Report: Chesapeake Bay dead zone size was average this summer, but lasted longer

Every summer, portions of the Chesapeake Bay see oxygen levels choked off when massive algal blooms decay – a natural phenomenon made even worse by nutrients dumped into the watershed, fueling those blooms.

The size and duration of such oxygen-deprived dead zones can vary each year, acting as a bellwether for bay health.

Now scientists say that, while the total size of summer dead zones didn’t change much from last year, its duration did: The season began earlier in the spring and lasted longer – a possible new normal for the bay because of global warming. Hot weather encourages algal growth.

“Overall, our model indicates that the total amount of hypoxia in 2018 was similar to 2017, but that the seasonal patterns in hypoxia were very different,” said Marjy Friedrichs at the Virginia Institute of Marine Science in Gloucester Point.

Hypoxia is when dissolved oxygen levels in the water column are too low to support aquatic life.

Earlier this year, scientists had forecast above-average hypoxia in the bay because winter and spring rains were dumping large amounts of nitrogen from fertilizers, wastewater and other sources into the watershed from the Susquehanna and Potomac rivers.

While low-oxygen conditions did start earlier and last longer overall, unusually strong winds in mid-July churned up bay waters so much that the dead zone volume nearly disappeared.

Aaron Bever of the environmental engineering consulting firm Anchor QEA calls that phenomenon “very atypical of historical dissolved oxygen conditions for mid-summer.”

Hypoxia rapidly rose again in early August, peaking at above-average in early September. But, again, strong winds churned up the bay and knocked the volume back.

VIMS tracks summer dead zones in the full bay every year from May through October, releasing its results in an Annual Chesapeake Bay Hypoxia Report Card, which Friedrichs and Bever started.

On average, Friedrichs said, about 7 percent of the bay’s total volume in summer is hypoxic, and about 20 percent of the volume of Maryland’s portion of the bay, where hypoxia is traditionally worse.

In Maryland, scientists at the Department of Natural Resources report that dissolved oxygen levels in their portion of the bay were comparable to long-term averages from 1985 to 2017.

The DNR found low hypoxia in late July due to rain and winds, then near-record high hypoxia in mid-September because of increased nutrient runoff and warm weather.

Scientists say this shows how weather events can vary hypoxia levels over a short period of time, even as hypoxia in the long-term has been decreasing.

Dead zone monitoring is funded by Virginia, Maryland and the U.S. Environmental Protection Agency’s Chesapeake Bay Program.

Contact Dietrich at 757-247-7892 or tdietrich@dailypress.com. Follow on Twitter at DP_Dietrich

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