Thin ice for lakes worldwide

There are about 100 million lakes worldwide, most of which lie above the 45th North latitude and usually freeze over in winter. In recent decades, lakes worldwide have warmed and the extent and thickness of ice on seasonally frozen lakes has decreased. However, the role of climate change in these trends has not been scientifically proven until now.

Climate change, not climate variability, responsible

A research team with IGB lake physicist Dr. Georgiy Kirillin has recently shown that these global changes in lake temperatures and ice cover are not due to natural climate variability. They can only be explained by the massive greenhouse gas emissions since the industrial revolution. To prove this, the research team compiled observations of lake temperature and ice cover. They compared these data with the results of their simulations by global lake and climate models. The researchers found correlations between the observed changes in lakes and the model simulations of lakes in a climate influenced by greenhouse gas emissions. The results were published in the journal Nature Geoscience. The team's future scenarios, calculated with mathematical models, indicate that lake temperatures will continue to rise if emissions continue to increase, at a similar rate to the average global air temperature, and that ice thickness and duration will decrease accordingly.

1 degree of air warming – almost 10 days less ice cover

"According to the model scenarios, for every 1° Celsius increase in air temperature, lakes are estimated to warm by 0.9 °C and lose 9.7 days of ice cover," said Georgiy Kirillin. The team also projected future development under different warming scenarios. In a low-emissions scenario, the average warming of lakes in the future is expected to stabilise at +1.5 °C above pre-industrial levels and the duration of ice cover will be 14 days shorter. In a high-emissions world, these changes could lead to a +4.0 °C increase and 46 fewer ice days. "This means that lakes will change significantly in winter," added Georgiy Kirillin.

Ice cover important for seasonal processes in the lake

Although winter is commonly considered a time of relative calm, ecosystem functions are often dynamic during this season. Moreover, there is growing evidence that conditions in winter set the stage for conditions in summer and vice versa, and that lakes need to be considered across a complete seasonal continuum. Ice cover fundamentally changes a lake by isolating it from the surrounding landscape and atmosphere. The thickness and optical properties of ice and snow regulate the amount of solar radiation entering the lake while shielding it from wind energy. Consequently, ice cover is an important factor regulating the mixing of water in lakes and structuring vertical thermal and chemical gradients. This is important for oxygen availability in the water column, for example. Organisms, from bacteria to fish, have adapted to the winter environment. IGB researcher Dr. Stella Berger has investigated how fluctuations in the ice cover of lakes affect basic ecosystem processes or the structure of the food web.

The lake-ice continuum: What chemical and physical processes act at the ice boundary between the environment and the lake?

"We know that lake ice covers will decrease, but we have lacked a conceptual framework to understand and predict the impact of such changes on ecosystem structure and function," explained Stella Berger. The research team combined scientific knowledge with three case studies and developed the concept of the lake-ice continuum: this provides the framework for understanding how conditions change along a continuum of energy fluxes mediated by winter climate and conditions mediated by ice and snow in a lake. How the lake behaves under the ice cover depends on the environmental conditions, but also on whether the ice is clear or snow-covered. Without ice coverage, more energy in the form of heat and wind and also more allochthonous material from the atmosphere and the surrounding landscape enters the lake in winter. The study was published in the journal JGR Biogeoscience.

Changes in biology: winter makes summer – and vice versa

"The supply, accumulation and conversion of nutrients and carbon in winter will create the conditions for primary producers in spring. Winter thus decides the food for organisms that produce organic substances from inorganic substances via photosynthesis or chemosynthesis. They are the basis of the food web in water bodies," Stella Berger explained. For example, high chlorophyll-a concentrations in winter were associated with low chlorophyll-a concentrations in the following summer, possibly due to the reduction of the nutrient pool as a result of higher winter productivity. "So if metabolic activities in the lake are really cranked up in winter due to low ice cover, this can reduce the food for organisms in summer," added Stella Berger. And she mentioned another example: "In a lake without ice cover, the water warms up faster, which might boost the development of thermophilic blue-green algae. This can cause the water quality to deteriorate."

Changes in species diversity

There will be "winners" and "losers" at all trophic levels, as annual patterns of biodiversity are partly maintained by specialised niches created by the contrasts between winter and summer. The loss of winter habitats will reduce the diversity and abundance of cold-loving organisms. This phenomenon is already observed in fish, so it is expected that major changes in winter duration will alter community composition throughout the year, which may lead to ecological cascading effects such as water quality degradation.