ANNAPOLIS, Md. – The Chesapeake Bay’s oxygen-starved “dead zone,” a closely watched indicator of the estuary’s health, started early this summer with worse than normal conditions before dramatically improving by early August, scientists recently reported.
Overall, scientists concluded this summer’s low oxygen, or hypoxic, conditions were near average. But, in this case, “average” masks conditions which, at times, were both significantly better and worse than normal.
Estimating the extent of the dead zone is important because almost all aquatic life requires some level of oxygen to survive, and reducing the affected area has been the driving force behind the region’s nutrient reduction efforts.
The nutrients phosphorus and nitrogen fuel algae growth. When there are more algae than can be consumed by fish and shellfish, the excess die and sink to the bottom where it is decomposed by bacteria in a process that removes oxygen from deep water areas.
Low oxygen conditions can squeeze aquatic life out of some areas and kill creatures that can’t move. Hypoxic conditions, usually found at the bottom, are those with less than 2 parts per million of oxygen. Areas near the surface usually have 6 ppm of oxygen or more.
Nutrient reduction efforts have slowly reduced the average size of the dead zone, but weather conditions play an important role in determining its size and duration.
Wetter than normal conditions, which took place for a portion of last spring, can drive more nutrients into the Bay and fuel algae growth, while factors such as temperature and wind play a role in determining the size and duration of hypoxic conditions.
Exactly how that played out this summer isn’t totally clear. Scientists from the Virginia Institute of Marine Science and Maryland Department of Natural Resources, who analyze summertime oxygen conditions using different techniques, reached somewhat different conclusions.
VIMS scientists reported that this summer had slightly worse than average conditions while DNR scientists found slightly better than average conditions.
Both agreed that the worst conditions were seen earlier than normal, in the first half of June. Usually, conditions are poorest in late July.
They also said conditions improved dramatically in early August when the remnants of Hurricane Debby passed over the region, bringing strong winds that mixed oxygen-rich surface water with oxygen-starved bottom water.
VIMS estimates show that as much as 20% of the Bay and its tidal tributaries suffered from low-oxygen conditions in mid-June when the dead zone was at its peak.
That means that nearly four cubic miles of water was off limits to most aquatic life, making it the largest area impacted since 2019, when a quarter of the Bay suffered from hypoxic conditions.
Their estimates also showed large areas of hypoxia persisting through July, but those were largely gone in early August in the wake of Debby, weeks earlier than normal. Overall, the VIMS data show that hypoxic conditions persisted for 96 days, the shortest period since 2017.
DNR scientists also found larger than normal hypoxic areas in early June but estimated that those conditions covered less than 10% of the Bay.
They reported better than average conditions in late June and July before disappearing after Debby. They said the early August oxygen conditions were the best observed for that time of year in nearly 40 years of monitoring.
But DNR also reported that hypoxia bounced back in late August and September, when oxygen conditions were again worse than average, before disappearing in October.
DNR’s estimates are based on samples drawn twice a month from Bay waters in Maryland and Virginia. Those provide an accurate snapshot of conditions at those times, though they may not capture changes that take place between sampling.
Also, mechanical problems and weather kept some areas from being sampled in July, which DNR scientists said may have resulted in an underestimate of hypoxic conditions at that time.
VIMS scientists produce their estimates using a computer model that draws on monitoring, weather and other data to estimate hypoxic conditions that take place between monitoring cruises and in places that are not monitored.
Another difference is that the DNR figures cover only the mainstem of the Chesapeake, while the VIMS estimates cover both the Bay and the tidal portions of its tributaries.
Despite some of the differences, the scientists did agree on this: Conditions overall fell within what would be considered an “average” range for the Bay.