Fish are struggling to breathe and algal blooms flourish in lakes around the world, thanks to a drastic decrease in oxygen levels. A new study published in the journal Nature examined oxygen in 400 lakes in the U.S. and Europe over the last 40 years.

Continue reading below
Our Featured Videos

Oxygen is called dissolved oxygen when it is in water, and the right amount of the element is crucial for a healthy ecosystem. But the researchers found an average 5.5% decrease of dissolved oxygen in surface waters and a startling 18.6% drop in deep waters.

Related: Warming in deepest parts of the Great Lakes could be irreversible

“Oxygen is one of the best indicators of ecosystem health, and changes in this study reflect a pronounced human footprint,” said co-author Craig E. Williamson, as reported in AP News. A biology professor at Miami University in Ohio, Williamson and the other study authors posit that decreased water clarity and warming temperatures are the culprits. Human-generated runoff from power plants, cars, sewage and fertilizer all adversely affect water clarity.

The reason it’s important to study both deep water and surface levels is because different organisms depend on different parts of a lake. Cold water fish dive deep in the summer, seeking cooler water temperatures. But if they can’t find enough oxygen in the lake depths, they won’t be able to survive.

“Those are the conditions that sometimes lead to fish kills in water bodies,” study co-author Kevin C. Rose of Rensselaer Polytechnic Institute told AP News. “It really means that a lot of habitats for cold water fish could become inhospitable.”

The study found that about one-quarter of the lakes have increased oxygen levels in their surface waters. But this is probably because of a blue-green algae growth spurt. Blue-green algae is toxic to fish and wildlife, and many a pet dog has tragically perished after drinking from ponds with algal blooms.

Nearly all the lakes included in the study were in Europe or the U.S., except for a few in New Zealand and one in Japan. Other lakes around the world may also suffer from a decrease in dissolved oxygen, but the study authors lacked enough data to evaluate their status.

Via AP News

Image via David Mark