Research published recently in Nature found that oxygen levels in lakes across the northern hemisphere are declining 2.8 to 9.3 times faster than in the world’s oceans – a trend which threatens freshwater fisheries, biodiversity, and drinking water quality.
Researchers analyzed over 45,000 dissolved oxygen and temperature profiles collected since 1941 from nearly 400 lakes around the globe.
University of Regina research in the study showed that rates of oxygen loss are particularly high in Canadian Prairie lakes of Saskatchewan where rapid warming, nutrient pollution, and blooms of toxic cyanobacteria combine to deplete oxygen from the bottom of lakes.
“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,” said Kevin Rose, corresponding author and professor at Rensselaer Polytechnic Institute in New York. “Loss of up to 18 per cent of deepwater oxygen since 1980 has a profound effect on the biology and chemistry of lakes”.
Peter Leavitt, a Canada Research Chair at University of Regina and lead investigator of the 28-year Qu’Appelle Long-Term Ecological Research program says that Prairie lakes may be especially vulnerable.
“Warming of surface waters reduces lake mixing and the flow of atmospheric oxygen into the deep waters where fish often hide to avoid surface heat. Loss of this deep oxygen suffocates native fishes, but can benefit some invasive species which in turn harm the lake’s food web,” says Leavitt.
In addition to warming, Prairie lakes have high levels of nutrients which increase the risk of algae and cyanobacteria blooms. Oxygen levels decline further when these blooms die, sink to the lake bottom, and decompose.
Although lakes make up only about three per cent of Earth’s land surface, they contain a disproportionate concentration of the planet’s biodiversity. Lead author Stephen F. Jane, who completed his PhD with Rose, said the changes are concerning both for their potential impact on freshwater ecosystems and for what they suggest about environmental change in general.
“Lakes are indicators or ‘sentinels’ of threats to the environment because they respond to signals from the surrounding landscape and atmosphere. We found that these biodiverse lakes are changing rapidly, showing that atmospheric warming has already impacted freshwater ecosystems,” Jane said.
In addition to biodiversity loss, declines in dissolved oxygen can increase greenhouse gas emissions from lakes, intensify pollution of surface waters with toxic metals, and favour blooms of toxic algae – all factors that affect human health.
“Climate warming and nutrient enrichment of prairie lakes have together caused an increase in the frequency and intensity of toxic algal blooms during the past 30 years,” says Leavitt. “Given that we expect both factors will intensify during the next 50 years, it seems likely that blooms will increase further and that oxygen will continue to be choked out of the water.”
“Widespread deoxygenation of temperate lakes” is an international collaboration led by Rose and Jane, with dozens of collaborators from the Global Lake Ecological Observatory Network (GLEON). The research was based in universities, environmental consulting firms, and government agencies around the world.
A copy of the Paper and its Supplementary information is available at https://www.nature.com/articles/s41586-021-03550-y
