Dead-Zone Report Card

Compare the annual severity of Chesapeake Bay hypoxia

Each year, the Virginia Institute of Marine Science and Anchor QEA, LLC release a retrospective seasonal analysis of the severity of hypoxia in the Chesapeake Bay. The Annual Chesapeake Bay Dead Zone Report Card summarizes dissolved oxygen concentrations in the Bay as estimated by the team's 3-D, real-time hypoxia forecast model. The modeling team also generates the same dissolved oxygen statistics for previous years for comparative purposes.

Download a printable pdf copy of the VIMS 2020 report card.

Report Card Metrics

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
Synopsis for 2020

Springtime nitrogen inflows in 2020 were 17% below the long-term average, resulting in the prediction that the amount of hypoxia would similarly be slightly less than average. However, cool windy weather helped mix and aerate Bay water in the spring, resulting in hypoxia beginning later than in previous years. As summer arrived, weak winds and very high temperatures allowed hypoxia to increase considerably, resulting in a a very large dead zone in late July. This midsummer peak is very similar to what occurred in mid-July 2019, but different from 2018 when strong winds temporarily eliminated the dead zone. In 2020, hypoxia decreased quickly in early August in response to Hurricane Isaias; however, hypoxia returned in early September until stronger winds and cooler temperatures prevailed, ending hypoxia in the mainstem of the Bay earlier than in previous years. Overall, the total amount of hypoxia in 2020 was estimated to be considerably lower than in the recent past, with hypoxia both starting later and ending earlier, as was also seen in periodic ship-based observations of dissolved oxygen.

Even with a springtime nutrient supply to the Bay that suggested hypoxia in 2020 should have been only slightly better than average, the overall severity of hypoxia was quite low and the duration quite short. This demonstrates how a cool spring and a large summer storm can impact the severity of Chesapeake Bay hypoxia from one year to the next.

Summed Annual Estimates

Here we describe the severity of hypoxia as estimated by the forecast model. We define hypoxia as dissolved oxygen concentrations less than 2 milligrams/Liter (mg/L).

Time Series Estimates

The maximum daily hypoxic volume was higher than in 66% of historical years. The Bay reached peak hypoxia on July 22nd, with an estimated volume of 11.2 km3.

Cool, windy weather helped mix and aerate Bay water in the spring, resulting in hypoxia beginning later than in previous years. As summer arrived, weak winds and very high temperatures allowed hypoxia to increase considerably, resulting in a a very large dead zone in late July. Hypoxia decreased quickly in early August in response to Hurricane Isaias; however, it returned in early September until stronger winds and cooler temperatures prevailed, ending hypoxia in the mainstem of the Bay earlier than in previous years.

The total hypoxic volume within the Chesapeake Bay in 2020 was smaller than in 8 of 10 historical years, and the lowest it has been in the last 5 years.

The 2020 hypoxia season was shorter than in 9 of 10 historical years. Duration is calculated as the number of days between the first and last day of hypoxic conditions exceeding 2 km3 in volume.