Another rare 2020 bright spot: Chesapeake Bay had shorter, less severe dead zone

A boat sits docked in Deltaville, which sits near the tip of Virginia's Middle Peninsula on the Chesapeake Bay. (Sarah Vogelsong/Virginia Mercury)

Thanks to a cool spring, early fall temperatures and the arrival of Hurricane Isaias in August, the Chesapeake Bay dead zone was less severe and lasted for a shorter time in 2020 than in previous years.

The annual dead zone report for the bay, issued by the Virginia Institute of Marine Science at William & Mary, bore out optimistic predictions put forward by scientists in June

One of the most obvious signs of ecological trouble, dead zones are parts of a body of water where animal and plant life cannot be sustained because of a lack of oxygen.

That condition, known as hypoxia, is primarily caused by excess nutrients in the water, with nitrogen playing a leading role in the process. Too much nitrogen leads to blooms of algae that deplete oxygen from water as they die and decompose. 

In the Chesapeake Bay, the resulting dead zone has been estimated in many years to cover as much as 40 percent of the area of the estuary’s mainstem. Efforts by bay states, including Virginia, to reduce nutrient pollution throughout the watershed under the Chesapeake Bay Watershed Agreement are designed to minimize and eventually eliminate dead zones.

A Virginia Marine Institute of Science “nowcast” of dissolved oxygen in the Chesapeake Bay on June 17. (Virginia Institute of Marine Science)

In a Wednesday news release, the Chesapeake Bay Program attributed this year’s small and short dead zone both to such long-term efforts and weather-related factors.

“The general trend we’re seeing is that hypoxia has been decreasing with time, as a result of reductions in the amount of nutrients flowing into the Bay over the past several decades,” said VIMS professor Marjy Friedrichs in a statement. 

But weather played a major role too. This year, “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,” the VIMS report card concluded. 

Researchers pointed especially to cool and windy weather in the spring that helped mix and aerate the Chesapeake Bay’s waters, as well as the early August appearance of Hurricane Isaias, which sharply shrank the dead zone that had ballooned in July. Cool temperatures in September and October also curtailed late summer hypoxic conditions. 

“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,” the authors wrote.