A Florida lake with low levels of dissolved oxygen.    Photo: USGS

By Amy Lupica, ODP Staff Writer

Oxygen levels in lakes are declining globally according to a new study, and experts say that it could have dangerous implications for the wildlife and humans that rely on freshwater ecosystems. Much like ocean stratification and acidification, this decline is caused by rapidly rising temperatures that prevent the water from absorbing gasses and often result in harmful algal blooms. But unlike oceans, lakes are losing oxygen at much faster rates. Advocates hope that by protecting 30% of all lands and waters by 2030, we can save these crucial freshwater ecosystems. 

Why This Matters: Lakes only cover 3% of the earth’s surface, but are some of the most biodiverse ecosystems in the world, providing food and water for millions of people around the globe.

• The Great Lakes watershed alone provides resources for almost 40 million Americans and Canadians. 
• Lakes already face numerous threats from invasive species and runoff pollution, triggering algal blooms that suffocate wildlife and render water undrinkable.

Experts say that these recently observed declines in oxygen may also lead to increased methane production from microorganisms. This creates a vicious cycle of warming and greenhouse gas production that protections against pollution and runoff can’t solve alone. 

Out of Breath

“Lakes are indicators or ‘sentinels’ of environmental change and potential threats to the environment because they respond to signals from the surrounding landscape and atmosphere,” said aquatic ecologist Stephen Jane at Rensselaer Polytechnic Institute. Researchers took 45,000 profiles of water and oxygen content from 393 temperate bodies of water across the globe. They found that from 1941 to 2017, there was an average drop of 5.5 percent in oxygen in surface-level waters. In deeper waters, the average reduction reached 18.6 percent. This is because, as the surface warms, less mixing of water layers occurs. Despite less warming in lower levels of water, oxygen is no longer circulating through the water effectively.

This same process, known as stratification, has been happening in our oceans as well, but lakes are now losing oxygen 2.75 to 9.3 times faster than oceans. “We found that these disproportionally more biodiverse systems are changing rapidly, indicating the extent to which ongoing atmospheric changes have already impacted ecosystems,” said Jane. As oxygen levels drop, however, some lifeforms will thrive. Unfortunately, these microscopic bacteria produce large amounts of methane and will only increase production as oxygen vanishes. In addition, many of the lakes surveyed showed increasing oxygen in surface waters, which is likely due to increased toxic algal blooms — sudden growth of blue-green algae.

Methane can be up to 86 times more effective at capturing heat in the atmosphere than carbon dioxide. Only recently, however, have scientists begun to study how aquatic ecosystems contribute to methane emissions around the world. An April 2021 study found that up to half of global methane emissions come from aquatic ecosystems, especially freshwater ones. Experts say that the key to fighting these rising freshwater emissions is smart water management practices and establishing protections for all lands and waters by 2030. Curt Breneman, dean of the School of Science at the Rensselaer Polytechnic Institute said, “We hope this finding brings greater urgency to efforts to address the progressively detrimental effects of climate change.”