by Kristen Minogue

Oysters in Chesapeake Bay face more dangers than overfishing and habitat loss. Over the last few decades they’ve also had to contend with crippling disease outbreaks. And according to marine ecologist Denise Breitburg, the wild day-night fluctuations in Bay waters aren’t helping.

Breitburg and her lab have spent the last three years examining Eastern oysters infected with Dermo, a slow but deadly disease whose impact in the Bay accelerated in the 1980s. Dermo is caused by a microscopic parasite called Perkinsus marinus that is acquired as oysters feed. Once inside, it multiplies until hundreds of thousands of parasites take over the oyster’s body. Even moderately infected oysters suffer slower growth and a diminished ability to reproduce.

Some of the biggest danger zones are the ones suffering from low oxygen, or hypoxia. Without enough oxygen, oysters’ capacity to fight infection drops. The problem with shallow water in some parts of the Bay—and many estuaries around the world—is that their oxygen levels swing drastically. Oxygen concentrations tend to soar during the day only to plummet at night. This phenomenon, known as diel-cycling hypoxia, is common in shallow waters where nutrients are high and winds and currents too low to mix the water well. This type of oxygen starvation may be the most threatening to oysters, according to Breitburg, because it occurs where they live.

“We don’t tend to find oysters in areas that are continuously low in oxygen,” she says. “But we do find them in areas that have these big day-night swings. So it’s very likely that this form of low oxygen is really problematic.”

Nutrients streaming off the land fuel the wildest fluctuations. While the sun is shining, massive algal blooms swollen by excess nutrients photosynthesize and flood the water with oxygen. But at night, when photosynthesis stops, plants, microbes and animals continue to respire, depleting the oxygen in the water and releasing CO₂—just as humans do when they breathe. Ideal summer oxygen concentrations in the Bay hover around 7 mg/L. At night some places drop to near zero.

Breitburg’s team is working to discover exactly how these fluctuations impact the disease and the benefits oysters give the Bay. Field experiments they conducted already showed that oysters in areas with low nighttime oxygen had a higher risk of Dermo infection. Now they are testing it in lab.

In a laboratory ominously named the “Room of DOOM” (Dissolved Oxygen Oyster Mortality), they are manipulating oxygen and pH (acidity) levels in 30 oyster aquaria. Each tank contains young, uninfected 1-year-old oysters. Another tank contains Dermo-infected 4-year-old oysters. The lab pumps water from the infected tank into the healthy tanks to see how many healthy oysters will contract the disease. Meanwhile five gas cylinders (normal air, air without carbon dioxide, oxygen, carbon dioxide and nitrogen) enable them to recreate the cycles Bay oysters experience in nature.

At first the team looked only at oxygen. The results were troubling: Oysters exposed to low oxygen, even for only a few hours each night, had much higher infection rates than ones that enjoyed constant, healthy levels. They also filtered less water, hampering their ability to help purify the Bay of excess algae.

This summer the lab is investigating a second piece of the puzzle: acidity. More acidic water also can reduce oysters’ ability to produce disease-fighting chemicals. Like oxygen, it operates on a day-night cycle. CO₂ from nighttime respiration can raise the acidity of shallow water tenfold.

Actually helping oysters recover will take a three-part strategy, according to Breitburg. First, protect oysters from fishing. Second, reduce nutrient runoff into the Bay, since it exacerbates the oxygen and acid cycles. Finally, restore large quantities of oysters in shallow areas, where the oysters themselves could clear the water enough to help fix the problem.

“During periods of low oxygen, oysters are filtering less, so they can’t provide ecosystem services important to the Bay’s health,” she says. “But restoring oyster populations could improve water quality in shallow Bay waters.”