Oxygen levels falling in lakes around world
Oxygen levels have dropped in hundreds of lakes in the United States and Europe over the last four decades, a new study found.
And the authors said declining oxygen could lead to increased fish kills, algal blooms and methane emissions.
Researchers examined the temperature and dissolved oxygen — the amount of oxygen in the water — in nearly 400 lakes and found that declines were widespread. Their study, published Wednesday in the journal Nature, found dissolved oxygen fell 5.5 % in surface waters of these lakes and 18.6% in deep waters.
The authors said their findings suggest that warming temperatures and decreased water clarity from human activity are causing the oxygen decline.
“Oxygen is one of the best indicators of ecosystem health, and changes in this study reflect a pronounced human footprint,” said coauthor Craig Williamson, a biology professor at Miami University in Ohio.
That footprint includes warming caused by climate change and decreased water clarity caused in part by runoff from sewage, fertilizer, cars and power plants.
Dissolved oxygen losses in Earth’s water systems have been reported before. A 2017 study of oxygen levels in the world’s oceans showed a 2% decline since 1960. But less was known about lakes, which lost two to nine times as much oxygen as oceans, the new study’s authors said.
Prior to this study, other researchers had reported on oxygen declines in individual lakes over a long period of time. But none have looked at as many lakes around the world, said Samuel Fey, a Reed College biology professor who studies lakes and was not involved in this study.
“I think one of the really interesting findings here is that the authors were able to show that there’s this pretty pronounced decline in dissolved oxygen concentrations in both the surface and (deep) parts of the lake,” Fey said.
The deepwater drop in oxygen levels is critical for aquatic organisms that are more sensitive to temperature increases, such as cold water fish. During summer months, they depend on cooler temperatures found deeper in the water, but if deep waters are low on oxygen, these organisms can’t survive.
Lakes examined in the new study were in the U.S. or Europe, except for one in Japan and a few in New Zealand. The authors said there was insufficient data to include other parts of the world.