Oxygen deficiency remains in the lake's ecological memory

When a lake is frozen, oxygen, which is vital for fish and other aquatic life depletes as the ice and snow prevent mixing with the air and limit oxygen production via photosynthesis. As the climate warms, the time a lake is covered with ice shortens, but the oxygen situation in all lakes has not improved.

An extensive study revealed that the oxygen content of small lakes does not increase, even though the ice cover is shorter. The situation is different for larger lakes over 10 ha. The results have been recently published in the scientific journal PNAS.

An international research team led by Joachim Jansen, a postdoctoral researcher at the University of Helsinki, used data collected from lakes between 1960 and 2022, including Finnish lakes.

In general, shorter ice cover increased the amount of oxygen, because the winter oxygen consumption period was shortened and the autumn mixing took longer, allowing more time for oxygen exchange with the air.

However, in small lakes, such as most Finnish lakes, oxygen is increasingly depleted in summer and insufficient oxygen is mixed with the surface water during the fall. This means that there is less oxygen available at the start of winter. As a result of this inter-seasonal ecological memory, the anoxic zones under the ice have expanded. The situation is particularly problematic in eutrophic lakes where oxygen consumption rates are high. In contrast, nutrient-poor lakes are less likely to develop oxygen deficits in summer and in winter.

The number of winter fish kills may decrease as the ice recedes only in larger northern lakes over 10 km2. 

“The results show that the effects of climate change are not unambiguously positive or negative – they depend on the size and characteristics of the lake,” says Joachim Jansen.

The study provides important information for the protection of lakes and the management of fish stocks in a changing climate.

Summary:

  • In big lakes (>10 km2) we see a positive trend in under-ice oxygen because of rapid shortening of ice cover duration, and little oxygen loss in summer.
  • In small lakes (<1 km2) we see a negative trend in under-ice oxygen because significant oxygen loss in summer is transferred to the winter season.
  • Lakes with sizes in between show no significant oxygen trend because the two drivers (ecological memory and ice cover shortening) cancel each-other out.

Reference:

J. Jansen, G.A. Weyhenmeyer, L.H. Härkönen, A.M. Paterson, P.A. del Giorgio, & Y.T. Prairie,  Divergent oxygen trends in ice-covered lakes driven by ice-cover decline and ecological memory, Proc. Natl. Acad. Sci. U.S.A. 122 (27) e2426140122, https://doi.org/10.1073/pnas.2426140122 (2025).
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