A vicious cycle of oxygen loss threatens water quality in lakes

Scientists have recently confirmed that the world’s lakes are rapidly losing oxygen. With a seven-year, whole-ecosystem study, a team of freshwater scientists at Virginia Tech has been one of the first to take the next step in asking: What does it mean for water quality that oxygen is declining globally?Sticky with sediment, the bottom waters of lakes are more than their deepest, darkest layer. They bury massive portions of the carbon, nitrogen, and phosphorus found in runoff rolling in from the land. As one of nature’s critical nutrient sinks, lakes earn their recognition as “sentinels” of their surroundings, said freshwater scientist Cayelan Carey.

“We think of lakes as sentinels because they truly integrate all of the changes that happen on land,” said Carey, an associate professor of biological sciences in the Virginia Tech College of Science and an affiliated scientist with the Fralin Life Sciences Institute. “Lakes do this really great job of receiving and processing all of this carbon, nitrogen, and phosphorus, preventing them from going downstream and reaching the ocean.”

But that work could be dismantled by anoxia, the loss of oxygenavailability, Carey’s team found in a study published this week in Global Change Biology. Dreaded by scientists for years and recently confirmed as widespread by data from hundreds of lakes, anoxia is sucking oxygen from the world’s fresh waters.

It’s a phenomenon linked to the warming of waters brought on by climate change and to excess pollutant runoff from land use. Warming waters diminish fresh water’s capacity to hold oxygen, while the breakdown of nutrients in runoff by freshwater microbes gobbles up oxygen.

In a seven-year field experiment that manipulated oxygen levelsin the bottom waters of a nearby reservoir, Carey’s team found that with anoxic conditions came effects they had expected: the sediments release a lot of nutrients and carbon. But they weren’t as prepared for the extent of the changes. They observed the lake going from a sink—which retains more nutrients and carbon than it exports—to a source of nutrients downstream, starting a cycle in which anoxia in one lake could beget anoxia in another.

“I had no expectation that there would be this much change in water chemistry,” Carey said. “And to see it consistently and to see it over the seven years of the study—the effect of anoxia was multiple orders of magnitude greater than what I originally predicted.”

FOR MORE INFORMATION: https://phys.org/news/2022-05-vicious-oxygen-loss-threatens-quality.html

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