since the 1980s have helped improve water quality in the Bay.