Nearly 400 lakes, primarily in the US and Europe, now have drastically lower oxygen saturation levels than they did in 1941, according to a study published June 2 in Nature. The authors report that as the climate continues to warm, the size of “dead zones,” low-oxygen areas of lakes and oceans ill-suited to support aquatic life, will also increase.

As water temperatures increase, oxygen becomes less soluble due to the water molecules’ kinetic energy. This not only creates hypoxic conditions that are unable to support aquatic life, causing massive die-offs, but also can enable methane, a greenhouse gas, to leak from sediment at the bottom of bodies of water.

“We know that most or many places around the planet are warming,” coauthor Kevin Rose of Rensselaer Polytechnic Institute in New York tells the Associated Press, “and so we would expect to see declining solubility.”

Oxygen levels are also declining due to diminished water clarity, brought on by fertilizer runoff, sewage, and other products of human activity. Cloudy water inhibits the growth of aquatic plants that normally provide oxygen for the lake. The change in the chemistry of the lake water creates a hospitable environment for algal blooms. After algae die, bacteria come in to eat them, consuming large quantities of oxygen in the process.

To find out more about how lakes have been changing over the past 80 years, researchers analyzed data from a number of databases on temperature and dissolved oxygen levels at the surface and deep waters of 393 temperate lakes across the US and Europe, along with a few in Japan and New Zealand. Oxygen levels in water at the surface of the lakes, on average, had dropped 5.5 percent over the years, while deeper waters saw losses averaging 18.6 percent, nine times greater than a 2017 study found for the oxygen decline in oceans since 1960, the AP reports.

“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,” Samuel Fey, a biologist at Reed College who was not involved in the work, tells the AP.

During hot summer months, some aquatic creatures go deeper in the water, where temperatures are cooler. With deep-water oxygen levels so low, previous studies have shown that animals can be pinned between suffocation and an overly warm environment, which affects metabolism and respiration rate. While some species can adapt to the harsher conditions, many cannot, and the ecosystem will be altered as they die off.

“Lakes are the indicators or ‘sentinels’ of environmental change and potential threats to the environment because they respond to signals from the surrounding landscape and atmosphere,” coauthor Stephen Jane of Rensselaer Polytechnic Institute tells the Brisbane Times

In a separate story, the AP reports that the National Oceanic and Atmospheric Association (NOAA) has predicted that the human-caused dead zone which appears every summer in the Gulf of Mexico will be of average size for the 2021 season, at 12,600 square kilometers. However, if a tropical storm comes through, the agitation of the water will help deliver oxygen deeper into the Gulf and could help mitigate the problem.