In shallow waters around the world, where nutrient pollution runs high, oxygen levels can plummet to nearly zero at night. Oysters living in these zones are far more likely to pick up the lethal Dermo disease, a team of scientists from the Smithsonian Environmental Research Center has discovered. Their findings were published online on Wednesday February 11, 2015 in the open-access journal PLOS ONE. Oxygen loss in the shallows is a global phenomenon, but it is not nearly as well known as the dead zones of the deep. Unlike deep-water dead zones, which can persist for months, oxygen in shallow waters swings in day-night cycles, called diel-cycling hypoxia. When algae photosynthesize during the day, they release oxygen into the water. But at night, when photosynthesis stops, plants and animals continue to respire and take oxygen from the water, causing dissolved oxygen to drop. Lack of oxygen can cripple the oysters' ability to fight off the parasite Perkinsus marinus that causes Dermo disease and slowly takes over the oysters’ bodies. "We usually think of shallow-water habitats as highly productive refuges from deep-water dead zones," says Denise Breitburg, marine ecologist at SERC and lead author of the study. "But if low oxygen makes even these shallow waters inhospitable for fish and shellfish, the whole system may suffer." In a field experiment, Dr. Breitburg and her colleagues suspended hundreds of eastern oysters (Crassostrea virginica) in underwater cages at each of 14 sites around Chesapeake Bay. Some oysters that were only a year old and did not show signs of being infected were used to test the vulnerability of new populations, especially where oyster restoration is concerned. Others that were older and had already been infected were used to test whether low-oxygen made the disease more severe.
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