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The Mysteries of the Bay at Night

OysterChesapeakeBayProgramIt’s like an episode of that old TV series, “The Night Gallery.”  You go into your house at night. And as the stars appear from the depths of space, and you fall asleep on your bed, all the oxygen is sucked from outside your windows.

It sounds creepy.  But this is essentially what happens to fish and shellfish in some shallow alcoves of the Chesapeake Bay when high pollution levels contribute to dramatic nighttime drops in oxygen levels.  The effects are often more subtle than fish kills, scientists say –- and may include spikes in acidity levels in the water, and greater vulnerability to disease for oysters.

The implications of this Midnight madness -– especially on efforts to replant and restore the Chesapeake Bay’s depleted oyster population –- is being studied in the “DOOM” lab.

That stands for “Dissolved Oxygen and Oyster Mortality.” The lab is run by the Smithsonian Environmental Research Center in Edgewater Maryland, south of Annapolis.

There, Senior Scientist Dr. Denise Breitburg explains why the setting of the sun changes the chemistry of shallow inlets of the Bay that are important breeding grounds and shelters for aquatic life.

She talks about the impact of nitrogen and phosphorus pollution from fertilizers, sewage plants and other sources -- which is also called “nutrient pollution” because it nourishes excessive growth of algal blooms.

“What we’re finding is that the Chesapeake Bay, like places around the world, actually has a real problem with huge day night fluctuations in oxygen concentrations in the shallow parts of the system,” Dr. Breitburg said.

“So that during the daytime, especially – and this is all really exacerbated when you have a lot of nutrients going in – during the time the plants are photosynthesizing, they are producing oxygen and they are taking up carbon dioxide.”

That positive oxygen balance reverses at night.

“When it’s dark out and the algae are not photosynthesizing, then the balance tips towards much more oxygen getting consumed,” Dr. Breitburg said.  “Everything from plants to fish to microbes are respiring, using up oxygen and releasing CO2 into the water. So oxygen concentrations at night can be really, really low.”

This is partly a natural phenomenon. But the oxygen deprivation at night gets worse with more pollution. And the buildup of carbon dioxide triggers a chemical reaction that creates carbonic acid, which increases the acidity of the water. More acidity is expressed as lower Ph levels.

“We can have a full Ph unit fluctuation between day and night,” Dr. Breitburg said.  “So that’s a 10 fold change in the amount of… essentially carbon dioxide that is getting into the water.”

These conditions appear to make oysters more susceptible to a chronic wasting disease, Dermo, caused by a parasite, Perkinsus marinus.

Other scientists have linked higher carbon dioxide levels in water to the burning of fossil fuels and climate change.  Research by Dr. Justin Ries of the University of North Carolina and others has suggested that high carbon dioxide levels in the ocean and resulting increased acidity may be causing some oysters to develop abnormally thin shells, and crabs to grow strangely large and thick shells.

All is not gloom and doom, however -– even at the DOOM lab.

The modernization of sewage treatment plants around the Chesapeake Bay, along with improvements to stormwater pollution runoff control systems and other factors, have somewhat reduced nitrogen and phosphorus pollution in recent years. Indeed, the oxygen starved areas of the Bay this past summer were the smallest in a quarter century.

That was an encouraging plot twist for the Bay’s “Night Gallery.”

By Tom Pelton

Chesapeake Bay Foundation

(Photo at top from Chesapeake Bay Program)


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