Global warming could lead to widespread oxygen depletion in the deep layers of lakes across the planet as early as the end of the 21st century. This is the conclusion drawn by the scientists at the Helmholtz Centre for Environmental Research in Germany, Aarhus University in Denmark, Uppsala University in Sweden, and other European research centers. Using complex climate scenarios and hydrodynamic models, they have shown that even a moderate increase in greenhouse gas emissions will trigger regular anoxia in many lakes – depletion of the oxygen within the bottom layers. The study provides a realistic picture of the potential climate change effects on freshwater quality which the future of ecosystems and thus, the future of humanity directly depend on.
Lakes are much more complex than they appear to be at first glance. In summer, they resemble a giant layered cake: on top is a warm, light layer of water – the epilimnion – and at the bottom, there is a cold, dense hypolimnion. A unique barrier between them is the thermocline which prevents water from mixing. Under normal conditions, this barrier breaks down in the fall, and oxygen from the surface reenters the depths. But global warming disrupts this mechanism.
To understand just how serious the consequences could be, the scientists analyzed 73 lakes across the world – from the polar Finnish lake Kilpisjärvi to the tropical Lake Bosumtwi in Ghana, from small bodies of water less than one square kilometer in area to the giant African Lake Kivu covering about 2,700 km². For each of them, the researchers calculated how water temperature, duration of summer stratification, and the rate of oxygen depletion would change up to 2099. To do this, they used an ensemble of three lake models and five global climate models, which allowed for a more reliable forecast.
The problem turned out to be related not solely to the temperature rise itself.
First, warmer water holds dissolved oxygen less effectively. Second, warming intensifies and prolongs summer stratification in lakes: in some bodies of water, the period the deep layers remain isolated from the surface could be nearly a month and become half longer by the end of the century than it is today. Third, heat accelerates activity of the microorganisms decomposing organic matter at the bottom while actively consuming oxygen. As a result, the deep layers begin to literally “suffocate.”
To assess the threat, the scientists introduced two indicators. The first is “time to anoxia,” meaning the number of days it takes for the oxygen content in a lake depth to fall below a critical level. The second is the “anoxic coefficient” indicating the portion of the warm season a lake spends with virtually no oxygen.
The situation will be the worst in eutrophic lakes – bodies of water with high nutrient levels, where algal blooms are already common today. According to the researchers’ calculations, by the end of the century, up to 90% of such lakes will regularly experience anoxia.
However, another finding proved particularly alarming: even relatively clean oligotrophic lakes are not immune to this threat. While today only 13% of such lakes face oxygen depletion by late summer, this proportion could rise to 57% by 2099 under the most severe climate scenario.
At the same time, as modeling has shown, the primary factor will be the climate itself, not just the pollution degree in the lake. For example, high-altitude Finnish lake Kilpisjärvi located above the Arctic Circle will remain relatively stable even by the end of the century. In contrast, New Zealand’s Lake Taravera, which is also considered relatively clean, may begin to experience oxygen depletion in its deep water layers nearly a month and a half earlier by the end of the century due to the warmer climate.
The scientists warn that their forecast may somewhat underestimate the scale of the problem. The fact is that the model does not account for a number of other factors capable of further accelerating lake degradation: increased rainfalls and the runoff of organic matter from fields, changes in food chains, and rising concentrations of the dissolved organic carbon which darkens the water and intensifies its warming.
At the same time, the researchers emphasize that humanity still has an opportunity to slow the progression of this scenario. Reducing phosphorus and nitrogen pollution in lakes, combating eutrophication, treating wastewater and cutting greenhouse gas emissions can significantly delay the onset of anoxia.
