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—one component of the Chesapeake Bay Environmental Forecast System—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 2021 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 2021

Springtime nitrogen inflows in 2021 were 19% below the long-term average, resulting in the prediction that the amount of hypoxia would similarly be slightly less than average. However, calm winds and slightly warmer temperature in May 2021 compared to 2020 resulted in hypoxia starting earlier this year. As summer arrived, moderate winds allowed hypoxia to increase through mid-July, resulting in a maximum size of the dead zone similar to the average historical size. This mid-summer peak is similar to what occurred in late July 2020, but smaller than 2019 when hypoxia was quite severe. In 2021, hypoxia decreased in early August, but a period of calm winds in late August allowed the amount of hypoxia to increase again. However, hypoxia decreased abruptly at the beginning of September as the remnants of Hurricane Ida stirred Bay waters. Stronger winds prevailed following Ida, but unusually warm fall temperatures and high precipitation counteracted some of the normal autumnal reduction in hypoxia, contributing to hypoxia ending in the mainstem of the Bay considerably later than in previous years. Overall, the duration of hypoxia in summer 2021 was relatively long, but the other metrics of severity were closer to typical mid-range values.

Springtime nutrient supply to the Bay suggested hypoxia in 2021 should have been slightly better than average; however, the duration of hypoxia was quite long, resulting in a slightly higher than usual overall severity of hypoxia. This demonstrates how relatively early onset and late termination can result in slightly more total hypoxia, even with typical hypoxia levels throughout the summer.

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).