Rising Water Temperatures ​Endanger Coastal ​Ecosystems

Increasing water temperatures are responsible for the accumulation of a chemical called nitrite in marine environments throughout the world.

Marine biologists James Hollibaugh and Sylvia Schaefer found that rising water temperatures could disrupt ocean food webs and lead to the release of more greenhouse gases . Credit: Andrew Davis Tucker/University of Georgia

It is a symptom of broader changes in normal ocean biochemical pathways that could ultimately disrupt ocean food webs, according to new research from the University of Georgia.

Nitrite is produced when microorganisms consume ammonium in waste products from fertilizers, treated sewage and animal waste. Too much nitrite can alter the kinds and amounts of single-celled plants living in marine environments, potentially affecting the animals that feed on them, said James Hollibaugh, co-author of the study published recently in Environmental Science and Technology. It also could lead to toxic algal blooms and create dead zones where no fish or animals can live.

"Rising ocean temperatures are changing the way coastal ecosystems—and probably terrestrial ecosystems, too—process nitrogen," said Hollibaugh, Distinguished Research Professor of Marine Sciences in UGA's Franklin College of Arts and Sciences. "Much of the global nitrogen cycle takes place in the coastal zone."

Hollibaugh and researcher Sylvia Schaefer found midsummer peaks in concentrations of nitrite alongside massive increases in numbers of the microorganisms that produce it in the coastal waters off Sapelo Island, Georgia, in data collected over the course of eight years. Although most researchers believe nitrite accumulation is a consequence of oxygen deficiency in a marine environment, Hollibaugh and Schaefer thought something else had to be driving the accumulation.

"The paradigm taught when I was in school was that hypoxia, or lack of oxygen, results in nitrite accumulation," Hollibaugh said. "But the Georgia coast does not go hypoxic. It just didn't fit."

After performing lab experiments that exposed the single-celled organisms known as Thaumarchaea to varying water temperatures, the researchers discovered that higher temperatures prompted the microorganisms to produce more nitrite.

"The microorganisms involved in this process are very tolerant to low oxygen levels," Schaefer said. "Typically, two groups of microorganisms work in really close concert with one another to convert ammonium to nitrate so that you don't see nitrite really accumulate at all, but we found that the activity of those two groups was decoupled as a result of the increased water temperatures."

To see if the pattern held beyond the island, Schaefer and Hollibaugh analyzed environmental monitoring data from 270 locations across the U.S., France and Bermuda, ultimately affirming the relationship between higher temperatures and nitrite accumulation.

Nitrite accumulation can also result in more production of nitrous oxide, a powerful greenhouse gas that has more of an effect on climate change per molecule than carbon dioxide, Hollibaugh said. That nitrous oxide production then increases global temperatures more, causing more nitrite accumulation and creating a positive feedback loop.

Read more: Phys.org

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• Coastal Engineering
• Ecosystem Management
• Coastal
• Climate Change
• Oceanographic Survey
• Environmental Impact
• Animal Health