Chesapeake Bay, the largest U.S. estuary, could have more corrosive water than expected in a layer 10 to 15 m beneath the surface, a new study finds. A decline in calcium carbonate (CaCO3)-shelled organisms—particularly oysters—may be hampering the bay’s ability to deal with acidity.
“Oysters and other bivalves provide a built-in Tums effect that naturally helps the bay deal with corrosive water,” says George Waldbusser, a marine ecologist with Oregon State University (Corvallis, Oregon). “They generate large amounts of [CaCO3] structures, which may be able to buffer the increasing amounts of carbon dioxide [CO2] entering the bay. Overharvesting and disease have reduced the number of oysters, however.”
Their study found pH levels in this stratified layer to be ~7.4, nearly a unit lower than surface waters, where the pH is ~8.2. Several factors likely caused this corrosive zone, including hypoxia from agricultural nutrients entering the bay and depleting oxygen levels, as well as the generation of hydrogen sulfide (H2S) in bottom waters mixing with other layers.
“This study shows for the first time that the oxidation of [H2S] and ammonia from the bottom waters could be a major contributor to lower pH in coastal oceans and may lead to more rapid acidification in coastal waters compared to the open ocean,” says lead author Wei-Jun Cai from the University of Delaware (Newark, Delaware).
Previous studies, including work by Waldbusser, have shown that agricultural nutrients entering Chesapeake Bay have progressively depleted oxygen levels in the bottom waters through a process known as hypoxia. In addition, these nutrients have caused the bay to acidify more quickly than offshore ocean waters, the researchers say. Animals need oxygen to live—and without it, they die. Bacteria, however, can “breathe” without oxygen, often producing H2S. This further increases oxygen demand and also enhances acidification, Waldbusser says.
“Hypoxia in this case leads to an amplification of acidification,” he explains. “If more oysters were there, they would help pull the food out of the water, reduce oxygen demand, and sequester carbon from the system. Now the acidification is such that we have to be concerned that it will make it harder for some marine organisms to produce their calcium carbonate shells. We don’t yet know what those thresholds are all around.”