VOA慢速英语2021--美欧数百个湖泊缺氧日益严重（在线收听）

Researchers say oxygen levels have dropped in hundreds of lakes across the United States and Europe over the last 40 years.

A new study suggests the oxygen loss can lead to more fish dying and increased algae growth.

Scientists examined the temperature and the amount of dissolved oxygen in nearly 400 lakes. Dissolved oxygen is the amount of oxygen gas contained in water. They discovered widespread drops in oxygen levels.

The study recently appeared in the publication Nature. It found that dissolved oxygen decreased an average of 5.5 percent in the surface water of the lakes and 18.6 percent in the deep water.

The researchers say their findings suggest that warming temperatures and decreased water clarity from human activity are causing oxygen levels to fall.

Craig E. Williamson is a biology professor at Miami University in the state of Ohio and a co-writer of the study. He told The Associated Press that testing for oxygen is one of the best ways to judge how healthy water systems are. The study's findings suggest "a pronounced human footprint," he said.

That footprint includes warming caused by climate change, the researchers said. In addition, decreased water clarity can be caused by runoff from human waste systems, fertilizers, cars and power plants.

Losses in dissolved oxygen in Earth's water systems have been reported before. A 2017 study of oxygen levels in the world's oceans showed a two percent drop since 1960. But less was known about lakes, which lost two to nine times as much oxygen as oceans, the study found.

In the past, other researchers had reported oxygen decreases in individual lakes over a long period of time. But those studies did not examine as many lakes around the world, said Samuel B. Fey. He is a biology professor at Oregon's Reed College who studies lakes. He was not involved in the latest study.

Fey said an important finding was that the researchers were able to show such a notable drop in dissolved oxygen levels. Also, the drops were found at both the surface and in deep parts of the lakes.

The deep water drop in oxygen levels may affect organisms that are more sensitive to temperature increases, such as cold water fish. During the summer months, they depend on cooler temperatures found in deeper water. But if deep waters are low in oxygen, the organisms struggle to survive.

"Those are the conditions that sometimes lead to fish kills in water bodies," study co-writer Kevin C. Rose said. This means that a lot of areas cold water fish depend on "could become inhospitable," he added.

Other organisms, Rose said, do better in warmer temperatures at the surface level and can get enough oxygen by remaining there.

About one fourth of the lakes studied showed increasing oxygen in surface waters. Rose said this is a bad sign because it points to increased algal blooms — the sudden growth of algae. In those lakes, dissolved oxygen was "very low" in deep waters, likely creating unlivable conditions for many species, the researchers said.

Words in This Story

algae – n. a plant with no stem or leaves that grows in or near water

pronounced – adj. very easy to notice

fertilizer – n. a substance or a special chemical that is added to soil to help the growth of plants

inhospitable – adj. not pleasant or easy to live in

bloom – n. a fast and excessive growth of water organisms

species – n. biology : a group of animals or plants that are similar and can produce young animals or plants : a group of related animals or plants that is smaller than a genus