Lakes stop freezing and why should we care?

Summer is long behind us, with winter on the doorstep. Lake area residents and limnologists may be asking themselves – will the lakes freeze over this year, when and for how long? Long-term measurement series indicate that it is likely to be later and for a shorter time than in the past, if at all. A synthesis of historical lake ice cover data, combined with global climate models, indicates that by 2080, more than 230,000 of the 1.4 million lakes larger than 0.1 square ^kilometers could be periodically or permanently ice-free. For lakes at lower latitudes, the number of days with ice cover could drop by as much as 80 percent. Just why should we even care? Because these changes have critical consequences for water quality, fisheries and biodiversity, weather and climate, as well as important cultural and socioeconomic aspects of human activity.

The ice cover on the lakes is disappearing, but does it matter?

Most of the world’s lakes freeze over, and the median duration of ice cover is 218 days. However, in response to climate change, the phenomenon is becoming less frequent and lasts shorter and shorter. According to scientists, the duration of ice cover has shortened by 31 days over the past 165 years, and thousands of lakes that have historically had ice cover every winter happen not to freeze at all [1]. Over the past 25 years, the rate of ice disappearance on lakes has accelerated significantly, by as much as 45 days per century in some regions of the Northern Hemisphere.

Although the problem of shortening winter ice in lakes due to climate warming is quite well documented [2, 3], including in Poland [4], its social, economic and environmental consequences are still poorly recognized [1]. One reason for this state of affairs is the relatively low availability of winter limnological research, which until recently was niche in nature. This was often due to certain logistical difficulties, but also to the misconception of winter as a period of dormancy and “rest” of the ecosystem.

The disappearance of the ice caps observed in recent years and technological advances have admittedly significantly stimulated the development of winter lake studies, but our knowledge of the effects of the phenomena occurring then is still very limited. And they are very significant, and in many ways, as demonstrated by American scientists on the basis of a comprehensive review of the issue that has just appeared in the Climate Change section of the journal Science [1]. The results of the work of Hampton and co-authors prove that the disappearance of ice on lakes is not at all indifferent to our functioning.

Ice on the lake is important for water quality

One of the consequences of the disappearance of ice cover on lakes is the reconstruction of geochemical cycles and ecological processes in them. A longer period of exposure to warm air and solar radiation causes the water to heat up faster, leading to a longer growing season and stronger summer thermal stratification. These phenomena, in turn, affect the internal dynamics of nutrients and contaminants, which are important for water quality. Faster-setting and longer-lasting thermal stratification and the lack of full spring mixing of the waters lead to rapid depletion of oxygen resources in the overlying layers (consumed, for example, in microbial decomposition processes of organic matter). When water at the sediment boundary is deprived of oxygen, nutrients (mainly phosphates) and other contaminants (e.g., some metals) trapped in the sediments are released and become bioavailable.

The mobilization of additional nutrient resources and higher temperatures, in turn, promote blooms of phytoplankton, including cyanobacteria. And the effects of their presence in lakes are well known to us. Cyanobacteria can produce toxins that adversely affect human and animal health, limiting recreational use and making it difficult to treat water for drinking purposes. Metals released from the sediment can directly threaten drinking water quality, especially if the intake is in or near an anaerobic zone. They can also bioaccumulate or biomagnify in the food chain, threatening consumers (including humans) who eat fish from these lakes. Of course, long periods of lake ice also promote deoxygenation of the bottom layers and can contribute to the release of substances bound in the sediments, but studies suggest that water quality problems resulting from shortened periods are more severe.

All this should effectively dampen unwarranted enthusiasm for higher summer temperatures – yes, the water in the lakes will be warmer, but in all likelihood dirtier and with more frequent blue-green algae blooms.

Importance of ice for biodiversity

Lake ice forms a specific ecological niche. The prevailing conditions there are characterized by low temperatures (<4°C), relatively stable throughout the water column, limited light availability and protection from ultraviolet radiation. At the ice-water boundary and in the ice itself, specific sympagic habitats (cryophilic organisms, associated with the ice) are formed, inhabited by specific microbial communities and increasing the biodiversity of seasonally ice-covered ecosystems.

The disappearance of the ice cap changes habitat conditions, which obviously affects organisms and thus taxonomic composition and diversity. First of all, it promotes range expansion and invasions of species adapted to warmer environments, including fish, invertebrates and plants [5], but also strongly affects the structure of phyto- and zooplankton [2] and bacterial communities, both in winter and during other seasons. Ice-associated organisms are an important food source for representatives of higher trophic levels, so their disappearance will have consequences for the entire trophic network. The rearrangement of biological assemblages can lead to a complete reorganization of ecosystems, likely resulting in a regime shift toward new ecological states [2].

The alternation of ice-covered winter periods and warm open water seasons allows cold- and warm-water fish with distinct ecological niches to coexist in the same lakes. Species native to seasonally freezing lakes have developed the necessary physiological and behavioral adaptations that organisms native to warmer regions lack. The disappearance of ice phenomena will favor species of warmer environments, leading to the withdrawal of cold-loving ones, with consequences for fish stock composition and diversity.

Some animals use the ice cover of lakes to implement critical life stages for survival, such as reproduction or migration. Therefore, the loss of ice and its associated niches can contribute to a reduction in the seasonal taxonomic diversity of biological assemblages on an annual or multi-year scale.

The importance of ice for the quality of fisheries management

Since the disappearance of the ice cover results in the reconstruction of ichthyofauna assemblages, it will not be without consequences for human fisheries management. Fish species inhabiting seasonally ice-covered lakes are often culturally and economically important. Many of them are important sources of food, and catch rates for some species (such as the common pike) are higher in winter than in summer.

Weaker ice cover, or lack thereof, can negatively affect winter fishing in a number of ways. First, access to fish in winter often depends on the longevity of the ice, and thinner and weaker cover can be dangerous and risk collapse. Second, altered ice conditions can affect the timing and success of fish reproduction. Shorter ice cover and higher winter temperatures have been linked to earlier spawning, reduced survival of eggs and larvae, and smaller gonads. Thus, fish that prefer a cooler habitat and can survive during winter periods, including culturally important cold-water salmonids, are most at risk from warmer winters.

In addition, the nutritional value of fish as a source of omega-3 and omega-6 fatty acids decreases as temperatures rise, due to changes in their food base. At higher temperatures, zooplankton and other primary consumers store less fatty substances [6, 7]. Consuming food with lower nutritional value can affect the metabolism and growth rate of fish at different stages of their development [8]. This also has consequences for humans, as fish not only lose their nutritional value, but also change their taste qualities. And this can affect consumer preferences and choices. Fish harvested from the same fisheries no longer taste the same as they used to.

The importance of ice for many socio-economic and cultural aspects of human activity

Few people are aware of how many cultural and social aspects of human activity are associated with lake ice, such as recreation (e.g. ice fishing), cultural identity, traditions and even aesthetic experiences. Many of these have quite real economic dimensions. For example, it was estimated that in 2011 anglers in the United States spent about $240 million on ice fishing equipment, and in Sweden this form of recreation generates about 0.03 percent of gross domestic product (about $880 million).

In some areas of the world, ice cover is an essential component of transportation infrastructure, especially for isolated northern communities, as well as some commercial ventures (e.g., ice roads are widely used in oil and gas exploration in the remote north).

The minimum thickness of ice safe for humans is about 10 cm of strong black ice. Historical data indicate that the period of time it lasted in this form on the lakes of the northern hemisphere averaged 152 days. When global air temperature rises by 1.5° to 3°C, this time is shortened by an average of 13-24 days, and at lower latitudes it can drop by as much as 80 percent. The quality of the ice is also changing toward white ice, which is mechanically weaker than black ice, increasing the risk of collapse and drowning. So climate change threatens the transportation infrastructure and migration routes provided by winter ice routes.

For high-mountain and alpine communities, the disappearance of ice on lakes is associated with a change in the environmental conditions that have shaped their cultural identity, social traditions and economy for centuries. Climate warming will mean a change in their way of life. Whether they will live better or worse is another question. But certainly different.

Importance of ice for climate phenomena

The presence or absence of ice on lakes is not without consequences for weather and climate phenomena. Ice cover effectively insulates lakes from the atmosphere, impeding gas exchange at the water-air interface. Lakes with longer ice cover tend to be colder at other times of the year as well, which slows temperature-dependent biological processes and has consequences for the carbon cycle and greenhouse gas emissions. There is evidence that ice cover increases carbon retention. It also delays atmospheric emissions of methane, which is converted to carbon dioxide by microbial oxidation in the presence of oxygen. Some studies also suggest that colder lakes with longer ice cover have lower annual emissions of nitrous oxide, another greenhouse gas [9].

Lake ice affects the global water cycle by preventing water loss through evaporation. The results of Zhao and co-authors [10] show that global water loss by evaporation from lakes increased at a rate of 3.12 ^km3 per year from 1985 to 2018, with 23 percent of the increase attributed to increased evaporation from open water due to the disappearance of ice cover on lakes. Although the loss also occurs above the ice cap, through sublimation, losses due to evaporation from open water are usually much greater.

Continuing trends of ice cap disappearance will increase the rate of evaporation on a global scale. And the water that evaporates from a lake may return to the Earth’s surface in the form of precipitation in a completely different, sometimes very remote location. This means that the lack of ice could be a significant contributor to lake water deficits and declining mirror levels, quite commonly observed recently in our country as well.

Ice cover on the lake matters to us

Aspects and effects of the disappearance of ice cover on lakes can still be listed in abundance, although many of them are not fully recognized. Multi-year measurement series clearly indicate that lakes freeze later, thaw earlier, or both occur. The rate of ice disappearance has clearly accelerated over the past 25 years, and we can expect this phenomenon to accelerate further in the face of future climate scenarios. And this will have significant implications for the physical, chemical, biological and social aspects of freshwater systems. As a result, we may find that our favorite lake splashes and ripples beautifully in the winter, but blooms with blue-green algae in the summer, and the fish caught in it are no longer what they once were.

In the article, I used, among others. z:

1. Hampton S. E., Powers S. M., Dugan H. A., et al. (2024). Environmental and societal consequences of winter ice loss from lakes. Science 386, eadl3211, 10.1126/science.adl3211.
2. Smol J. P., Wolfe A. P., Birks J. B., et al. (2005). Climate-driven regime shifts in the biological communities of arctic lakes. PNAS, 102 (12) 4397-440, https://doi.org/10.1073/pnas.0500245102
3. Woolway R.I., Dokulil M.T., Marszelewski W., et al. (2017). Warming of Central European lakes and their response to the 1980s climate regime shift. Climatic Change 142: 505-520.
4. Choiński A., Ptak M., Skowron R., Strzelczak A., (2015). Changes in ice phenology on Polish lakes from 1961 to 2010 related to location and morphometry. Limnologica, 53, 42-4. https://doi.org/10.1016/j.limno.2015.05.005.
5. Rahel F.J., Olden J.D., (2008). Assessing the effects of climate change on aquatic invasive species. Conserv Biol. 22(3): 521-33. doi: 10.1111/j.1523-1739.2008.00950.x.
6. Hayden B., Harrod C., Sonninen E., Kahilainen K. K., (2015). Seasonal depletion of resources intensifies trophic interactions in subarctic freshwater fish communities. Freshw. Biol. 60, 1000-1015. doi: 10.1111/fwb.12564.
7. Schneider T., Grosbois G., Vincent W. F., Rautio M., (2017)Saving for the future: Pre-winter uptake of algal lipids supports copepod egg production in spring. Freshw. Biol. 62, 1063-1072. doi: 10.1111/fwb.12925.
8. Taipale S. J., Pulkkinen K., Keva O., et al. (2022). Lowered nutritional quality of prey decrease the growth and biomolecule content of rainbow trout fry. Comp. Biochem. Physiol. B Biochem. Mol. Biol. 262, 110767. doi: 10.1016/j.cbpb.2022.110767.
9. Kortelainen P., Larmola T., Rantakari M., et al, (2020). Lakes as nitrous oxide sources in the boreal landscape. Glob. Change Biol. 26, 1432-1445. doi: 10.1111/gcb.14928.
10. Zhao G., Li Y., Zhou L., Gao H., (2022). Evaporative water loss of 1.42 million global lakes. Nat. Commun. 13, 3686. doi: 10.1038/s41467-022-31125-6.