Chesapeake Bay ‘dead zone’ near long-term average: Analysis

Chesapeake Bay Program partners said Wednesday that the Bay’s “dead zone” in 2024 was near the long-time average taken from 1985 to 2023.

Dead zones are areas of low oxygen that form when high levels of nitrogen and phosphorus enter the water through polluted runoff and feed naturally occurring algae. This drives the growth of algal blooms, which eventually die, sink and decompose, removing oxygen from the surrounding waters faster than it can be replenished.

This creates low-oxygen — or hypoxic — conditions at the bottom of the Bay that limit habitat for crabs, oysters, fish and other wildlife.

Dead zone conditions began forming from late April to early May, reaching a peak sometime in June to July and ending in October.

The announcement aligns with the Chesapeake Bay dead zone forecast made earlier this year, in which researchers called for a near-average dead zone.

“The past few years of near or below average Bay-wide hypoxia levels are a good sign that Chesapeake Bay nutrient management actions are working, but these must be continued if we are to outpace water quality degradation due to climate change,” said Dr. Marjy Friedrichs, research professor at the Virginia Institute of Marine Science.

In the Chesapeake Bay, hypoxia levels are estimated from late spring to early fall by the Maryland Department of Natural Resources and the Virginia Institute of Marine Science, each using a different combination of monitoring and modeling tools to make their estimates.

This year, the VIMS model estimated that the total volume of hypoxia was somewhat above the long-term average taken between 1985 to 2023, with higher peaks in mid-June and mid-July, but a lower volume of hypoxia in the water occurring in August.

According to DNR’s estimates, which are based on semi-monthly monitoring cruises measuring dissolved oxygen, hypoxia levels were higher than the long-term average during the early June cruise but were below average for the rest of the summer.

Typically, the volume of hypoxic water in the Bay is greatest in late July and early August, but DNR and VIMS both noted that this year some of the highest levels of hypoxia occurred unusually early, in early June.

As the remnants of Hurricane Debby passed over the region in early August, hypoxia dramatically decreased, in part due to strong winds mixing oxygen from the surface of the Bay into its deeper waters. Hypoxic conditions then remained relatively low in September and into early October when hypoxic conditions ceased.

“In this milestone 40th year of Chesapeake Bay Program water quality monitoring, the program and partnership continue to show its importance in improving the health of the Bay by guiding and tracking progress towards restoration with products such as the annual dissolved oxygen monitoring, forecasting and modeling,” said Mark Trice, program chief of water quality informatics with DNR’s Resource Assessment Service.

The United States Geological Survey provides data on the levels of streamflow and nitrogen and phosphorus entering the Chesapeake Bay. According to a preliminary assessment from the USGS, nitrogen pollution entering the Bay from around the watershed were near the historical average from January to May, which likely contributed to the dead zone also being near average this summer.