Our winters are getting milder and as a result, many lakes freeze less and less frequently. However, this has far-reaching consequences, as ice cover brings about fundamental changes to water and habitats – and most aquatic organisms have adapted to it. Scientists have now investigated in more detail what the consequences would be for limnic ecosystems if there is no winter rest under the ice.
There are about 100 million lakes around the world. Most of these lie in cold regions beyond the 45th parallel north, meaning that these lakes regularly freeze in winter. Ice cover insulates lakes from the surrounding landscape and environment, affecting fundamental physical, chemical and biological processes in these waters. “Sea ice affects the transfer of energy, heat, light and material between lakes and their surroundings, creating conditions that are dramatically different from those of open water,” explain Emily Cavaliere of the University of Saskatchewan and her colleagues. Huh.
different world under the snow
The research team examined the consequences of fluctuating or absent winter snow cover on these processes within the framework of three case studies. “We know that lake ice cover will decrease, but so far we lack a conceptual framework to understand and predict the effects of such changes on ecosystem structure and function,” said the Leibniz Institute for As co-author Stella Berger of Freshwater Ecology explains. and Inland Fisheries (IGB) in Berlin. The team’s sea-ice continuum concept allows changes and effects in an ice-covered lake to be described in greater detail.
For example, the thickness and optical properties of ice and snow control the amount of solar radiation entering the lake while shielding it from the wind. As a result, ice cover also controls water mixing and affects the vertical thermal and chemical gradient of the lake. It is important, for example, for the availability of oxygen in the water column. Also, changed water temperatures have an effect: “In a lake without ice, the water heats up more rapidly in spring, which can result in the growth of heat-loving blue-green algae. As a result, The water quality may deteriorate,” says the researcher.
effect the following summer
Another factor is nutrients: “The supply, accumulation and conversion of nutrients and carbon in winter creates conditions for primary producers in spring,” explains Berger. Winter therefore decides on the food that is available to the algae in spring and summer and thus forms the basis of the food web. For example, the team found that high chlorophyll concentrations in the winter coincided with low chlorophyll levels in the following summer. Obviously, the high productivity of algae in mild winters means there is a lack of nutrients in the coming months. “So if the metabolic activities in the lake are actually increased in winter due to reduced ice cover, it could reduce the amount of food for living creatures in the summer,” Berger says.
All this affects not only the primary producers in the lake, but also the entire habitat of the water body through the food chain. As the scientists point out, if ice cover continues to decrease, there will be winners and losers at all levels of the food web. The latter mainly includes organisms whose seasonal activity is strongly adapted to the water body’s winter rest period and the strong contrast between summer and winter. Because they occupy a specific place in the ecological composition of the limnic environment, ice loss will particularly harm these species. This phenomenon can already be observed in fish, the research team explains.
This means for the future: as a result of climate change and increasingly mild winters, the composition of the biocenosis in lakes will change – even in our latitudes. According to Cavaliere and colleagues, the exit of some predominantly cold-loving species would create ecological cascade effects, resulting in changes across the entire spectrum of species.
Sources: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB); Articles: Biogeology, doi: 10.1029/2020JG006165
