Climate change is sweeping the lake of oxygen around the world. Oxygen levels in most of the world’s richest freshwater lakes have plummeted in recent decades, and even faster than the “shocking” drop seen in planetary oceans. While reporting their study in the journal Nature this week, an international team of scientists led by the Rensselaer Polytechnic Institute looked at more than 45,000 dissolved oxygen and temperature profiles collected from about 400 lakes around the world since 1941, most of them found in temperate regions.
The dissolved oxygen in this freshwater environment has decreased by 5.5 percent on the surface and 18.6 percent in deep water since 1980, about two to nine times more than in the oceans. Kevin Rose, study author and professor at Rensselaer Polytechnic Institute, said in a statement, “All complex life depends on oxygen. It is a support system for aquatic food nets. And when you start to lose oxygen, your species may lose it.” Oxygen depletion is drive by physics. In surface water, the concentration of dissolved oxygen decreased by 0.11 mg per liter over the decade as surface water temperatures increased by 0.38°C (0.68°F) over the decade.
Rose explained, “The amount of oxygen saturated or the amount of water that water can hold decreases with increasing temperature. That’s a known physical relationship and it explains most of the surface oxygen trends we see.” Further down, warmer temperatures cause stronger stratification between the water layers, so that more oxygenated surface layers cannot replenish the deeper layers. Further down, warmer temperatures cause stronger stratification between water layers, so that more oxygenated surface layers cannot replenish deeper layers.
As the authors of the study explained, these lakes were more likely to pollute with nutrient-rich runoff from agriculture. Combined with warmer temperatures they felt an explosion of cyanobacteria, which pumps oxygen through photosynthesis. Although and large lakes have a worrying drop in oxygen levels. This is bad news for fish and bad news for us. First, biodiversity is likely to affect due to its dependence on oxygen. Second, oxygen mediates many more aspects of water quality such as microbial growth that can affect our drinking water supply.
On top of that, a decrease in oxygen levels can lead to bacteria thriving in an environment without oxygen. Many of these bacteria produce the powerful greenhouse gas methane, which in turn contributes to further climate change.