Climate change is sapping the world’s lakes of oxygen. Recent decades have seen oxygen levels in most of the world's temperate freshwater lakes deplete rapidly, even faster than the "shocking" drops seen in the planet’s oceans. If the trend continues, this unforeseen side effect of the climate crisis could further threaten biodiversity and drinking water quality worldwide.
Reporting their study in the journal Nature this week, an international team of scientists led by the Rensselaer Polytechnic Institute looked at a total of over 45,000 dissolved oxygen and temperature profiles collected since 1941 from nearly 400 lakes around the globe, most of which were found in the temperate zone.
Dissolved oxygen in these freshwater environments was found to have declined by 5.5 percent at the surface and 18.6 percent in deep waters since 1980, around two to nine times greater than those seen in the oceans.
"All complex life depends on oxygen. It's the support system for aquatic food webs. And when you start losing oxygen, you have the potential to lose species," Kevin Rose, study author and professor at Rensselaer Polytechnic Institute, said in a statement.
The depletion of oxygen is driven by physics. In surface water, dissolved oxygen concentrations declined by 0.11 milligrams per liter per decade as surface water temperatures increased by 0.38°C (0.68°F) per decade.
"Oxygen saturation, or the amount of oxygen that water can hold, goes down as temperatures go up. That's a known physical relationship and it explains most of the trend in surface oxygen that we see," explained Rose.
Deeper down, the warming temperatures are causing stronger stratification between the waters’ layers, resulting in the deeper layers not being replenished by the more oxygenated surface levels.
There are some exceptions, however. A small number of the studied lakes actually saw rising oxygen levels despite being hit by warming temperatures. As the study authors explain, these lakes were likely to be polluted with nutrient-rich runoff from agriculture. Combined with the warming temperatures, they experienced an explosion of cyanobacteria, which pump out oxygen through photosynthesis.
By and large, though, lakes saw a worrying decrease in oxygen levels. That's bad news for fish and bad news for us.
Firstly, biodiversity is likely to be impacted due to its reliance on oxygen. Secondly, oxygen mediates many other aspects of water quality, such as the growth of microbes, which could affect our drinking water supplies. On top of that, declining oxygen levels are likely to see a blossoming of bacteria that thrive in environments without oxygen. Many of these bacteria produce the potent greenhouse gas methane, thereby contributing to further climate change.